The radical ions and photoionization of bile pigments

The semireduced, semioxidized, and OH(.)-adduct radicals of bilirubin (BR) and biliverdin (BV) have been characterized using pulse radiolysis techniques. