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Design of Thermotropic Liquid Crystals with Micellar Cubic Mesophases Amphiphilic N-(2 3-Dihydroxypropyl)benzamides.

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L-Phenylalanine ethyl ester (0.4 g) and dicyclohexylcarbodiimide (0.4 g) were
added 4-(-pyrenyI)butyric acid (0.5 g) in acetonitrile (50 mL). The mixture
was stirred at morn temperature (4 h), and excess DCC deomposed. The solvent was removed under reduced pressure, and the reaction mixture subjected
to acid hydrolysis (1 N HCI, 12 h, room temperature). Pure product was
isolated after chromatography on silica gel. 'HNMR (270 MHz, DMSO):
b = 8.06-8.34 (m, 9H), 7.2 (m, 5H), 4.5 (exchangeable NH, 1 H), 2.0 (m, 1 H),
2.1-2.5 (m, 6 aliphatic H); IR (KBr): G = 3300 (OH), 1703 (acid C=O),
1640 cm-' (amide C=O); the absorption maxiama at 313,326, and 343 nm as
well as the fluorescence maxima at 376, 396, and 417 nm (340 nm excitation)
correspond to those of the pyrenyl chromophore. The circular-dichroism maxima are at 336. 351, and 360 nm.
A. Wolfe, G. H. Shimer, Jr., T. Mechan, Biochemistry 1987, 26, 6392; A. M.
Pyle, R. M. Rehman, C. V. Kumar, N. J. Turro, J. K. Barton, J. Am. Chem. Soc.
1989, f l f , 3051. The equation used is ([BSA]/A&.,) = ([BSA]/As) +(l/A&K)
where Atap = I&, -- E J , A& = [cb - E , ] , K is the binding constant, E. =
(absorbance at 343 nm)/[Py-Phe], E,, = extinction coefficient of the bound form
(2.21 1 x 1 0 4 ~ - cin-I),
'
and E( is the extinction coefficient of free Py-Phe.
The reaction mirture (0.1 mL) was irradiated at 344nm for various
lengths of time with a xenon lamp source and a monochromator ( 3 . 0 ~
lo-* einsternsrnin-' at 340 nm). Filters were used to remove stray UV light,
and the band pass on the monochromator was adjusted to 10 nm. Irradiated
solutions of the protein and probe were dried under reduced pressure, and the
residue was redis!;olved in the sample buffer (0.024mL) made of glycerol
(1 mL), sodium dodecyl sulfate (3 mL, 10% aqueous solution), tris(hydroxymethy1)aminomethane hydrochloride (1.25 mL. 0.5 M), bromophenol blue
(0.6 mL, 0.1 % solution), and deionized distilled water (4.5 mL). The gels were
run by applying .a voltage of 60V until the dye (Coomassie blue) passed
through the stacking gel. The voltage was then increased to 110 V, and the gels
were run for a total of 2 h; see H. Schagger, G. V. Jagow, Anal. Biochem. 1987,
166, 368.
P. A. King, E Jamison, D. Strahs, V. E. Anderson, M. Brenowitz, Biopkys. J .
1993.64, A179; Nacleic Acids Res. 1993,21,2473; P. A. King, V. E. Anderson,
I
Am. Chem. Soc.
J. 0. Edwards. G. Gustafson, R. C. Plumb, J. W Suggs, .
1992, f 14. 5430.
the phase type. Depending on the sign of the interface curvature
either normal phases (type 1;the interface curvature i s directed
away from the regions with stronger cohesive interactions) or
inverted phases (type 2 ; the interface is curved toward the regions with stronger cohesive forces) can occur.
In contrast to the large number of lyotropic systems with
cubic mesophases, relatively few thermotropic compounds with
cubic phases are k n ~ w n . [ ~In. ~most
] cases, bicontinuous structures with body-centered cubic lattices (Ia34Im3m),which represent intermediate phases between columnar and smectic
phases, have been proposed for these thermotropic cubic
phasesr4! More recently different types of thermotropic cubic
phases have been observed for amphiphilic carbohydrate
derivative^.^^. 61 For some of these compounds inverted micellar
cubic phases with primitive cubic lattices (Pm3nor P43n) have
been found.[61Because this type of primitive cubic phase has
never been observed in thermotropic systems, their more detailed investigation should be of special interest ;['I however, the
transition temperatures of these carbohydrate derivatives are
rather high such that their study is hampered by decomposition.
In an effort to overcome these difficulties, we decided to decrease the number of attractive hydrogen bonds in the head
group regions.
Here we describe the first stable model compounds for these
carbohydrate derivatives. These are amphiphiles in which one,
two, or even three lipophilic alkyl chains were grafted to a hydrophilic N-(2,3-dihydroxypropyl)benzamide group (1-3). As
"Wm
1, R' = R2= H
2, R' = C,H2,+10, R2= H
3,R' = Rz= C,&+fO
C"~Z".lO
RZ
Design of Thermotropic Liquid Crystals with
Micellar Cubic Mesophases: Amphiphilic
N-(2,3-Dihydroxypropyl)benzamides* *
Konstanze Borisch, Siegmar Diele, Petra Goring,
Horst Kresse, and Carsten Tschierske*
Cubic mesophases are common in surfactant/solvent and
lipid/solvent systems and are ordered supermolecular arrangements with isotropic physical properties. They have attracted
considerable interest because of their potential use in drug release systems, as templates for the preparation of mesoporous
silicates, and as models for cell fusion processes.['] Several different cubic mesophases are observed in these systems depending on the concentration of the surfactant.['] They can occur as
intermediate phases either between lamellar and hexagonal
columnar phases (bicontinuous cubic phases, V-phases) or between the hexagonal columnar phases and the micellar solutions
(discontinuous cubic phases, I-phases). The first type can be
regarded as interwoven networks of branched columns, the second one consists of closed micelles arranged in a cubic lattice.
The interface curvature between hydrophilic regions and
lipophilic regions was recognized as the key factor determining
[*I Prof. Dr. C. Tschierske, K. Borisch
lnstitut fur Organische Chemie der Universitat
Kurt-Mothes-Strasse 2, D-06120 Halle (Germany)
Fax. Int. code +/345)5527030
e-mail: coqfx(u.mlucom.urz.uni-halle.de
Dr. S. Diele. P. Goring, Prof. Dr. H. Kresse
Institut fur Physikalische Chemie der Universitat Halle (Germany)
[**I This work was scpported by the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie.
Angew. Chem. Inl. Ed. Engl. 1997.36, No. 19
in the case of related D-glucamides,[61three different types of
mesophases were found depending on the number of alkyl
chains: compound 1 with only one dodecyloxy chain, n = 12,
forms a smectic A, phase, a hexagonal columnar phase (Col,,)
is found for the double chain compound 2, and a cubic phase
was detected for compound 3 with three dodecyloxy chains
(Table 1). The X-ray diffraction pattern of this cubic phase can
Table I. Transition temperatures T of the compounds 1-3. n =12, and lattice
parameter a of their mesophases [a].
l,n=12
2,n=12
3, n = I 2
H
Cl,H,,O
C,,H,,O
H
H
C,,H,,O
cr 89 S , 132 is
cr 98 Col,, 148 is
cr 69 Cub,,126 is
4.03 (85)
4.18 (105)
7.45 (90)
[a] Abbreviations: cr = crystalline solid, S , = smectic A-phase. Col,, = hexagonal
columnar mesophase, Cub,, = inverted micellar cubic mesophase (space group
Pm3n or P43n), is = isotropic liquid. The designation of the Col and Cub phases is
given as subscripts using descriptors of lyotropic phases as proposed by Tiddy [2c].
The polarilipophilic interfaces are curved in the direction of the polar regions
(negative curvature) and thus represent type 2 phases (inverted phases)
be indexed on the basis of a primitive cubic lattice (Pm3n or
P43n;a,,, =7.45 nm at T = 90°C; Table 2). Thus, simply by
changing the number of lipophilic chains grafted to the aromatic
linking units the supermolecular organization of these molecules can be changed from Iayerlike through columnar to cubic.
In this respect the thermotropic mesomorphism of these compounds, which depends on the number of lipophilic chains, is
analogous to the lyotropic mesomorphism of surfactant/solvent
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
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Table 2. Bragg reflections of the cubic mesophase of compound 3, n = 12, at
T = 70 "C.
