Time-Resolved CIDNP Study of Intramolecular Charge Transfer in the Dipeptide Tryptophan-Tyrosine

Abstract

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) and laser flash photolysis (LFP) techniques have been used to study the kinetics and mechanism of the photochemical reaction between 2,2‘-dipyridyl (DP) and the dipeptide l-tryptophan-l-tyrosine (TrpH-TyrOH) in acidic aqueous solution (pH = 3.8). Analysis of the geminate CIDNP pattern reveals that the quenching of the protonated triplet dipyridyl TDPH+ results in the formation of both tryptophan and tyrosine radicals from the dipeptide with comparative efficiency. The total quenching rate constant of triplet dipyridyl by TrpH-TyrOH was found to be (2.5 ± 0.3) × 109 M-1 s-1. The radical transformation TrpH+•→TyrO• via intramolecular electron transfer (IET) leads to an increasing tyrosyl radical concentration, growth of the tyrosine CIDNP signals, fast decay of the CIDNP signal of TrpH, and inversion of the CIDNP sign from emission to enhanced absorption for DP. The nuclear spin−lattice relaxation times of the radicals formed in the react...