PHOTOCHEMISTRY OF BIOLOGICAL MOLECULES-V. A MECHANISTIC STUDY OF THE PHOTOLYSIS OF CYCLOALANYLALANINE IN THE SOLID STATE

Abstract— Earlier studies of the photodamage induced by 254 nm irradiation of linear alanine peptides in the solid state have been supplemented by an investigation into the gaseous photoproducts from the cyclic dipeptide, 3,6-dimethyl-2,5-diketopiperazine. The trans and cis isomers have been prepared and the photoproducts compared with those from the DL-mixture. The conformation of the molecule does influence the yield of gaseous products. CO was produced by peptide bond rupture with concomitant release of hydrogen. CO₂ was also produced. The use of N- and C-deuterated analogues together with relevant crystallographic and EPR data has enabled a detailed study of the mechanism of photode-gradation to be made, from which it is concluded that the methyl protons are not inert but rather are the major source of the hydrogen observed on photolysis.