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Poly--amino acid fibres.

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Die Angewandte Makromolekulare C h m i e 22 (1972) 107-131 (Nr. 299)
From the Department of Polymer Science, Faculty of Science,
Hokkaido University, Sapporo, Japan
Poly-a-amino Acid Fibres
By JUNZO NOGUCHI
*) * *), SEICHII
TOKURA,
and NORIONISHI
(Eingegangen am 3. Juni 1971)
SUMMARY:
Poly-a-amino acid fibre being rich in /?-form has a silky quality. The fibre being
rich in a-form becomes woolly. When there is a strong interaction force through side
chains of poly-a-amino acid molecules, the polymer solution is not spinnable regardless of the film forming ability. As it is difficult to spin a strong fibre directly from a
polymer in a-form, the spinning solution should contain some parts of random coil
in the a-form and the fibre molecules should be oriented in a parallel with the axis by
stretching. Finally, most of the remaining p- or random-partsshould be retransformed into the a-form. The p-form of poly-L-alanine is most stable and most analogous to silk, but most of p-form in poly-L-leucine stretched fibre is transformed into
the a-form in boiling dioxane and the woolly character appears. The character of polya-amino acid fibres seems to depend on the contents of a- and p-form. Poly-L-leucine
fibre in the a-form is stable in boiling water and it seems to be unnecessary to introduce a cystine bridge into the fibre to maintain a woolly character. By copolymerizing a small amount of methionine into a polymer, the dyeability of fibres is remarkably improved.
XUSAMMENFSSUNG :
Die an p-Form reiche Poly-a-aminosiiure-Faserhat seidenartige Eigenschaften.
Ihgegen ist eine a-reiohe Faser der Wolle ahnlich. Wenn starke Wechselwirkungen
durch Seitenketten von Poly-a-aminosauremolekiilen
bestehen, ist die Polymerlosung nicht spinnbar, obwohl sie die Fahigkeit hat, einen Film zu bilden. Da es schwierig ist, eine starke Faser in a-Form direkt aus dem Polymeren zu spinnen, sollte die
Spinnlosung einige Teile statistisch gehartete Molekiile in der &-Form enthalten, und die Fasermolekule sollten durch Strecken parallel zur Achse angeordnet
werden. SchlieBlich sollten die meisten der iibriggebliebenen @- und a-Teile wieder in
die &-Formzuriickverwandelt werden. Die ,!?-Formdes Poly-L-Alaninsist am stabil
sten und der Seide am iihnlichsten. Aber fast alle 8-TeiIe von gedehnten Fasern aus
I’oly-L-Leucin werden in kochendem Dioxan in die &-Formverwandelt, und die
*
**
Department of Polymer Science, Faculty of Science, Hokkaido University, Sapporo, Japan.
Presented a t the Scientific Symposium “The Physics and Chemistry of Fibre
Materials”, Miinchen, 3rd and 4th June 1971.
107
J. NOGUCHI,
S. TOKURA,
and N . NISHI
wollartigen Eigenschaften zeigen sich. Die charakteristischen Eigenschaften der
Poly-a-aminostiure-Fasern scheinen auf dem Gehalt an a- und p-Formen zu beruhen.
Die Poly-L-Leucin-Faserist in der a-Form stabil in kochendem Wasser, und eine
Cystin-Bruckein der Faser scheint nicht notig zu sein, um den wollartigen Charakter zu bewahren. Durch Copolymerisation mit einer kleinen Menge von Methionin
wird die Anfarbbarkeit der Fasern bedeutend erhoht.
1. Essential conception of protein fibres
Recently, several kinds of synthetic fibres have been developed and have
come t o share the market with natural fibres. Though the synthetic fibres have
been utilized in the various fields according to their characteristics, there is
still great room for improvement t o copy the silky or woolly characters. The
protein chemistry has been advancing remarkably in this century and the conception of protein structure has clearly been established. An animal fibre has
to be revaluated from the standpoint of protein chemistry.
Natural protein and poly-a-amino acids have common polypeptide backbone
as a primary structure (Fig. 1). Though the amino acids in natural proteins are
NH3CHCONHCHCO-
I
RI
I
- - - - - - - - - - -NHCHCOOH
I
Rz
Component a-amino acid
Ri
*
NHzCHCOOH
I
Ri
R
Fig. 1. Primary structure of proteins (polypeptides).
108
Poly-a-amino Acid Fibres
limited to L-configuration, D- or DL-amino acids should be also included in
the field of synthetic poly-a-amino acids.
a-helix
a-helix
/?-coil
random coil
/?-coil
/”
Fig. 2. Secondary structure of proteins (polypeptides).
Therefore, various secondary structures which greatly influence the character
of polypeptide, exist in the structures such as a-helix, random coil and stretched
,&coil (Fig. 2). It makes a big difference in the formation of characters of silk
and wool that silk is in a ,!?-form1and wool is in an a-form2. I n the a-form, the
hydrogen bond bridges are intramolecular by amide bonds of polypeptide. The
intermolecular hydrogen bond forms /I-form. However, the amide bond of polypeptide chains is considered to be free in the random coil and to be related t o
the dyeability of fibre.
109
J. NOGUCHI,
S. TOICURA,
and N. NISHI
a
Fig. 3. Tertiary structure of proteins (polypeptides).
Moreover, the ionic bond among amino acid residues and the S S bridge formed by cysteine residue, maintain the whole shape of the molecule. Such a high
order of protein structure is called as the tertiary structure of protein (Fig. 3).
While the essential properties of proteins depend on the primary, secondary and
tertiary structures, those of protein fibres are mainly influenced by the primary
and secondary structures. The chemical fibres have generally been spun by the
orientation of linear molecules and this conception seems to have been applied
to the regenerated protein fibres without any consideration on the conformational change in the spinning process. However, poly-a-amino acids have shown
clear conception between the spinnability and the molecular conformation of
the proteinlike polymer.
2. General aspects on ply-a-amino acids
Among the polyamides (-NH(CH2),CO--),
prepared from o-amino acids
(NH2(CH2)nCOOH),the representative synthetic fibre is “Nylon” and amino
acids of n 2 5 are generally utilized. They are polymerized easily to high polymers by heating, but those of n < 5 cannot be obtained in high molecular
weight only by heating. Recently, a series of polymers of n < 5 has been prepared by other methods. I n the case of n = 1, so-called 2 nylon, the resulting polymer is a poly-a-amino acid which contains D- or L-type of a-amino acid and
polypeptide conformation like a protein as described above. Natural proteins
correspond to poly-L-a-amino acids and are estimated as an isotactic stereocopolymer of L-a-amino acids from the form of polymerization. As “Nylon”
contains the repeated structure of amide bonds ( 4 0 - N H - )
in the main
chain of the molecule as that of proteins and contains the ,!?-formin its structure, silk-like properties are partly obtained.
