Photolysis of the aqueous thymine system: I. Excitation at 1849 Å

Abstract

1. The 1849 Å photolysis of thymine in aqueous solution occurs with a quantum efficiency varying from 8·10 −2 to 0.2, depending on experimental conditions, in contrast to the photolysis at 2537 Å which occurs with a quantum efficiency of 4·10 −4. 2. Products of photolysis have been identified as thymine hydroxyhydroperoxides (4:5 and 5:4), thymine glycol (4:5) and hydrogen peroxide. These, together with thymine loss, have been determined quantitatively under a variety of conditions. 3. Mechanisms of photolysis are deduced. At low thymine concentration, reaction is due to water photolysis, and complete stoichiometry is obtained. Kinetic analysis shows that direct photolysis of thymine also occurs. Direct photolysis is accounted for by two processes, (a) photoionization, and (b) addition of O 2 to an excited state. These primary processes are correlated with the far ultraviolet absorption spectrum of thymine.