1.19
1.32
1.41
2.68
1.88
2.13
2.22
2.36
2.64
2.12
1.32
1.45
1.68
1.88
2.13
2.22
2.37
2.65
2.71
200
210
21 1
220
310
320
321
400
420
421
works can act as proton conductors, the conductivity is expected
to decrease at the transition from the hexagonal columnar to the
inverted micellar cubic mesophase, whereas it should remain
approximately constant at the transition in a bicontinuous cubic
phase. Indeed, a strong decrease in the specific conductivity is
found at the transition from the hexagonal columnar phase to
the cubic phase (Figure 2), which strongly supports the inverted
micellar structure of this cubic phase.
=..
.
j
systems. In analogy to lyotropic systems it can be expected that
by increasing the number of chains the interface curvature between the segregated regions of the lipophilic chains and the
polar N-(2,3-dihydroxypropyl)amide groups increases and gives
rise to the transition from a smectic phase (S,) through an
inverted hexagonal columnar phase (Col,,) to a cubic phase
consisting of closed inverted micelles (Cub,,).
The comparison of several homologous compounds 3 with
alkyl chains of different length (Figure 1) indicates that the
84°C
64°C
I
1
10-7!
=.
, . ,
0.0027
.
,
. , . , .
0.0029
T-'lK-'-
0.m1
,
=.
.
,
,
0.0033
Figure 2. Dependence of the specific conductivity K (in AV-lm-') of compound
3, n = 8, on the temperature. Measurements were carried out in a microcapacitor
( A = 2 cm', d = 0.02 cm) without orientation (f=1 K Hz).
cubic phase is destabilized on decreasing the chain length and a
hexagonal columnar phase is found for the short-chain derivatives. This is in accordance with the proposed inverted micellar
structure of the cubic phase. Compound 3, n = 8, exhibits a
cubic-columnar dimorphism. This provided the opportunity to
investigate the structural changes that occur at the transition
from the thermotropic columnar to the thermotropic cubic
phase.I8]The hexagonal lattice parameter of the Col,, phase is
ahex= 3.3 nm at T = 60°C. The cubic phase has a primitive
cubic lattice (Pm3n or P43n) with a lattice parameter a,,,, of
6.9 nm at T =75 "C.
In order to confirm the inverted micellar structure of the cubic
phase of 3, n = 8, proton conductivity investigations were carried out. If the cubic mesophase is built up by closed inverted
micelles, the hydrogen bonding networks between the head
groups should be located inside the micelles and therefore they
should be isolated from each other by the lipophilic regions of
the alkyl chains. In a bicontinuous cubic phase and in columnar
phases however, the hydrogen bonding network should be extended over large distances. Since these hydrogen bonding net2088
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
What shape are these closed micelles in the cubic phases? If
one assumes that spherical micelles exist and that these are arranged in a primitive cubic cell, their diameter should be equal
to the lattice parameter (a,,, = 6.9 nm). The length of a single
molecule of 3, n = 8, as estimated from CPK models under the
assumption of an all-trans conformation of the chains is, however, only 2.2 nm and is thus significantly smaller than the radius of the spherical micelles. This contradiction between molecular size and cell dimensions is often observed in cubic phases.
Therefore, Fontell et al.[91suggested an alternative structure for
lyotropic micellar cubic phases (Cub,,) located between the micellar solution and the normal hexagonal phase in surfactant/
solvent systems. They proposed that the cubic unit cell is built
up by eight short rodlike aggregates with an axial ratio of approximately 2: 1. One of these rod-shaped micelles is placed in
each corner of the unit cell, one in the center, and two on each
face of the
The micelles centered at the corners and in the
center should be statistically disordered or freely rotating,
whereas those on the faces of the cell are only rotationally disordered.'']
We propose that the thermotropic cubic phases of the compounds 3, n = 8-12, have the same structure (Figure 3). How-
Figure 3. a) Structure of the thermotropic cubic phase Cub,, of the compounds 3,
n = 8-12, based upon the proposal by Fontell et al. [9]. a) Arrangement of the
micelles in the primitive cubic lattice. The rodlike micelles are indicated as open
circles, the statistically disordered micelles at the comers and in the center are
indicated by filled circles. b) View upon a face or the cubic lattice. c) Schematic
representation of a rodlike micelle.