110
Poly-a-aminoAcid Fibres
But the essential properties are far from the characters of protein fibres.
:Poly-a-amino acid fibres are expected to show the properties similar to silk or
wool, because the chemical structure of poly-a-amino acids is similar to protein.
:Recently, a-amino acids were chemically polymerized into high molecular
weight up the order of natural proteins3. Though several methods for the polymerization of a-amino acids have been developed, the only way to give a high
polymer having spinnability and film forming ability, is so far the LEUCHS’
method4 :
NHzCHCOOH
I
R
--f
CO-0
I
----f
I
NH
CO
\CH/
. . . (NHCHCO)n.. .
I
R
I
R
a-amino acid
a-amino acid NCA
poly-a-aminoacid
D- or L-a-amino acids give isotactic polymers: a-helix, ,!?-coil or random
coil in the same handed sense. DL-a-amino acids give atactic polymers: random coil in the mixture of opposite handed senses.
Several poly-a-amino acids obtained by the LEUCHS’method have been studied to obtain synthetic fibres. Poly-a-amino acid fibres including poly-alanine
were studied by the researchers of du Pont Co. Ltd. during 1945-1952 and
poly-y-methyl-glutamate were also studied by Courtauld’s researchers led by
Professor C. H. BAMFORD
during 1955-1962. Their studies on the poly-a-amino
a,cid fibres have been followed in Japan by many investigators of Universities,
National institutes, Fibre Industries and the companies supplying amino acids.
The results have revealed to some extent in many patents, but the details were
scarecely published in papers5. The results obtained mainly in our laboratory
will be shown in this paper. The scale of experiments, therefore, is small and
the conditions of spinning will be further improved by the industrial field. The
poly-a-amino acids of high molecular weight which give film forming ability
and spinnability are expected in the a-form. On the other hand, high molecularweights are hardly expected in the polymerization of ,!?-or random-form in
solution. It is necessary to select the solvent and to use highly purified
solvents in the polymerization of amino acids. Moreover, if either D- or
L-amino acid is polymerized, it gives isotactic polymers which form a-helix
or ,!?-coil in lev0 or dextro sense. However, DL-amino acids give atactic
block polymers which form random coils by the distraction of the helix due
to the migration of diagonally opposite helix. Even if there is the fibre
from a DL-copolymer, the content of ,!?-formis not sufficient to form ,!?-structure among polypeptide chains and poor water resistance of the fibre is obtain111
J. NOGUCHI,
S. TOKURA,
and N. NISHI
ed. Moreover, there are two groups of amino acids which polymerize in a-form
and /I-form: Ala, Leu, Lys, Glu, Phe, Met, give a-helixe type polymerization
and high polymers with film forming ability and spinnability.
Gly, Val, Ser, Cys give B-coil type polymerization and low molecular weight
products without spinnability as well as random type.
I n the spinning of poly-a-amino acid fibres, the strength and the luster of
fibre depend on the content of B-form which is obtained by the elongation of
fibre after the spinning. It seems to be important that amino acids are polymerized in the a-form and the fibres are transformed into the p-form from aform or random form by the elongation in the spinning bath after the spinning of the fibre.
This simple information can be applied usefully to the spinning of protein
fibres. I n the poly-a-amino acids polymerized in a-form, poly-L-phenylalanine6
of high molecular weight is obtained, but it is impossible t o spin fibres owing
to the lack of proper solvents to dissolve in the spinning process. Poly-Lleucine7 is obtained as a clear gel of a-form in benzene and the clear gel forms
a liquid crystal below 50°C which retains the tixotropic properties. The benzene solution of poly-L-leucine forms films but has no spinnability. Poly-ybenzyl-L-glutamate is polymerized in the a-form and forms films easily, but is
poor in spinnability as well as poly-8-carbobenzyloxy-L-lysine.
Poly-a-amino
acids containing long side chains such as poly-DL-a-aminolauric acid can be obtained in a high molecular weight in petroleum benzine and forms a soft film
though lacking spinnabilityg. Generally, when the interaction force among side
chains of poly-a-amino acid molecules is so strong that the molecules associated with each other, only film forming ability is given without spinnability. As
mentioned above, DL-a-amino acids are polymerized in random coils so that it
is hard to obtain high polymers sufficient to prepare films or fibres.
Poly-a-amino acids of high molecular weight are not always expected, even
in dioxane or dimethylformamide known as a helix solvent for NCA polymerization, because the molecular weight of poly-a-amino acids depends on a variety
of amino acid NCA and on the polarity of the solvent. The high molecular
weight is expected by the use of rather nonpolar solvents and also by applying
high concentrations of NCA in the polymerization. But NCA’s are hardly soluble in such nonpolar solvents generally. Even if the poly-a-amino acid is obtained in a clear solution with the use of a helix solvent, the isolation of the polymer from the solvent sometimes reduces the solubility greatly owing to the
change of the secondary structure. It is most important to select the solvent to
give a clear solution without any precipitate in the polymerization and, to be
spinnable in high concentrations of polymer. A few poly-a-amino acids which
are polymerized through a stable a-helix should transfer a part of their helical
112
Poly-a-amino Acid Fibres
structure into random coils in the spinning process by the addition of a random
solvent such as dichloroacetic acid. It is possible to prepare the fibres of alanineg,
glutamateg, leucine7 and methioninel0 as far as above conditions are satisfied.
Since DL-amino acid NCA's don't polymerize generally to high polymers, it is
necessary to use either L-amino acids or D-amino acids as starting material.
Poly-a-amino acids which polymerize through /?- or random form are also lacking in spinnability because of their low molecular weight. On the other hand,
poly-a-amino acids having [T]red" 2 in dichloroacetic acid are generally
spinnable. I n the spinning of poly-a-amino acids, it is very important not only
to put the fibrous molecules side by side through nozzles as seen in the preparation of cellulose fibres and other synthetic fibres, but to take the change of the
secondary structure of molecule into consideration.
The melt spinning is unable to apply to poly-a-amino acid fibres owing to the
decomposition of polymer without melting. Also dry spinning hardly produces a
fibre of fine quality because of too short time to change the secondary structure of molecule in the spinning process. For example, in the spinning of polyy-methyl-L-glutamate,the fibre coagulates before the full deformation of the secondary structure so that the quality of the fibre is unsatisfactory. Therefore,
on the application of dry spinning to poly-a-amino acid fibres, the above mentioned defect should be improved. Thus, the most suitable process, in the preparation of poly-a-amino acid fibres seems to be wet spinning. Collagen fibre is
unable to get a fine quality except the heat treatment a t 40 to 60 "C which is the
deformation temperature. When the elongation step is applied to the coagulation in a hot water bath (60 to 80 "C) and heat treatment over 100 "C is followed
in a wet state, a fibre of fine quality is expected. The qualities of the fibre such
as luster, feeling, strenght and etc. depend seriously on the spinning conditions.
