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Demixing Phenomena in High Polymers and Their Technical Importance.

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C O N F E R E N C E REPORTS
Structure and Mode of Action of Aldolases
By B. L. Horecker 1*I
Fructose diphosphate aldolase from rabbit muscle has a
molecular weight of 160000 and appears to be built up
according to the OL& principle from four sub-units of equal
molecular weight. The function of the various sub-units is
not yet known. The active center of the enzyme contains an
active lysine residue that forms a Schiff base with the substrate
dihydroxyacetone phosphate. Reduction of this intermediate
with sodium borohydride enables the active center to be
labeled. A peptide containing this center could be isolated;
it proved strongly hydrophobic in character. The fact that
the corresponding peptide from rabbit-liver aldolase has a
similar primary structure carrying numerous analogous
amino-acid substituents provides evidence for the genetically
independent origin of the two enzymes.
Investigations with reagents for the specific rearrangement
of individual amino acids evidence further essential amino
acids alongside the active lysine.
N-Ethylmaleimide reacts with sulfhydryl groups.The presence
of the substrate inhibits this reaction and, consequently, the
deactivation of the enzyme. The sulfhydryl groups may be
oxidized to disulfides by o-phenanthroline. Since this reaction
is also inhibited by the substrate, the sulfhydryl group must
lie in the active center. Reaction with acetylimidazole
provided evidence for the presence of essential tyrosine
residues, and essential histidine residues were shown to be
present by photooxidation. The properties of the aldolase
that has been modified by acetylation or by photooxidation
resemble those of transaldolase and, like the latter, such
modified aldolases d o not promote proton exchange between
the substrate and water.
On the basis of the above and other observations, the
following possible steps are discussed for the reaction of
aldolases: 1. Formation of a Schiff base. 2. Reaction of the
6-phosphate group of the fructose diphosphate with a
positively charged group in the active center, during which a
negatively charged group is released. The latter abstracts a
proton from the O H group o n C-4 and thus initiates the
aldol cleavage. 3. The resulting Schiff base carbanion reacts
with a proton of the water via the histidine residue.
The aldol cleavage of fructose 6-phosphate with transaldolase
is promoted by a histidine residue at the active center. This
group retains the proton as long as the Schiff base structure
remains intact.
[VB S8 IEI
Lecture at Gottingen (Germany) on May 29th. 1967
German version: Angew. Chem. 79, 948 (1967)
nitrile) 01 it arises by insolubility of the polymer in the
monomer (example : bulk polymerization of vinyl chloride).
In discontinuous copolymerization it is only with azeotropic
mixtures "1 that the copolymer has the same composition
as the starting mixture. In all other mixtures the composition
of the copolymer changes during the course of the reaction
and inhomogeneous copolymers are formed which tend
to demix. Azeotropic mixtures very seldom contain more
than two different monomers, although recently some have
been found with three, four, or five different monomers.
Block copolymers occasionally demix to give ordered
structures, some even showing Bragg reflections in visible
light. Block and graft copolymers often cause a better
compatibility of the corresponding homopolymers, which
in principle results from intimate distribution of the two
phases in each other: the copolymers act, as it were, as
emulsifiers of a polymeric oil-in-oil emulsion. Almost all
high-impact polystyrenes and ABS polymers are to be
considered as such emulsions, consisting of a soft rubber
and a hard polystyrene or poly(styrenejacrylonitri1e) phase.
The properties of such shock-resistant masses depend on
which of the two components forms the coherent phase of
the emulsion. When, e.g., high-impact polystyrene is manufactured by polymerization of styrene in presence of polybutadiene dissolved in styrene, it is the polybutadiene that
first forms the coherent phase; but as reaction continues, the
volume of the polystyrene phase progressively increases and
this leads to phase reversal so that the coherent phase then
consists of polystyrene.
Lecture at Freiburg (Germany) on June 9th, 1967
[VB 92 IE]
German version: Angew. Chem. 79, 949 (1967)
[*I Dr. H. Gerrens
Badische Anilin- & Soda-Fabrik AG.
Ammoniaklaboratorium
67 Ludwigshafen (Germany)
[I] A monomer mixture is termed azeotropic when the copolymer formed from it has the same composition as the monomer
mixture over the whole range of conversion.
Chemistry of Sulfur Ylides
By A . Hochrainer [*I
Individual sulfur ylides have been known for a long time,
but it is only in recent years that their chemistry has been
intensively studied 11JJ. Sulfonium ylides ( I ) are formed
from oxosulfonium ylides (2) :
[*I Prof. Dr. B. L. Horecker
Albert Einstein College of Medicine
1300 Morris Park Avenue
Bronx N . Y . 10461 (U.S.A.)
Sulfur ylides can be prepared in four general ways:
Demixing Phenomena in High Polymers and Their
Technical Importance
By H . Gerrens[*]
Incompatibility between two polymers is the rule, compatibility the exception. In the chemical technology of high
polymers, demixing phenomena are sometimes avoided and
sometimes consciously utilized. Precipitation polymerization
begins with a homogeneous starting mixture, from which the
polymei is precipitated during the polymerization. The
precipitation is either achieved by adding a precipitant to the
monomer (example: copolymerization of styrene and acrylo-
962
1. From the corresponding sulfonium salts and a base.
2. By direct condensation of CH-acidic compounds with
sulfoxides (only with very strongly CH-acidic compounds;
owing to electron delocalization in the carbanion of the
ylide this yields particularly stable ylides).
3. By rearrangement of an ylide already present (particularly
by replacement of one or both of the hydrogen atoms o n the
carbanion [Rl and R2 in ( I ) or (2)l).
4. By a few special processes, of which some appear capable
of generaIization, e.g., reaction of diazo compounds with
sulfides (yielding sulfonium ylides) or with sulfoxides (yielding oxosulfonium ylides) 131.
Angew.
Chem. internat. Edit. 1 VoL 6 (1967) / No. 11
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