0570-0833/97/3619-2088$17.50+ S O / O
Angew. Chem. Inr. Ed. Engl. 1997.36, No. 19
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-
ever, they are built up by inverted micelles and appear in the
absence ofany sohent. Thus, the cubic lattice of the compound
3, n = 8, should be built up by short rodlike micelles, each
consisting of approximately 44 individual molecules.[' 'I The
proposed model (Figure 3) requires that two micelles are arranged side by side at each cell face of the cubic lattice (edge
length 6.9 nm). Therefore the short diameter d, of these rods
(d, = a,,,/2) should not exceed 3.45 nm. In order to organize 44
molecules in each of these closed cylindrical micelles (see Figure 3) with a diameter of about 3.4 nm, these micelles should be
about 5 nm long ( d J . Thus, an axial ratio d,:d, of the order of
about 1.5: 1
Remarkably the short diameter (d,) of the rodlike micelles in
the cubic mesophase is nearly identical with the diameter of the
columns in the hexagonal columnar mesophase of 3, n = 8,
formed at lower temperature (ahex= 3.3 nm at 60 "C). Thus, we
conclude that the short rodlike micelles of the cubic mesophase
result from the collapse of the extended columns of the hexagonal columnar phase into small segments. Probably the arrangement of eight rodlike micelles in such a cubic lattice represents
an energetic minimum that could be stabilized by quadrupole
interactions['] and allows an efficient packing of the molecules.
If spherical micelles are formed, their packing coefficient in this
arrangement would amount to only 0.52. Therefore, it is expected that spherical micelles preferably arrange in a body-centered
(0.68) or in a fae-centered lattice (0.74), which allow more
efficient packing.
A related model was recently proposed for thermotropic cubic phases of some: dendrirner~.'~]
A nearly spherical shape was
assumed for the rnicelles of these cubic
However,
since both compounds differ in their chemical structure, they
could not be compared directly. Further detailed investigations
are necessary to elucidate the general principles for the directed
design of thermotropic cubic mesophases.
Received: January 13, 1997
Revised version: May 30. 1997 [Z9988IE]
German version. Angew. Chem 1997. 109,2188-2190
Keywords: amphiphiles * cubic mesophases * hydrogen bonds
liquid crystals micelles
-
[I] G Lindbloni. L Rilfors, Biochim. Biophys. Actu 1989, 988, 221.
[2] a) V. Luzzati. A. P. Spengt, Nature (London) 1967,215,701, b) J. M. Seddon,
R. H. Templer, Philos. Trans. R. Soc. London A 1993, 344, 377; c) G. I. T.
Tiddy, Phys. Rep. 1980, 57. 1.
[3] a) D Demus. G. Kunicke, J. Neelsenand. H . Sackmann, Z . Nuluforsch. A
1968. 23. 84; b) G. Etherington. A. J. Leadbetter, X. J. Wang, T. Tajbakhsh,
Liq Cryst 1986, 1, 209; c) G . Lattermannn, G. Staufer, Mol. Cryst. Liq. Cryst.
1990, 191, 199; d) CJ. Stebani, G . Lattermann. R. Festag, M. Wittenberg, J. H.
Wendorff. J Muter Chem. 1995,5,2247; e) W. Weissflog, G . Pelzl, I. Letko, S .
Diele. Mol. Crrst. Lip. Cryst. 1995, 260. 157.
[4] a) Y. Fang, A . M. Levelut, C. Destrade, Liq. Crysf. 1990, 7 , 265; b) A . M .
Levelut. Y Fang. Colloq. Phys 1990, 299; c) D. W. Bruce, B. Donnino, S . A.
Hudson, A. M. Le5elut. S. Megtert, D. Petermann. M. Veber, J Phys. II 1995,
5. 289. and references therein.
[5] a) K. Praefcke. B. Kohne, A. Eckert, J. Hempel, Z. Naturforsch. B 1990, 45,
1084; b) S. Fischer, H. Fischer, S . Diele, G . Pelzl, K. Jankowski, R. R.
Schmidt, V. Vdl. L q . Cryst. 1994, 17, 855.
[61 a) K. Borisch. S. Dicie, P. Goring, C. Tschierske, J Chem. Sor Chem. Commun.
1996, 237; b) Liy. Cryst. 1997. 22, 427.
[71 After submission of this work a Pm3n phase was proposed for dendrimers:
V.S . K . Balagurusamy. G. Ungar. V. Percec, G . Johansson, J Am. Chem. Soc.
1997. 119. 1539.
[a] The hexagonal-cubic transition in normal lyotropic systems was recently studied: Y. Rancon. J. Charvolin, J Phys. Chem. 1988,92,2646; P. Mariana, L. Q.
