An electron spin resonance study of .gamma.-irradiated frozen aqueous solutions containing N-acetylamino acids

Abstract

The results of ..gamma.. irradiation of a number of N-acetylamino acids at 77 K in frozen D/sub 2/O and H/sub 2/O solutions are reported. The radicals produced by the irradiation, their relative amounts, and their stability were studied as a function of temperature. The results found for ices of N-acetylalanine illustrate the reaction mechanisms found. At low temperatures (120 K) the peptide anion and decarboxylated species dominate the spectrum. Upon warming to 190 K the anion converts to the amide and fatty acid radical by secondary deamination: CH/sub 3/CO/sup -/NDCH(CH/sub 3/)CO/sub 2//sup -/ + D/sup +/ ..-->.. CH/sub 3/COND/sub 2/ + .CH(CH/sub 3/)CO/sub 2//sup -/. Further warming to 220 K results in abstraction of the intermediate radicals from the parent compound to form the ..cap alpha..-carbon radical: CH/sub 3/CHCO/sub 2//sup -/ + CH/sub 3/CONDCH(CH/sub 3/)CO/sub 2//sup -/ ..-->.. CH/sub 3/CH/sub 2/CO/sub 2//sup -/ + CH/sub 3/CONDCCH/sub 3/CO/sub 2//sup -/. Results found for N-acetylglutamic acid show similar reactions and also show evidence for electron attachment to the two carboxyl groups as well as the peptide linkage. An investigation as a function of pD shows that the site of electron attachment is strongly pD dependent with the carboxyl groups being favored atmore » low pD and the peptide linkage favored at high pD. 9 figures, 1 table.« less