Polyacrylonitril seems to be a synthetic fibre having similar spinning conditions
as poly-a-amino acid fibres, because polyacrylonitril stays as a coil in the solution, and the strength and luster of polyacrylonitril fibres are known to be increased by the heat setting a t 110 "C immediately after the spinning a t the wet
state as well as poly-a-amino acid fibres. Since the quality of poly-a-amino acid
fibres seems to be attributed to the secondary structure, the spinning condition is most important to decide the character of fibre. Therefore, there is room
to modify the properties of poly-a-amino acid fibres by the change of a- and
/I-contents in fibres.
3. Preparation of polyamino acid fibres
.Among the poly-a-amino acid fibres, poly-alanine and poly-glutamate have
been mainly studied from the stand point of silkyfibres5>9.As mentioned above,
the silky quality depends on the /?-contents in the poly-a-amino acid fibres.
113
J. NOGUCHI,
S. TOKURA,
and N. NISKI
From this point of view, poly-alanine fibres are superior to poly-L-glutamate fibres in the silky quality, because the former have shorter side chain length than
the latter, ,and thus there is a difference of resistivity against the deformation
into j3-form upon side chain length. Since most of the data on the preparation
of poly-L-alanine and poly-y-methyl-L-glutamate fibres are not reported in details, the results applied in our laboratory will be explained. Amino acid materials should be powered and dried as far as possible, because the mass parts cause
the contamination of impurities by the prolonged time course in the smooth reaction between phosgen and amino acid into NCA. Carefully purified solvents
should be used for the polymerization.
Poly-L-alunine fibre9
L-Ala NCA is easily prepared from L-Ala and phosgen by the normal route :
NH2. CHCOOH
I
CH3
COCl2
--+
50 "C
co-0
I
I
NH CO+ZHCI
\ /
CH
L-alanine
CH3
L-alanine NCA
The key points of NCA preparation are the quick saturation of phosgen in the
solvent and the separation of hydrogen chloride from NCA with quick distillation of the solvent. The following treatment with ether or acetonitril efficiently
removes hydrogen chloride. Poly-L-alanine is prepared by heating the NCA in
toluene a t 70 to 80 "C without any initiator as a gel separated from solvent. The
molecular weight of poly-L-alanine thus obtained is rather higher than that of
poly-L-alanine prepared with use of initiator and the spinnability and film forming ability are attached t o po1y;L-alanine free of initiator. Tetrachloroethane is
also a good solvent for the polymerization. However, poly-L-alanine is separated from the solvent in a gel form so that the reaction mixture is unable
to be used as the spinning solution. Once the polymer is separated with the
addition of alcohol, it is dissolved again in dichloroacetic acid a t the concentration of 4 to 5% of polymer.
The polymer is spun into hot water (60°C) and the thread is stretched
twice a t 70 to 80 "C (Fig. 4).Silky fibre is obtained by washing and boiling
with water (one hour) followed by drying a t 100 "C in air. As poly-L-alanine
shows the a = j3 transition a t about 60°C in water, the spinning condition is
based on this transition. Although the poly-L-alanineforms exellent fibres, the
solvent is limitted owing t o be sparingly solubility in other solvents except
114
Poly-a-amino Acid Fibres
10
6
\\
1'
3'
Fig. 4.
I
5
I
//
/ /
I
I /
4
Spinning apparatus of poly-L-alanine. 1 : Hot water (60°C), 2: spinning
solution, 3: pump, 4: nozzle, 5 : 1st coagulation bath of 60°C hot water,
6 : roller, 7 : back roller, 8 : 1 m length of 70-80 "C hot water bath, 9 : front
roller, 10: hot roller (llO'C), 11: winder.
dichloroacetic acid. It would be worth studying to seek such a common solvent
for polymerization and spinning as seen in the case of poly-L-glutamate fibre
from the economical point of view. The apparatus is corroded by dichloroacetic acid.
Poly-y-methyl-L(D)-glutamatefibreg
The conditions of preparation of glutamate NCA and the polymerization of
glutamate NCA have been improved by Azinomoto Co. Ltd.12 and Kyowa
Hakko Kogyo Co. Ltd., in Japan. When glutamate reacts with phosgen a t the
boiling point of dichloroethylene, pure NCA is obtained and the NCA is polymerized into high polymers in methylenchloride without any turbidity. Once
the polymer is separated from the solvent, it hardly becomes soluble in the
same solvent. Dichloroethylene is not a good solvent for the spinning, because
of the difficulty in coagulation. y-Methyl-L-(D)-glutamateNCA is polymerized in methylenchloride a t 50°C with the use of triethylendiamine as an
initiator.
7T
9
7
6
I
I
10
8
Fig. 5.
Spinning apparatus of poly-L-methyl glutamate. 1 : Spinning solution,
2 : pressure, 3 : nozzle, 4 : coagulation bath (1,5 m length, 20 "C acetone),
5 : washing bath of water, 6 : back roller, 7 : front roller, 8 : 1 m length of
4 0 ° C water bath, 9 : hot roller (llOOC), 10: winder.
115
J. NOGUCHI,
S. TOPURA,and N. NISHI
The clear solution of 5.5% polymer in the mixed solvents (methylenchloride:
dioxan 80:20 v/v) is spun into acetone a t room temperature and the thread
is stretched up to 1.7 times of length in hot. water (60°C). The procedures of
treating it with a hot roller a t 110 "C increased a silky luster of the fibre (Fig. 5 ) .
According to the Courtauld's Patentl3, the concentration of the spinning
solution is optically isotropic up to 10% of polymer concentration and optically anisotropic over 14% of it. This phenomenon should be remarked in the
preparation for spinning solutions, because the formation of liquid crystals is
used to interfere with the spinnability. As the y-carboxyl group of the glutamic
acid has to be blocked by an ester group before the preparation of NCA, the
dyeability of the fibre is not satisfactory enough in comparison with protein
fibres and the YOUNG'S
modulus of poly-glutamate fibre is less than that of silk.
These defects seem to be due to the long side chain of glutamate. Poly-glutamate fibre contains a large portion of j3-form with a small portion of a-form
and the ratio of both forms remains almost constant even after the treatment
with dioxanel4, in spite of the reduction of silky luster. I n our country, glutamic acid is supplied from petroleum chemistry and fermentation chemistry.
