O-C

H—C +

Ή

Λ MODELS. Stereophotographs of models show L-glutamic acid (top) and D-glutamic acid in extended conformations. For three-dimensional effect, hold mirror upright between each pair of pictures with mirrored surface facing left. Look down mirror so that left eye sees left picture in mirror and right eye sees right picture. Front-faced mirror gives best effect. These pictures appear in full color in Biochemistry, February, page 729, the first ACS journal to use full-color stereophotographs

glutamic acid is replaced by a methyl group, the enzyme becomes strictly L-specific. It cannot bind the alpha-methyl derivative of D-glutamic acid. This, too, can be explained in terms of the proposed conformation. A model of «-methyl-D-glutamic acid in this conformation shows that the methyl group is on the side opposite the methyl group of the L-acid. This sug­gests that the methyl group prevents close approach of D-acid to enzyme. In contrast, an alpha-methyl on L-glutamic acid sticks harmlessly out on the side of the molecule opposite the enzyme. This supports the proposed conformation of the substrate on the enzyme.

Finally, Dr. Meister and his group extended this methyl-substitu­tion technique to test the proposed conformation. Their theory indicates that only one of the four possible isomers of β-methylglutamic acid would be stereochemically acceptable to the enzyme. It predicts this isomer

to be iforeo-/J-methyl-r>glutamic acid. Experiment verifies this. They also find that of the four isomers of γ-methylglutamic acid, again only the predicted one, an L-isomer this time, is a substrate.

These observations support Dr. Meister's hypothesis concerning the conformation of the enzyme-bound substrates. According to this hypo­thesis, L-glutamic acid is oriented on the enzyme in an extended conforma­tion. In this conformation, the alpha hydrogen is directed away from the active site of the enzyme. The car-boxyl and amino groups of D-glutamic acid (also in an extended conforma­tion) are bound to the same sites on the enzyme as the corresponding groups of L-glutamic acid. The alpha hydrogen of D-glutamic acid, however, is oriented toward the enzyme. This work shows how substrates combine with glutamine synthetase, and why the enzyme acts the way it does with various substrates.

Electric Field Raises Polymerization Rate Japanese polymer chemists at Kyoto University are accumulating still more data showing that electric fields in­crease the rate and degree of polymer­ization of certain vinyl monomers. In their latest work, Prof. Ichiro Sakurada and Dr. Norio Ise find that an electric field significantly raises the rate and slightly increases the degree of cat­alytic cationic polymerization of a-methylstyrene. The effect is propor­tional to field strength. Similar stud­ies with styrene, however, show no field influence.

In earlier studies, the Kyoto chem­ists found that an electric field in­creases the rate of catalytic cationic polymerization of p-methoxystyrene and isobutyl vinyl ether. They found a similar effect in the radiation-induced cationic polymerization of styrene. An electric field also in­creased the degree of polymerization of p-methoxystyrene and isobutyl vi­nyl ether. However, electric fields do not influence radical polymerizations of styrene or methylmethacrylate.

The Japanese work is mainly aca­demic and is aimed at clarifying poly­merization mechanisms, according to Dr. Ise (who is being advised in the project by Prof. Seizo Okamura). "We think we can clarify the nature of growing chain ends through study of the electric field effect/' Dr. Ise says. But he notes that there is a secondary, practical side, too. The field effect could prove economical in upgrading polymerization systems without addi­tional catalyst expense.

Of the monomers studied so far, p-methoxystyrene is affected the most. In a field of 1 kv. per centimeter, the ratio of the initial rate of polymeriza­tion with field (RpE) to that without (Rpo) is 1.34. At the same field strength, the ratio is 1.28 for isobutyl vinyl ether, 1.25 for a-methylstyrene, and 1.00 for styrene.

The ratio of degree of polymeriza­tion with and without field (at 1 kv. per centimeter) is 1.75 for p-meth-oxystyrene, 1.04 for isobutyl vinyl ether, 1.05 for a-methylstyrene, and 1.00 for styrene. These data cor­respond to a concentration of 1.0 X 10 - 5 moles per liter of catalyst (io­dine) and 0.8 mole per liter of mono­mer.

All these reactions were carried out using ethylene dichloride as a solvent. In addition, styrene was polymerized

FEB. 28, 1966 C&EN 37

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under gamma irradiation. In a field of 1 kv. per centimeter, RpB/Rpo *S

1.3. The field effect is proportional to

field strength. With a-methylstyrene, for example, RpB/Rpo rises linearly from 1.3 at 1 kv. per centimeter to about 2.2 at 5 kv. per centimeter. Other monomers tested at Kyoto act similarly (except for styrene) when cationically polymerized. Also, RpE/Rpo is independent of both cat- fi alyst and monomer concentration.

