Influence of DNA binding on the formation and reactions of tryptophan and tyrosine radicals in peptides and proteins.

The rate constant of the one-electron oxidation of the tryptophan (Trp) or tyrosine (Tyr) residues by Br- X 2 radical anions is strongly decreased when the peptides are bound to DNA. Oxidation by N X 3 is much less affected by binding. These results can be explained by electrostatic repulsion between the charged polyphosphate backbone and the Br- X 2 radicals. Once oxidized, the interacting aromatic residues react with the DNA in a first order process with a rate constant of the order 10(3) s-1. These results have been extended to the single strand binding protein: the product of gene 32 of phage T4 (gp 32). The pulse radiolysis study suggests that one Trp residue of the protein oxidized by the Br- X 2 radicals reacts with the DNA in the complex while one Tyr residue is buried upon association. It is also shown that the exposure of Trp and Tyr residues to radical attack depends on whether the T4 SSB protein is bound to native or heat-denatured DNA.