Flash photolysis study of aliphatic amino acids and peptides in aqueous solution

Abstract

The intermediates produced in the photolysis of oxygen-free aqueous solutions of a number of aliphatic amino acids and peptides were observed spectrophotometrically using the fast-reaction technique of flash photolysis. Included among the compounds examined are the N-acetyl derivatives of glycine, alanine, sarcosine, glutamic acid and glycylglycine; the esters and amides of these N-acetyl compounds; diketopiperazines; the amino acids glycine, alanine and β -alanine; and finally the oligopeptides di-, tri- and tetraglycine. The direct optical excitation of these compounds was found to lead primarily to a photo-induced decarboxylation reaction: The transient spectra of the radicals produced have been identified. The quantum yields of these processes were found to be directly proportional to the pK_a of the carboxyl groups of the corresponding ground-state molecules, and hence to the concentration of the non-ionized carboxylic acids. The φ 's of these processes for the ionized acids were close to zero. The dependence of φ upon pH is correlated to the absorption spectra of these compounds. The quantum yields of the corresponding esters were lower but independent of pH. No intermediates were observed from excitation of the amine derivatives. Other photolytic reactions are suggested. The photo-decarboxylation of alanine and diglycine were found to be monophotonic, while that of N-acetyl alanine, N-acetyl diglycine, and tetraglycine were found to be biphotonic. A triplet excited state precursor is indicated for the latter group of compounds. These and other results are discussed.