The recrystallization from conglomerates of D- and L-glutamic acids is employed as a method for the resolution of synthetic DL-glutamate in industry
and D-glutamate is also used for the fibre as well as L-glutamate. Therefore,
the starting materials of poly-L(D)-glutamate and the technics upon fibre
productions have been already settled. The industry will be able to respond
whenever some special uses are developed. I n fact, the dope of poly-y-methylL(D)-glutamate in dichloroethylene is already supplied to our market as a
coating material for synthetic leather.
Poly-L-methionine fibrelo
L-Methionine NCA is not easy to get in crystals in a good yield, but the
aceton solution of syrupy methionine NCA is purified easily by passing it
through a column of active charcoal absorbed by silver oxide. Dry acetone is
most suitable for this purpose, because other solvents initiate the polymerization reaction in the process of purification with the column and the polymer
inhibits the flow through the column. The syrup after the removal of acetone
is able to be polymerized in methylenechloride into a clear solution and the
resulting polymer is spinnable into fibres. The stretching process to 1.4-1.9
times of length is proceeded in hot water of 65°C and the filaments are treated
with a hot roller of 110 "C. Dichloroethylene makes the polymer solution turbid
and tetrachloroethane gives a very viscous but clear solution. However, both
of them are unspinnable into fibres, because no proper solvents for coagulation
116
Poly-a-aminoAcid Fibres
were found. When the fibre is boiled in dioxane, it shrinks to less than half
of full length and threads stick each other by melting.
Poly-L-leucine fibre7
L-Leucine NCA is polymerized in benzene into a clear solution a t 70°C in
a. sealed bottle. The solution forms a gel below 50°C and the viscous solution
of 4% polymer in benzene (2,700 poise a t 70°C) has no spinnability in spite
of film forming ability. When the viscosity of 4% polymer solution is reduced
to 100 poise by the addition of 5% of dichloroacetic acid (v/v of benzene),
spinnability is obtained. Though poly-L-leucine keeps a-helix in a sol state,
the random coil seems to appear to some extent by the addition of dichloroacetic acid, The major part of the polymer molecule seems to be put back to
a-helix upon the spinning into isopropyl alcohol before the elongation step
and the fibre takes an oriented form of a mixture (a- and ,%form)with elongation. According to a du Pont patentl5, the mixture of chloral and formic acid
is used instead of dichloroacetic acid.
Leucine fibre containing cysteine as a wool model
Natural wool contains about 5% moles of cystine (corresponds to 10%
nioles of cysteine). However, in the preparation of copoly (L-Leu, S-Cbzo-LCys), the percent molar ratio must be higher than 9713 (each shows yo moles)
to make a clear spinning solution. Thus copoly (L-Leu97, L-Cyss), which is
composed of oriented a- and ,&forms, contracts to 72% in length by boiling
in dioxane with the transformation of its structure and a woolly character
appears in a-form. These leucine fibres with or without cystine having woolly
character contain the stable a-structure and no super contraction as in the
case of wool is observed.
Glutamate fibre containing methionine-8-methylsulfonium ions16
Poly-y-methyl-L-glutamate fibre is inferior to the natural fibre in dyeability.
To improve this defect, the hydrazine treatment17 or copolymerization with
methionine is devised.
The fibre of copoly (y-methyl-L-Glu97,L-Me@)is treated with dimethylsulfate to change the methionine residue into methionine-S-methyl-sulfonium ion.
The modified fibre has a fine and silky luster. The dyeability of the fibre is
greatly improved to acidic dyes. The streched fibre with silky luster contains a,
B and random structure, and the shrinked fibre about 75% of original length
also consists of a and B structure, inspite of loss of silky luster.
117
J. NOGUCHI,
S TOKURA,
and N. NISHI
-NH-CH-CO-
I
CH2
I
CH2
I
COOCH3
-NH-CH-CO-NH-CH-CO-
I
-NH-CH-CO-
I
CH2
NHzNH2
___.,
I
I
CH2
CO-NH-NH2
-NH-CH-COI
CH2
I
CH2
I
COOCH3
Leucine fibre containing methionine-8-methyl-sulfoniumions
To improve the dyeability of poly-L-leucine fibre, the fibre of copoly (L-Leu97,
L-Me@)is treated with 3% dimethyl sulfate in dioxane. The fibre contracts to
about 70% of the original length and the dyeability against acidic dyes and
the woolly character with a-form appear by only one treatment.
4. Properties of ply-a-amino acid fibres
The properties of poly-a-amino acid fibres which appeared in patents are
listed in Table 115, 18, 19, 20, 21, 22, 23, 24.
The appearance of chemical fibres similar to animal fibres is expected by the
studies of poly-a-amino acid fibres similar to their chemical structure. The
poly-a-amino acid fibres burn without melting and show no remarkable difference between wet and dry tenacities like natural fibres. The properties of
poly-a-amino acid fibres such as lightness, strength, luster, feeling and dyeability depend on the spinning conditions even in the same polymer. Among the
poly-a-amino acid fibres, poly-L-alanine fibre having b-structure is most similar to silksl 11. The properties of poly-L-alanine fibres are promissing as synthetic silk; the structure of the crystalline part of poly-L-alanine fibre is similar to that of wild silk yarn11 and yellow discoloration is less than that of raw
silk. Poly-L-leucine7 fibre containing a- and ,%form in a stretched fibre shows
silky luster and the intermediate property of wool and silk in the load-elogation curve. The dyeability of poly-L-leucine fibre is not satisfactory, but the
fibre is very light compared to other synthetic fibres. The fibre also resists to
the heat deformation and burns without melting. It rather looks like feather
than wool, but by the treatment in boiling dioxane, the fibre contracts about a
118
Poly-a-amino Acid Fibres
Table 1. Poly-a-amino acid fibres appeared in patents.
Poly-a-amino acid
Poly-e-acetoxy-DLamino-n-caproic acid
Foly - DL -1eucine
Folyamine
1
I
Viscosity
Denier (d)
filaments
Tenacity Elongation
(gld)
(Yo)
[rll 0.56
66/23
3.0
8
118l20
0.8
1.4
Poly-B-methyl-DL-aspartate
41/?
0.6
1.5
Poly-DL-alanine
Poly-DL-leucine
Poly-DL-alanine
Poly-L-alanine
Poly -D-alanine
Poly - L-alanine
Poly -D-alanine
62/30
1.0
38
113120
1.5
9
78/30
3.5
6
117130
2.3
8
-
2.0
7
I
Poly-u-amino-L-n-butylic
acid
Poly-L-leucine
Poly-y -methyl-L-glutamete
Poly-y-ethyl-L-glutamate
102/30
1.5
17
33/35
2.07
13
1.7
3.1
11.8
1.5
2.9
8.3
half in full length and woolly character appears by the transformation into the
u-form from the mixed state of a- and B-structure.
