Intramolecular electron transfer in tryptophan-tyrosine peptide in photoinduced reaction in aqueous solution

The formation of chemically induced dynamic nuclear polarization (H-1-CIDNP) and its time evolution in course of the photochemical reactions of 2,2'-dipyridyl in its triplet state with the tryptophan-tyrosine dipeptide and with a 1: 1 mixture of N-acetyl tryptophan and N-acetyl tyrosine is studied in acid (pH 3.0), neutral (pH 7.4), and basic (pH 11.4) solution. Time resolution on the microsecond scale allows us to establish the full reaction scheme and to determine all relevant transient intermediates. The stationary value of spin polarization is found to depend strongly on secondary reaction steps and does only in part reflect the primary process of triplet quenching. Degenerate electron exchange, spin relaxation in paramagnetic intermediates, and radical pair reactions in the bulk are identified from the polarization kinetics. In particular, the increase of tyrosine quenching efficiency and the influence of intramolecular electron transfer from the tyrosine moiety to the transient tryptophan radical are revealed when the amino acids are linked in the dipeptide in comparison with the case that they are separated in the 1:1 mixture. The strong effect of linkage on the amplitude and the kinetics of spin polarization is discussed with regard to application of this method to studying structure and folding kinetics of proteins.