Electron spin resonance study of γ-irradiated frozen aqueous solutions containing N-acetylamino acids

The results of γ irradiation of a number of N-acetylamino acids at 77 K in frozen D2O and H2O 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: CH3CO-NDCH(CH3)CO2- + D+ → CH3COND2 + .CH(CH3)CO2-. Further warming to 220 K results in abstraction of the intermediate radicals from the parent compound to form the α-carbon radical: CH3CHCO2- + CH3CONDCH(CH3)CO2- → CH3CH2CO2- + CH3CONDCCH3CO2-. 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 at low pD and the peptide linkage favored at high pD. 9 figures, 1 table