Photochemical Coupling of 5-Bromouracil (BU) to a Peptide Linkage. A Model for BU-DNA Protein Photocrosslinking

Article abstract of DOI:10.1021/ja00240a032

Photocrosslinking of DNA containing 5-bromouracil (BU) in place of some of the thymine to an associated protein has potential for studying protein nuclei acid interactions.Experiments with model compounds are described which indicate some of the electronic and molecular details of the crosslinking.Earlier experiments suggested that the BU chromophore in its ?,?^* singlet state reacted by C-Br bond homolysis probabaly leading to BU-DNA single strand breaks and that the triplet state was responsible for crosslinking of BU-DNA to associated proteins probably via initial electron transfer.Only intermolecular coupling of BU to cysteine, glutathione, tryptophan, and tryptophan derivatives had been observed previously.The important conclusions established here are (1) that the BU chromophore has a poorly resolved, possibly n-?^* transition in the region of 308 nm, (2) that the photochemistry of the BU chromophore is significantly wavelength dependent, (3) that the ¹n,?^* state can be selectively populated with a XeCl excimer laser or the 313-nm mercury band isolated with a monochromator, (4) that the ¹n,?^* state does not undergo C-Br bond homolysis in competition with intersystem crossing to the triplet manifold, and (5) that the triplet state couples to a peptide linkage in proximity with loss of HBr.These conclusions were established by studying the photoreduction of BU in 2-propanol-d and the photocyclization of (5-bromo-6-uracyl)-N-ethylacetamide (1) and N-<(5-bromo-6-uracilyl)acetyl>-D,L-threonine N-ethylamide (2) to furans 3 and 6 in buffered water, both as a function of wavelength.