The radiation degradation of poly(amino acid)s, amino acid copolymers, N-acetylamino acids, ethylene-α-olefin copolymers, polyethylenes and reduced poly(vinyl chloride)s has been investigated.
Homopolymers of amino acids with a variety of side-chains have been used as models to investigate the effect of molecular structure on the radiation sensitivity of polypeptides. In this study, (1) the volatile radiolysis products and their yields have been identified and measured by gas chromatography (GC) and gas chromatography/ mass spectrometry (GC/MS), (2) free radical intermediates and their yields at 25°C, at -196°C, and on subsequent warming, have been investigated by electron spin resonance (ESR) spectral analysis and (3) yields of main-chain scission and crosslinking have been measured by studying the molecular weight changes on irradiation using gel permeation chromatography (GPC).
For poly[amino acid)s with aliphatic hydrocarbon side-chains, the main reactions were: (i) scission of the entire side-chain, and (ii) main-chain scission and fragmentation leading to elimination of a segment of the chain. When a carboxyl group was present in the side-chain, the main reaction was decarboxylation instead of main-chain scission. Aromatic protection was observed in polyphenylalanine and more effectively with the phenol group in polytyrosine. It has been proposed that γ-irradiation of polypeptides causes ionization to give the polymeric radical cation, which undergoes chain scission to produce an amino cation and an alkyl radical, and that fragmentation occurs via the cation.
The yields of volatile products, free radicals, main-chain scission and crosslinking from two random copolymers of glutamic acid and tyrosine after irradiation have been measured. The G values were substantially lower than the values corresponding to linear relationships between the homopolymers, attributable to protection by the aromatic groups. The number of glutamic acid units protected by a tyrosine unit has been calculated for decarboxylation, CO production and main-chain scission. The results suggest that a number of protection mechanisms may be involved.
N-acetylamino acids have been considered by previous workers to be useful model compounds for the study of the effects of ionizing radiation on polypeptides. In this study, yields of (1) volatile products and (2) free radicals from γ-radiolysis of a number of N-acetylamino acids in the polycrystalline state have been determined at 25°C. Carbon dioxide was found to be the major product. There was evidence for some scission of the N-Cα bond but deamination was found to be of much lesser importance than decarboxylation.
The radicals formed on γ-radiolysis of N-acetylvaline at -196°C, at 25°C and on subsequent warming over the range -196 to 167°C have been determined by ESR spectral analysis. A general mechanism for the radiation degradation of N-acetylamino acids has been suggested.
The yields of alkanes and alkenes from γ-irradiation of a series of copolymers of ethylene with propene, 1-butene and 1-hexene, and low- and high-density polyethylenes at temperatures from 25 to 175°C, have been measured to investigate (1) the nature and frequency of short-chain branching in polyolefins and (2) the mechanism of the radiation degradation.