PHOTOCHEMICAL COUPLING OF 5‐BROMOURACIL TO TRYPTOPHAN, TYROSINE AND HISTIDINE, PEPTIDE‐LIKE DERIVATIVES IN AQUEOUS FLUID SOLUTION

Abstract

Abstract Direct irradiation of 5‐bromouracil (BU) in aqueous fluid solution in the presence of tryptophan (trp), tyrosine (tyr) or histidine (his) derivatives using a XeCl excimer laser at 308 nm yielded photocoupling of BU to the aromatic ring of each amino acid. Irradiation of BU at 308 nm most likely results in excitation of the n‐φ* transition, intersystem crossing to the triplet manifold, and coupling via electron transfer from the aromatic amino acid. The coupling observed was regiospecific between the 5‐position of uracil (U) and the 2‐position of the indole and phenol rings and the 5‐position of the imidazole ring of the respective amino acids. Quantum yields of photocoupling to BU ranged from 1 × 10 ‐3 to 7 × 10 ‐3 and paralleled known rates of electron transfer and ionization potentials of the aromatic rings. The photocoupling between BU and some of the aromatic amino acid peptide‐like derivatives possibly mimics photocrosslinking of BU‐DNA to associated proteins, a potentially useful photoreaction for studying nucleic acid‐protein interactions. Formation of crosslinks of the type proposed here might be detected by the characteristic fluorescence emission of the uracil amino acid adducts.