Pulse- and Gamma-Radiolysis of Aqueous Solutions of Tryptophan

Absorbing transient species are formed by the reaction of ·OH, H·, and ${\rm e}_{{\rm aq}}^{-}$ with tryptophan. The rate constants for the reaction of ·OH and H· with tryptophan are 1.25 ± $0.3\times 10^{10}\ M^{-1}\ {\rm sec}^{-1}$ and 7.4 ± $1\times 10^{9}\ M^{-1}\ {\rm sec}^{-1}$, respectively. Hydroxyl radicals react largely by addition to the indole ring. The spectra are interpreted in terms of various reaction sites each giving rise to discretely absorbing isomers. Variations in the spectra under acid conditions are interpreted as due to protonation of the ·OH and H· adducts. The transients decay by second order reactions giving further absorbing species. The rate constant for decay of trp-·OH is 2k = 7.7 ± $0.9\times 10^{8}\ M^{-1}\ {\rm sec}^{-1}$ at pH 7. Similar rate constants are obtained for the other radicals derived from tryptophan. At pH 1 the H· and ·OH adducts decay at the same rate with a composite rate constant, 2k, of 1.1 ± $0.1\times 10^{9}\ M^{-1}\ {\rm sec}^{-1}$. Gamma radiolysis of deaerated $2\times 10^{-2}\ M$ solutions at pH 1 gives $G({\rm NH}_{3})=0.51$, $G({\rm H}_{2}{\rm O}_{2})=G({\rm RO}_{2}{\rm H})=0.63$, $G({\rm H}_{2})=0.37$, $G({\rm CO}_{2})\sim 0$, G(-trp) = 0.7. The loss of tryptophan was also measured under various conditions and a mechanism has been proposed, involving recombination and the reactions of the radicals with the irradiation products, to account for the small loss of tryptophan in the gamma radiolysis of deaerated solutions.