Solvent Effects on the Intramolecular Hydrogen-Atom Transfer between Tyrosine and Benzophenone. Diverting Reaction Mechanisms in Protic and Nonprotic Media

The intramolecular hydrogen-atom transfer between tyrosine and triplet-excited benzophenone was studied by nanosecond laser-flash photolysis in a number of nonprotic and protic solvents. The reaction rates were found to be strongly solvent dependent with a range from <10 5 s −1 to 5 × 10 7 s −1 . Eventual contributions from solvent-dependent conformational equilibria on the reaction rates were found to be of minor importance. In contrast, the rate-enhancing and rate-retarding effects of the solvents could be attributed to specific solvent−solute interactions with the hydrogen donor and the hydrogen acceptor. Opposite effects were exerted by nonprotic and protic solvents. The sharp decrease of the reaction rates in nonprotic solvents with the hydrogen-bond acceptor ability of the solvent is referred to hydrogen bonding of the tyrosine to the solvent, TyrOH → S. It could be quantified in terms of an intramolecular kinetic solvent effect model. In turn, a sharp increase of the reaction rates with the hydrogen-bond donor ability of the solvents is observed in protic solvents, which is referred to solvation effects on both chromophores, SH → O(H)Tyr and SH → bp. Implications of these diverting solvent dependencies on the reaction mechanism in protic and nonprotic solvents are discussed.