Though poly-L-methionine fibre has silky luster, the YOUNG’S
modulus and
load-elongation curves are rather close to those of woollo. The major part of
polymethionine fibre is in a-helix, containing a small amount of B-form. PolyDL-methionine film is said to be permeable with moisturel4, but the water absorption as seen in wool is not observed in this fibre. It is also unsuitable to a
practical fibre owing to its slight smell of methionine. On the other hand, poly-y-methyl-L-glutamatefibre being rich in @-formrather than a-form is fairly
similar to silk in its properties916. The properties of these fibres are listed in
the following tables.
As shown in Table 2, poly-L-alanine fibre is close to silk in the properties of
modulus, t e n d recovery, moisture regain and
tenacity, knit strength, YOUNG’S
absorption of moisture. Poly-L-leucine fibre has specifically low density and is
very light.
I n Table 3, the properties of woolly fibres prepared from poly-L-leucine are
compared with that of wool.
The properties of poly-L-leucine fibres are almost the same except YOUNG’S
modulus. Any crimping is not observed by boiling them in water, although wool
119
g \
Fibre
Density
Moisture regain (yo)
Absorption of
moisture (yo)
9.5
8.7 (20°C
80% RH)
16 (20°C
1 0 0 ~R
o H)
1.27
1.7
2.4 (20°C
80% R H )
3.5 (2OOC
lOOyo RH)
1.29-1.33
72
(5% strain)
90
(5% strain)
1.26-
94
(3% strain)
1.026
1.53
13.5
35
2.87
9.7
102
100
(3% strain)
2.17
18.5
41.2
2.06
2.02
17.0
19.7
2.23
2.02
4.34
2.75
1.80
1.90
Poly-L-Leu
12.8
16.8
2.08
2.01
9.3
11.9
1.70
Poly-y-CHsL-Glu
2.00
Poly - L - Ala
Properties of stretched fibres (Silky).
Elongation ratio
Denier
Tenacity (g/d)
Dry
Wet
Elongation (yo)
Dry
Wet
Knit
Strength (g/d)
Elongation (yo)
YOUNG'S
modulus (g/d)
Tensile
recovery (yo)
Properties
Table 2.
1.19
1.o
2.1 (20°C
95% RH)
5.7 (20°C
1 0 0 ~ RH)
o
93.4
(3% strain)
87.3
(5% strain)
69.6
(10% strain)
1.39
17.3
25.4
20.8
25.7
1.91
2.06
4.81
1.90
Poly-L-Met
1.38
0.4
0.5 (20°C
95% RH)
3.3 (20°C
lOOyo R H )
72.8
(10% strain)
53.3
98.9
(5% strain)
1.37
7.2
13.5 (20°C
80% RH)
30 (2OOC
lOOyo R H )
-
88
( 3yo strain)
62 65
(6% strain)
48 49
(10% strain)
90.0
3.92
-
3.66
-
20.5
-
3.98
-
-
Silk
-
31.6
-
4.02
-
4.20
-
Polyester
84 (2% strain)
36 (20% strain)
1.047
0
81 (2% strain)
34 (20% strain)
1.037
0
0
1.043
31 (20% strain)
15.1
22.6
16.8
0.58
85
0.63
26
71
117
0.61
0.54
5.40
L-Met 97 :3)-Smethyl sulfoniutn
Copoly(L-Leu,
0.60
56
53
127
0.64
0.83
0.56
0.64
55
97
5.31
Copoly(L-Leu,
L-cys 97 :3)
2.62
Poly-L-Leu
Properties of shrinked fibre (Woolly).
Denier
Tenacity (g/d)
Dry
Wet
Elongation (yo)
Dry
Wet
Knit
Strength (g/d)
Elongation (yo)
YOUNG’S
modulus
(g/d)
Tensile
Recovery (yo)
Density
Crimping by
boiling water (yo)
Properties
Tabelle 3.
1
1.32
93.5 (3% strain)
86.8 (5% strain)
25.4
.-
6.54
Wool
-
2
0-
2
J. NOGUCHI,
S. TOEURA,
and N. NISHI
shows super contraction. The a-helix in poly-L-leucine seems to be so stable
that there is no further contraction. The tenacities are less than half of wool
and are far inferior to those of the stretched fibre containing ,@-form,but the
YOUNG’S
modulus of copoly (L-Len,L-Cys)is near to that of wool.
To estimate the contents of a- and ,&forms in poly-a-amino acid fibres, the
infrared absorption spectra and the X-ray diffraction patterns were studied25.
As shown in Table 4, on the contents of a- and B-form in the shrinked fibres,
,@-formdecreases and a-form increases in the order of poly-L-methionine > poly-l-leucine > poly-y-methyl-l-glutamate.
The X-ray diffraction patterns in Table 5 show also that the ,@-form
of shrinked fibres is less than that of the stretched fibres.
Fig. 6 shows the load-elongation curves of poly-a-amino acid fibres compared with silk and wool.
4
4
6
5
L
0
10
20
50 60
El o ngat i o n ( 1
30
40
70
80
Fig. 6. Load-elongation curves of poly-a-amino acid fibres. 1 : Silk, 2: Poly-L-leucine fibre (without shrinkage), 3: wool (64’s merino), 4: Copoly (L-Leu,
L-Met 97 :3)S-methyl sulfonium fibre (Shrinkage), 5 : Copoly(L-Leu, LCys 97 :3) fibra (shrinkage),6 : Poly-L-leucinefibre (shrinkage).
The curves of poly-L-leucine and related fibres with shrinkage (rich in a-form)
are the same and that of wool lies in the intermediate between the stretched
and the shrinked poly-L-leucine fibres. Therefore, it will be possible to prepare
the similar woolly elongation curve by varying the ratio of a- and ,%form.
modulus and tan 6 against temperature are shown in
The dynamic YOUNG’S
Fig. 7. The tendency of the fibres related to poly-L-leucine is almost the same
122
Absorp.
Fibre
\
Absorp.
‘y
1
Shrinked
Stretched
I
Shrinked
Copoly(y-CH3-L-Gl~g7,
L-MeV)
Stretched
Poly-y-methyl-L-glutamate
Shrinked
Stretched
Shrinked
Copoly(L-Leu97,L-Cys3)
Stretched
Poly-L-leucine
I
Shrinked
Stretched
Shrinked
Copoly(L-Leu97,L-Me@)
Stretched
Poly-L-methionine
B
kl
3.
