H and¹³C Hyperfine Coupling Constants of the Tryptophanyl Cation Radical in Aqueous Solution from Microsecond Time-Resolved CIDNP

Abstract

Relative values of the 1H and 13C isotropic hyperfine couplings in the cationic oxidized tryptophan radical TrpH*+ in aqueous solution are determined. The data are obtained from the photo-CIDNP (chemically induced dynamic nuclear polarization) enhancements observed in the microsecond time-resolved NMR spectra of the diamagnetic products of photochemical reactions in which TrpH*+ is a transient intermediate. The method is validated using the tyrosyl neutral radical Tyr*, whose 1H and 13C hyperfine couplings have previously been determined by electron paramagnetic resonance spectroscopy. Good agreement is found with hyperfine coupling constants for TrpH*+ calculated using density functional theory methods but only if water molecules are explicitly included in the calculation.