Photochemistry of protein and nucleic acid constituents: electron spin resonance and spin-trapping with 2-methyl-2-nitrosopropane

Abstract

Our recent studies of the direct uv-photolysis of aliphatic and aromatic peptides, DNA constituents, and their 5-haloderivatives in aqueous solution and the photo-induced reactions of benzoylperoxide with amino acids, peptides, fatty acids, and pyrimidines in dimethylsulfoxide-containing solutions are reviewed.2-Methyl-2-nitrosopropane was used for spin-trapping and characterization of free radicals generated photochemically with light in the wavelength range of 220–313 nm in aqueous or aprotic solvents. Direct photolysis of aliphatic dipeptides and of phenylalanine peptides produced mostly decarboxylation radicals, while for tyrosine peptides both decarboxylation and deamination radicals were spin-trapped; for tryptophan di- and tripeptides, deamination radicals were predominantly produced, while for long chain polypeptides, main-chain scission was observed.When pyrimidine bases were photolysed, radicals consistent with the addition of an H-atom or an OH-radical at the C(5) position of the 5,6-double bond could be detected. The general reaction pattern in the photolyses of 5-chloro, bromo, or iodouracil was the homolytic cleavage of the carbon–halogen bond, while for 5-fluorouracil, the α-fluoro radical was spin-trapped.Dibenzoylperoxide was found to photoinduce the free radical generation in amino acids, peptides, and fatty acids exposed to ultraviolet light, which is not absorbed by these compounds, that is, λ = 313 ± 10 nm. The most predominant reaction is the decarboxylation of the terminal acid moiety. This process is explained by an electron transfer from the acid to the photoexcited peroxide or its fragmentation products. Pyrimidine bases, such as cytosine and thymine, can be oxidized under these conditions to generate C(5) centered radicals.