INTRAMOLECULAR ELECTRON-TRANSFER IN THE DIPEPTIDE, HISTIDYLYTROSINE -A PULSE-RADIOLYSIS STUDY

The technique of pulse radiolysis has been used to investigate the pos siblity of intramolecular charge transfer in the dipeptide histidyltyr osine, following one-electron oxidation of one of its amino acid resid ues. The radical anion, Br-2(.-), was found to react with the dipeptid e at pH 6.0 with a bimolecular rate constant of 2.3 +/- 0.2 x 10(7) dm (3) mol(-1) s(-1) suggesting that it reacts very selectively with the histidine moiety. Spectral observations at, or close to the end of s t his reaction show only the presence of a tyrosinyl free radical (TyrO .), however, indicating that fast (>10(6) s(-1)) intramolecular charge transfer has taken place between histidine radicals (His(+.)) and tyr osine (TyrOH). This finding was supported by the direct observation of the rate of formation oi TyrO . in experiments with the free amino ac ids, histidine and tyrosine, under conditions where Br-2(.-) reacted s electively with histidine. The bimolecular rate constant for the react ion between His(+.) and TyrOH was found to be 2.4 +/- 0.5 x 10(6) dm(3 ) mol(-1) s(-1). Taken together, the results of the study indicate tha t His(+.) is a relatively strong oxidising agent where (E (His(+.)/His ) > 770 mV at pH 6.0. (C) 1998 Elsevier Science Inc.