Radiolysis of model polypeptides in the solid state: radiation sensitivity and side chain structure

Homopolymers of the amino acids gly, ala, val, phe and tyr have been used as models to investigate the effect of molecular structure on the degradation of polypeptides by ionizing radiation. The main reactions were-(i) scission of the entire side chain leading to its hydrogen adduct, (ii) fragmentation of the side chain, (iii) main-chain scission and fragmentation leading to a reduction in molecular weight and elimination of a segment of the chain. ESR studies confirmed hydrogen abstraction to form the α-carbon radical (except for polytyrosine) with G(R·) comparable with G(total ammonia), but these radicals were shown to be stable up to the temperature at which the volatile products were removed (100°C). The radicals formed from polytyrosine were unstable and decayed rapidly at room temperature. It is proposed that main-chain scission of the N-C bond occurs to form polymer cations and radicals, and that fragmentation occurs to form polymer cations and radicals, and that fragmentation occurs via the cation. High yields of aliphatic acids reported by previous workers were found to be due to residual, hydrogen-bonded water in the polymer. Aromatic protection was observed in polyphenylalanine and more effectively with the phenolic group in polytyrosine