Amaral. L. Saturni. H Delacroix, J Phys. II 1994, 4 , 1393
[9] K. b-ontell. K . K. Fox, E. Hansson, Mot. Crysf.Liq. Cryst. Lett. Seci. 1985,f.
9 ; K. Fontell. Colloid Poljm. S C I .1990, 268, 264.
[I01 The micelle in the center contributes as a whole micelle, each of the 8 micelles
in the corners contribute 1:s. and each of the 12 micelles at the faces contribute
I / ? to the unit cell.
Anxeu Cben7 Inr Ed Engl 1997.36, No 19
[ l l ] By using the equation n = V,,,,N,,/Mp and by assuming a density of
p = 1 gcm-' it can be calculated that each cubic unit cell contains about 350
molecules.
[I21 A value of 1.35'1 was obtained by fluorescence quenching experiments of
normal cubic mesophases of lyotropic systems (type 1): L. B.-A. Johansson, 0 .
Soderman, J. Phjs. Chem. 1987, Y l , 5275.
[13] Two different kinds of micelles were found: spherical and tetrahedral
distorted
Absolute Sense of Twist of the C12-CI3 Bond
of the Retinal Chromophore in Bovine
Rhodopsin Based on Exciton-Coupled CD
Spectra of 11,12-DihydroretinaI Analogues**
Qiang Tan, Jihong Lou, Babak Borhan, Elena
Karnaukhova, Nina Berova, and Koji Nakanishi*
Rhodopsin, the photoreceptor responsible for dim-light vision in vertebrate species, is located in the outer segment of rod
cells. This membrane protein is composed of seven transmembrane a-helices and contains the 1I-cis-retinal chrornophore 1
(Figure I), which is attached to the &-aminogroup of Lys(K)296
on helix G through a protonated Schiff base (PSB) .I1.Glu 113
serves as a counterion to stabilize the positive charge of the
PSB.[31The 1I-cis trans isomerization triggered by light initiates changes in the protein conformation leading to a cascade
of events referred to as the visual transduction process.[41The
structure of the protonated Schiff base (PSB) of 11-cis-retinal,
with the long conjugated polyene side chain, five methyl groups,
the cyclohexene ring, and the C11 -C12 double bond, together
with the nonplanar 6-s-cis and 12-s-trans arrangements, is ideally suited for the very subtle regulation of the pigment absorption
maximum.[51Depending on the fit of the pigment within the
rhodopsin binding site, this single chromophore can cover the
entire range from the UV region up to wavelengths of around
640 nm, a range most suited for the environment surrounding
the ~ r g a n i s m . ' ~ ]
The UV/Vis and circular dichroism (CD) spectra of bovine
rhodopsin in octyl glucoside are shown in Figure 1. Native
rhodopsin (A,,, = 500 nm) exhibits two positive Cotton effects
in its CD spectrum at 480 nm (A& = 2.8, a-band) and 337 nm
(A&= + 9.8, /?-band). A rhodopsin incorporating a retinal analogue in which planes B and C are coplanar due to a five-membered ring (linkage of C10 and 13-methyl group) exhibit CD
spectra with a negligible a-band. Similarly, the pigment formed
from a retinal with coplanar A and B planes as a result of a
bridged five-membered ring between the 5-methyl group and C8
showed weak p-band. Based on these findings, Ito and co-workersC6]have assigned the origin of the a- and /?-bands to distortions around 12-s-trans and 6-s-cis bonds, respectively. Although the two positive Cotton effects reflect the interaction
between the twisted chromophore and its environment, at this
stage it is not possible to gain further information regarding the
sense of twist around the 6-s-cis and 12-s-trans bonds, that is,
the arrangement of planes A, B, and C in Figure 1. We have
--f
+
[*I
[**I
Prof. K. Nakanishi, Q. Tan, J. Lou, Dr. B Borhan, Dr. E. Karnaukhova.
Prof. N . Berova
Department of Chemistry, Columbia University
New York, NY 10027 (USA)
Fax: Int code +(212)932-8273
e-mail: kn5@columb1a.edu
This study was supported by grants from the U. S National Institutes of
Health (GM 36564 and 34509).
Q WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
0570-0833/97/3619-2089 S 17 SO+ 50'0
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crystals, thermotropic, amphiphilic, design, mesophases, benzamide, dihydroxypropyl, micellar, liquid, cubic
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