Intramolecular Electron Transfer in the Dipeptide, Histidyltyrosine: A Pulse Radiolysis Study

The technique of pulse radiolysis has been used to investigate the possiblity of intramolecular charge transfer in the dipeptide histidyltyrosine, following one-electron oxidation of one of its amino acid residues. The radical anion, Br2·−, was found to react with the dipeptide at pH 6.0 with a bimolecular rate constant of 2.3 ± 0.2 × 107 dm3 mol−1 s−1 suggesting that it reacts very selectively with the histidine moiety. Spectral observations at, or close to the end of this reaction show only the presence of a tyrosinyl free radical (TyrO·), however, indicating that fast (>106 s−1) intramolecular charge transfer has taken place between histidine radicals (His+·) and tyrosine (TyrOH). This finding was supported by the direct observation of the rate of formation of TyrO· in experiments with the free amino acids, histidine and tyrosine, under conditions where Br2·−reacted selectively with histidine. The bimolecular rate constant for the reaction between His+· and TyrOH was found to be 2.4 ± 0.5 × 106 dm3 mol−1s−1. Taken together, the results of the study indicate that His+· is a relatively strong oxidising agent where (E (His+·/His) > 770 mV at pH 6.0.