Addendum: Radiolysis of Aqueous Glycine

Abstract

At the National Institutes of Health we have been looking into some of the details of the radiolysis of aqueous glycine. We have concentrated on two aspects of the problem: first, the identification of additional products in an effort to obtain a more complete material balance; and second, the specific mechanism for the formation of certain products. Products containing basic functional groups are retained on Dowex 50-H and separated by gradient elution with HC1. Ammonia, methylamine, aspartic acid, DL-diaminosuccinic acid, and meso-a,l-diaminosuccinic acid have been previously reported. In addition, we have now identified DL-erythro-f3-hydroxyaspartic acid and DL-threo-#-hydroxyaspartic acid. On Dowex 50 these are eluted as one peak ahead of the aspartic acid, but when rechromatographed on Dowex 2-formate they are easily resolved into two peaks of approximately the same size. In neutral 1 M glycine (pH 6.3) the combined yield of these products is 0.17 molecule per 100 ev absorbed. In the pH range from 2 to 5 the yield is less, approximately 0.1 molecule per 100 ev. We can propose no mechanism for the formation of these acids, but do believe that we have ruled out one of the more obvious schemes-that is, formation as a secondary product by the addition of an electron to the carbonyl group of glyoxalic acid followed by neutralization and dimerization with the NH2CHCOOH radical. The yields in both acid and neutral solutions are independent of dose and unaffected by the addition of substantially large amounts of glyoxalic acid prior to irradiation. Organic acids not retained on Dowex 50 can be retained and separated with gradient elution on Dowex 2-formate. Acetic, formic, glyoxalic, and succinic acids have been previously reported. We have, in addition, identified glycolic acid. In 1 M neutral glycine this acid is produced with a G value of 0.08 in the absence of air and of 0.17 in the presence of air. Again, we have no mechanism for the formation of this acid, but believe that an obvious scheme-that is, the addition of an OH radical to -CH2COOH-is ruled out, since glycolic acid exhibits the same dependence on the glycine concentration as the other products studied. Another organic acid has been separated but not identified. It is produced in