title: 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)=9.0x10(4) s(-1) and 1.8x10(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)=1.2x10(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.
authors.affiliation: Department of Chemistry, 6-335, 77 Massachusetts Avenue, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA.
authors.forename: Steven Y
authors.initials: SY
authors.lastname: Reece
authors.forename: Joanne
authors.initials: J
authors.lastname: Stubbe
authors.forename: Daniel G
authors.initials: DG
authors.lastname: Nocera
mesh_terms: D004151:Dipeptides / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D019906:Nuclear Magnetic Resonance, Biomolecular; D011522:Protons; D013056:Spectrophotometry, Ultraviolet; D014364:Tryptophan / Q000737:chemistry*; D014443:Tyrosine / Q000737:chemistry*
chemical_list: D004151:Dipeptides; D011522:Protons; D014443:Tyrosine; D014364:Tryptophan