Interpretation. As they gather more data, the Kyoto chemists are re­inforcing their interpretation of what causes the effect. They reason that in ionic polymerization, where growing chain ends contain ion pairs as well as free ions, application of an electric field raises the degree of dissociation of the ion pairs, yielding still more ions. Since free ions propagate much faster than ion pairs, the net result of the electric field is to raise the rate and degree of polymerization.

However, in radical polymeriza­tions, where ionic species aren't in­volved, no field effect should be ex­pected. And the Japanese work bears this out. Even at much higher field strengths (as high as 20 kv. per cen­timeter), no effect is apparent.

The Japanese chemists have de­rived equations for rate and degree of polymerization that are generally a function of the degree of dissociation of ion pairs at the growing chain ends. For relatively large degrees of disso­ciation, these expressions can account for the field effect observed. This is the case, for instance, with a-methyl­styrene, p-methoxystyrene, and iso-butyl vinyl ether, Dr. Ise says.

When the degree of dissociation is small, though, the relations become al­most independent of the degree of dis­sociation. And a small increase in the degree of dissociation caused by an electric field does not significantly in­fluence the polymerization. This ex­plains why styrene is relatively little affected. And it indicates that styrene is less dissociated than the other mono­mers studied, Dr. Ise feels.

Possibilities. Before concluding that the effect is due to electric field, the chemists had to eliminate other possibilities. For example, Joule heat stemming from the passage of electric current through the polymerizing solu­tion could conceivably cause the ob­served effect. Experiments with and without electric fields at various tem­peratures showed that polymerization

Initial Rate of Polymerization of «Methylstyrene Increases Linearly with Strength of Electric Field

1 2 3 4 5 6 Field Strength (kv./cm.)

rates actually drop with increasing temperature for p-methoxystyrene and α-methylstyrene. Thus, Joule heat, whatever its role, does not cause the increased rates.

Another possibility is that electro-initiated polymerization (through elec­trolysis of trace impurities such as wa­ter) triggers the effect. To check this, the Japanese chemists subjected ethyl­ene dichloride solutions of p-methoxy­styrene without iodine catalyst to high electric fields. No polymer formed.

Still another and related possibility is that the field effect stems from elec­trolysis of iodine. However, calcula­tion shows that the amount of elec­tricity carried by iodine during poly­merization is not enough to account for the amount of additional material polymerized. Also, the field effect is almost independent of the quantity of electricity passed through trie system.

Further, infrared spectra of poly­mers produced with and without an electric field show no structural dif­ferences. This means that the field effect is not caused by polymerization mechanisms different from those at work in conventional vinyl polymeri­zations. The chemists plan to extend their studies to anionic systems. They will use styrene and n-butyllithium catalyst in toluene.

Impurities. The Japanese use a

38 C & E N FEB. 28, 1966

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rather simple setup in their work. The experimental apparatus consists of a glass tube that has a thermometer and a pair of platinum electrodes at the ends. Several versions have been used. In one, for example, the elec­trodes have an area of 1 square centi­meter and are 10 centimeters apart. In another, the electrodes are 1 centi­meter apart, but have an area of 10 square centimeters. The tube is de­signed to provide as uniform a field as possible. The volume between elec­trodes is at least 80% of the total solu­tion volume.

The trickiest problem in this work is purity. Ionic impurities, for example, cause a relatively high current flow with resultant Joule heat and unreli­able results. So the Japanese take pains to remove material that could screen the effect they are looking for. After the solvent is purified, the im­purity level is further lowered by pre-electrolyzing the catalyst solution (without monomer) under high volt­age (15 kv.) for 10 minutes.

BRIEFS

The gyromagnetic ratio of the free positron has been measured to one part in 100,000 by physicists H. R. Crane and Arthur Rich at the Univer­sity of Michigan (Ann Arbor). This ratio is the ratio of spin to magnetic moment. Knowing the ratio accu­rately is important in testing modern quantum electrodynamics, the two sci­entists explain. In 1963, Dr. Crane measured the gyromagnetic ratio of the electron to an accuracy of one part in 100 million. Thus far, his measure­ments on the positron support the view that positrons and electrons are identi­cal, except that they have opposite charges and magnetic moments.

Scientists at NIH have used a direct culture procedure to isolate myco­plasma from blood and bone marrow specimens. Most previous isolations of mycoplasma from patients with leu­kemia and other diseases have been carried out in tissue cultures that fre­quently harbor mycoplasma as a con­taminant. Four out of 10 specimens from patients with acute leukemia were positive for mycoplasma. In the National Institutes of Health study, control cultures of 10 bone marrow specimens from heart surgery patients were mycoplasma negative.

40 C & E N FEB. 28, 1966