FL
Diffract.
\I
Diffract.
\I
Table 5 .
1
Shrinked
Stretched
Shrinked
Stretched
Shrinked
I
Stretched
I
Shrinked
Poly-L-leucine
Copoly(L-Leu97L-Cy53)
I
I
I
Copoly(y-CH3-L-Glu97,L-Met3)
Stretched
Poly-y-methyl-L-glutamate
X-ray diffraction patterns (in A).
1
I
Shrinked
5
?:
Stretched
I
Shrinked
0.
1
Copoly(L-Leu97,L-Met9
Stretched
Poly -L-methionine
Poly-a-amino Acid Fibres
0
50
100
150
200
250
Temp.( ‘C 1
Fig. 7.
Temperature dependance on the dynamic viscosity of fibres. (- - - - ) :
E’(Dynamic Young’s modulus), (----)
: tan 6. 1 : Poly-L-leucine
fibre (stretched), 2: Copoly(L-Leu,L-Met 97: 3) (stretched), 3: Copoly(LLeu, S-Z-L-Cys97 :3) (stretched),4: Poly-L-leucine(shrinked),5: Copoly(L-Leu, L-Met 97 :3)S-methyl sulfonium (shrinked), 6 : Copoly(L-Leu,
L-Cys 97 :3) (shrinked).
on the stretched or the shrinked state. The response to temperature up to
200 “C seems to be excellent fibres.
5. The f u t u r e problems of ply-a-amino acid fibres
Poly-a-amino acid fibres are expected as an animal-like fibre which has various properties as that of silk, wool and the intermediates. The quality of
Poly-L-alanine fibre9 is most similar to that of silk, but the only economical
method for preparation of optically active L- or D-alanine has not yet been
establised, althougki DL-alanine is supplied easily from petroleum chemistry.
The industry is waiting to start whenever the problem has been resolved. Po1y-L-glutamate fibre99 1% 12 has most promising future in the market terms of
the supply of materials and techniques for each step, if there is some demand.
125
J. NOGUCHI,
S. TOEURA,
and N. NISHI
Poly-L-leucinefibre7 seems to be hopeful as a woolly or feather like fibre, so that
it will be worthwhile to study the application for the industrial purpose because
of its easy preparation and the special light and woolly characters compared to
other poly-u-amino acid fibres. L-Leucine is easy to crystallize from aqueous
solution and the optical resolution of DL-leucine is not so difficult as that of
glutamate. The preparation of L-leucine by fermentation from glucose is also
more efficient than that of alanine.
To improve the dyeability of poly-u-amino acid fibres, the chemical treatment
of side chain or the copolymerization with a small amount of methionine followed the conversion into S-methyl-sulfonium ion was appliedl6. Though the
methionine copolymer is promoted remarkably to dying, it seems to be unsuitable owing to the smell of methioninelo. However, applying this principle,
poly-u-amino acid fibres would be generally improved the dyeability by copolymerization of a small amount of basic amino acid.
Poly-DL-alanine fibre23 is not so expected owing to the defect on water resistance even if it is polymerized into high polymer. The blended polymer of
L- and D-homopolymers is said to give a good fibre, but no study to polymerize
DL-monomer into L- and D-homopolymer independently, has succeeded in the
same reaction mixture. To improve the spinning condition for poly-L-alanine,
it seems that the block copolymer of L- and DL-alanine would increase not
only the dyeability but the solubility for the solvents other than dichloroacetic
acid :
- (DL -Ala)k - (L-Ale), - (DL-Ah), - (L-Ah), Moreover, the studies on the model of transformation between water soluble
form of random coil and water insoluble form of j3-coil might give a valuable
information like the spinning of natural silk, and the door is wide open to a new
wet spinning method in the future. Also natural proteins should be reexamined
as one of poly-u-amino acids a t wide sense. Protein is soluble in water generally
and insoluble in most of organic solvents. However, the protein soluble in
organic solvents is easily obtained by the conversion of water soluble protein
into multipolypeptidyl-protein which attaches branched poly-a-amino acids on
a protein molecule by the reaction with NCA in a neutral water.
The crude NCA can be used in this experiments and the procedure is simple.
The modified protein is able to give a woolly character to the chemical fibres
a t the most 10% in the contents. The viscose fibre which pendanted casein
with epichlorohydrin, appeared in the market as a woolly viscose. Polyacrylonitril fibre blended with multipoly-glutamyl-casein or multipolyacrylonitrilcasein showed also a woolly character26. Since it is possible to introduce the
woolly character by the blend of protein with chemical fibres before spinning
and it would be possible to obtain a cheap protein from the fermentation
126
Poly-a-amino Acid Fibres
NH2
I
F O r X i i H
n-9-10
EOOH
7.0 water
Protein
-COOH
NCA
COOH
Cellulose
NH(C0CHNH):H
I
R
Cellulose
of petroleum products in the near future, natural protein should be also considered as a source of chemical fibres on the standpoint of poly-a-amino acid.
6. Experimental
Solvents
Crude dioxane is refluxed with a small amount of conc. HCl, distilled and refluxed again with metalic sodium until melt surface of Na becomes bright like a
mirror. Then it is distilled and kept in the atmosphere of nitrogen with ti small
amount of Na. Toluene and benzene are purified by the ordinary method after
washing with conc. HzS04. Methylenchloride is purified after successive washing and
drying with aqueous potassium permanganate, conc. sulfuric acid, water, sodium
carbonate, water and drying on calcium chloride. Finally it is kept on a molecular
sieve. Ethylacetate is kept on calcium hydride after the distillation.
Procedure
(49
20 g of poly-L-alanine ([v]red = 2) was dissolved in 288 ml of dichloroacetic acid
with stirring at 60°C. The resulting 4.26 yo solution of poly-L-alanine in dichloroacetic acid was debubbled in vacuo after filtration through a flannel and a calico clothes with air pressure. Using the spinning apparatus shown in Fig. 4, 200 ml of the
spinning solution a t 60°C was spun through a goldplatinum nozzle (0.09 mm x 50
127
J. NOUUCHI,
S. TOKURA,
and N. NISHI
holes) into a hot water bath at 60"C with compressed air at the rate of 2.83 ml/min.
The coagulated thread in the f i s t coagulation bath was lead up to rollers a t the rate
of 8 m/min and then stretched into twice length immediately in a hot water bath of
80 "C (back roller ; 8 m/min and front roller; 16 m/min), followed to wind up through
a hot roller of 110°C. The poly-L-alanine fibre was washed with water to remove
dichloroacetic acid completely and then treated with boiling water for one hour to
set up into the ,8-form. Thus a silky fibre was obtained.
