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 k_X⋅=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<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⋅=1.2×10⁵ 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.