pH dependence of charge transfer between tryptophan and tyrosine in dipeptides

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 kX⋅=9.0×104 s−1 and 1.8×104 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