Procedure (2)9
The original solution of poly-y-methyl-L-glutamate ([T)]red = 1.92) which was
polymerized in the mixed solvent of methylenechloride and dioxane (80 : 20 v/v)
at a concentration of 7.2 yo,was diluted to 5.5 yo with the same solvent and filtered
followed to the debubbling under the normal pressure. Resulting clear solution was
spun into acetone through a nozzle (0.09mm X 50 holes) at the rate of 2.83 ml/min
with the pressure of 0.5 kg/cmZ at room temperature.The coagulated filaments were
led to back roller a t the rate of 13 m/min after the passing through water and then it
was stretched to 1.7 times of length in a water bath a t room temperature by front
roller (rate of 22 m/min) followed to heat treatment with hot roller of 110 "C.A silky
fibre was obtained containing a small amount of a-form additive to the p-form of a
large portion.
Procedure (3)7
15 yo benzene solution of L-leucine N C A (w/v) was polymerized in a sealed bottle
a t 70 "C for 3 days without any initiator. The clear gel was diluted to 4% solution by
benzene (2,700poiseat 70'C) andthen 5 Yo of dichloroacetic acid (v/v of benzene) was
added to reduce the viscosity to 100 poise a t 7OOC. The spinning solution kept at
70 "C was spun into isopropylalkohol a t 60-65 "C and then stretched in hot water as
described in the section of poly-L-glutamate. The spinningconditionswere as follows:
Molecular weight of poly-L-leucine: MW = 70,000.
Spinning solution :
Coagulation bath:
Elongation bath:
Rate of elongation:
Heat treatment:
Nozzle :
Spinning pressure:
4 yo of polymer in the mixture of benzene and dichloroacetic
acid (95: 5 v/v) with 100 poise of viscosity a t 70°C.
ethanol of 65°C and hot water of 60-65OC.
hot water of 65°C.
1.9 (back roller, 12 m/min and front roller, 23 m/min).
hot roller of 100°C.
0.09 mm x 50 holes of Au-Pt alloy.
0.5-1.5 kg/cm2.
The stretched fibre was boiled in dioxane for 2 hours and contracted to about
52 yo of the original length.
Procedure ( 4 ) l o
L-Methionine N C A (25 g) in methylenechloride (100 ml) was polymerized with
use of 1/200 equimolar triethylenediamine as a n initiator at room temperature for
2 days. The reaction mixture was then kept for other 2 days at 40"Cand finally heat-
128
Poly-a-aminoAcid Fibres
ed at 70°C for 1 hour to complete the reaction. The clear solution was spun into
acetone at room temperature by use of the apparatus shown in Fig. 5. The spinning conditions were as follows :
Molecular weight of poly-L-methionine: MW = 30.000.
Spinning solution:
21.2 g of polymer in 100 ml of methylenechloride at room
temperature.
acetone at room temperature.
Coagulation bath :
Elongation bath:
hot water of 65 "C.
Rate of elongation: 1.7-1.9 (backroller, 5-8m/minandfront roller, 10-15 m/min).
Heat treatment:
hot roller of 100°C.
0.1 mm x 50 holes of tantalum nozzle.
Nozzle :
Spinning pressure : 0.6 kg/cm2.
Procedure ( 5 )
A mixture of L-leucine NCA (53.6 g ) and S-benzyloxycarbonyl-L-cysteine
NCA
(2.88 g) was dissolved in benzene (420 ml) and polymerized at 70°C for a few days
in a sealed bottle. The resulting gel was diluted with benzene (340 ml) and dichloroacetic acid (80 ml) at 70"C, and then the clear solution of 70°C should be spun into
isopropyl alkohol at 70 "C immediately after the filtration and debubbling, because
the molecular weight of copolymer tends to be reduced by dichloroacetic acid in the
spinning solution. The tendency is not so remarkable for the homopolymer. The
spinning conditions were as follows :
M-olecular weight of copolymer: MW = 28,000.
9 yo of copolymer in a mixed solvent (benzene : dichloroaceSpinning solution :
tic acid = 100 : 8.4 v/v) with viscosity of 52 poise at 70°C.
Coagulation bath : isopropyl alkohol of 70°C.
Elongation bath:
hot water of 65 "C.
Rate of elongation: 1.47 (back roller, 9.5 m/min and front roller, 14.0 m/min).
hot roller of 100°C.
Heat treatment:
0.09 mm - 50 holes of tantalum.
Nozzle :
Procedure ( 6 )
L-leucine NCA (52.6 g) and L-methionine NCA (1.90 g) are dissolved in benzene
(492 ml) and copolymerized at 70°C in a sealed bottle for a few days. The spinning
conditions were as follows :
Molecular weight of copolymer: MW = 27,000.
Spinning solutions : 8.2% (w/v) of polymer solutions in the mixture of dichloroacetic acid and benzene (8.3 : 91.7 v/v) with 40 poise of viscosity
at 70°C.
Coagulation bath:
isopropyl alcohol of 65 "C.
Elongation bath:
hot water of 65°C.
Rate of elongation: 1.47.
70 yo.
Shrinkage :
hTozzle:
0.09 mm - 50 holes of tantalum nozzle.
129
J. NOGUCHI,
S. TOKURA,
and N. NIsm
The authors are very much indebted t o ASAHIKASEIKOGYO
Co.. LTD.and
FUJIBOSEKICo. LTD.for the measurement of fibre qualities. Also they wish
SAKURADA
for his ent o express their hearty thanks t o Professor Dr. ICHIRO
couragement throughout this work and t o Professor Dr. WERNERKERN,University of Mainz, for his kindest arrangement of this publication.
Discussion
P. C. LIMBURG,
Arnhem: How stable are the solutions of the polymers in the
a-form?Do they withstand shear stresses, such as those encountered in flow through
orifices, without transformation into the insoluble (stretched) #?-form? Do you know
anything about the energy barriers between the a and #? forms?
J. NOGUCHI
: The stability depends on the natures of the polymer and solvents.
Thus, poly-L-leucine in the a-form is very stable in benzene and fibres of the polymer redissolve in benzene. The time-scale of the a+#?,transformation would play
an important part in determining the behaviour of a solution flowing through an
orifice; we have no direct experimental evidence on this matter. - The energy
barrier again depends on the polymer; 2-3 kcal/residue is probably a typical figure.
H. DE VRIES,Arnhem: Can you explain the changes in fibre structure which
occur between 200" and 250°C (compare figure 7)?
J. NOGUCHI
: Thank you very much for your pointing out. The legends of Fig. 7
were miswritten in the opposite sign and should be corrected. We have not yet
studied these changes in detail.
