PH dependence of charge transfer between tryptophan and tyrosine in dipeptides

Abstract

Time-resolved absorption spectroscopy has been employed to study the directionality and rate of charge transfer in W–Y and Ac-W–Y dipeptides as a function of pH. Excitation with 266-nm nanosecond laser pulses produces both W⋅ (or [⋅WH] +, depending on pH) and Y⋅. Between pH 6 and 10, W⋅ to was found to oxidize Y with k X⋅=9.0×10 4 s −1 and 1.8×10 4 s −1 for the W–Y and Ac-W–Y dipeptide systems, respectively. The intramolecular charge transfer rate increases as the pH is lowered over the range 6>pH>2. For 10<pH<12, the rate of radical transport for the W–Y dipeptide decreases and becomes convoluted with other radical decay processes, the timescales of which have been identified in studies of control dipeptides Ac-F–Y and W–F. Further increases in pH prompt the reverse reaction to occur, W–Y⋅→W⋅–Y − (Y −, tyrosinate anion), with a rate constant of k X⋅=1.2×10 5 s −1. The dependence of charge transfer directionality between W and Y on pH is important to the enzymatic function of several model and natural biological systems as discussed here for ribonucleotide reductase.