MIGUELMASRIERA,
Barcelona: ( 1 ) Which is more helpful in studying the change
of n- to #?-form,infra-red spectroscopy or X-ray diffraction? (2) Have these synthetic fibres been examined by electron microscopy?
J. NOGUCHI:
(1) For our purposes, infra-red spectroscopy is simpler to apply
and has proved more helpful. We have used the former technique extensively in
semi-quantitative determination. (2) We have not used electron microscopy.
2
R. E. MARSH,
R. B. COREY,and L. PAULINO,
Biochim. Biophys. Acta 16 (1955), 1.
R. B. COREYand L. PAULING,
Internatl. Wool Text. Res. Conf. Australia (1955)
B-249.
5
R. B. WOODWARD
and C. H. SCHRAMM,
J. Amer. Chem. SOC.69 (1947) 1552.
H. LEUCHS,
Ber. Deut. Chem. Ges. 39 (1906) 857.
W. PRICHARD:
US. P. 2516145 (1950), C. A. 45, 645; R. N. MACDONALD:US.
P.
2534283 (1950), C. A. 45,3198; US.P. 2560584 (1951), C. A. 46,788;US. P. 2572843
(1951),C.A.46,778;US.P.2572844 (1951),C.A. 46, 779; US. P. 2630423 (1953),
C. A. 47, 5130; US. P. 2644808 (1953), C. A. 48, 5881; US. P. 2650214 (1953),
C. A. 48, 1064; US. P. 2671772 (1954), C. A. 48, 6740; US. P. 2789973 (1957),
C. A. 51, 10084; W. BAIRD,E. G. PARRY,and S. ROBINSON:
Brit. P. 646033
US. P. 2540855 (1951), C. A. 45, 4491;
(1950), C. A. 45, 5177; C. W. TULLOCK:
130
Poly-a-amino Acid Fibreg
us. P.
2600596 (i952), c. A. 47, 917; us. P. 2652389 (i953), c, A. 49, 2 1 ~ :
E. KATCHALSKI
and J. BLUMENFELD:
US. P. 2578428 (1951), C, A. 46, 2848;
A. C. FARTHING:
Brit. P. 651513 (1951); Brit. P. 651914 (1951), C. A. 46,2571;
W. E. HANBY,S. G. WALEY,and J. WATSON:US. P. 2598372 (1952), C. A. 46,
9343; Brit. P. 675298 (1952), C. A. 46, 9344; US. P. 2628886 (1953), C. A. 47,
5128; R. E. MIEGEL:US. P. 2612487 (1952), C. A. 47, 7536; US. P. 2729621
US. P. 2647907 (1953), C. A. 48,
(1956), C. A. 50, 6808; G.A.RICHARDSON:
10062; US.P. 2653946 (1953), C. A. 48, 10063; J.B.OTT: US.P. 2653947
(1953), C. A. 48, 10062; R . B . WOODWARD:
US.P. 2657972 (1953), C.A. 49,
1364; B. GRAHAM:
US. P. 2692247 (1954), C. A. 49, 2088; Courtaulds Ltd.:
Belg. P. 577609 (1959); Belg. P. 591015 (1960), J. KIRINO,K. HACHINO,
and
Y. HASHIMOTO,
Bull. Sericult. Exp. Sta. 17 (1962) 523; Azino-mot0 Co. Ltd.:
Japan P. S 39-8397 (1964); J. NOGUCHI,
M. ITAYA,
Y. SUZUKI,and M. KAGAWA,
Annu. Rep. Fib. Res. Inst. Osaka 17 (1964) 30; J. NOGUCHI,Review of the
meeting for the poly-a-amino acid fibres (the Society of Polymer Science, Japan, Bridgestone Hall, Tokyo 1964) p. 1 ; S. SAKURAI,
ibid., p. 7.
W. E. HANBY,S. G. WALEY,and J. WATSON,J. Chem. SOC.1950, 3009.
‘1 J. NOGUCHI,T. NAKAMURA,
T. HAYAKAWA,
and C. OHIZUMI,Kogyo Kagaku
Zasshi 70 (1967) 1254; US. P. 2789973 (1957), C. A. 51, 10084.
H. TANIand J. NOGUCHI,
Chem. High Polym. 8 (1951) 51.
(I J. NOGUCHI,M. ITAYA,Y. SUZUKI,and M. KAGAWA,
Annu. Rep. Fib. Res.
Inst. Osaka 17 (1964) 30.
J. NOGUCHI,N. NISHI, M. ITAYA,
and S. TOKURA,Kogyo Kagaku Zasshi 69
(1966) 745.
11 C. H. BAMFORD,L. BROWN,A. ELLIOT, W. E. HANBY, and I. F. FROTTER,
Nature 173 (1954) 27; R. E. MARSH,R. B. COREY,and L. PAULING,
Acta Crystallogr. 8 (1955) 710.
1* Azinomoto Co. Ltd.: Japan. P. S 41-12517 (1966), C. A. 66, 19050; S40-3290
(1965), 542-7382 (1967), S42-22073 (1967), C. A. 68, 40913; 539-5213 (1964).
13 Courtaulds Co. Ltd. : Brit. P. 931 678 (1963).
l4 R. N. MACDONALD:
US. P. 2650214 (1953), C. A. 48, 1064.
l5 R. N. MACDONALD:
US. P. 2789973 (1957), C. A. 51, 10084.
J. NOGUCHI,N. KOBAYASHI,K. TATSUKAWA,
and N. NISHI, Kogyo Kagaku
Zasshi 71 (1968) 1748.
17 Azinomoto Co. Ltd.: Japan. P. S38-17095 (1963).
18 Courtaulds Ltd. : Belg. P. 577 609 (1959).
l9 R. E. MACDONALD:
US. P. 2630423 (1953), C. A. 47, 5130.
20 B. GRAHAM:
US. P. 2636873 (1953), C. A. 48, 1020.
21 C. W. TULLOCK:
US. P. 2652389 (1953), C. A. 49, 2116.
22 B. GRAHAM:
US. P. 2692247 (1954), C. A. 49, 2088.
23 R. E. MIEGEL:US. P. 2729621 (1956), C. A. 50, 6808.
24 Courtaulds Ltd.: Belg. P. 591015 (1960).
25 C. H. BAMFORD,A. ELLIOT, and W. E. HANBY, “Synthetic Polypeptides”,
Acad. Press Inc. Publ., New York 1956.
26 J. NOGUCHI,
S. KURIHARA,
Y. YOSHIDA,and N. NISRI, Kogyo Kagaku Zasshi,
70 (1967) 1032.
131
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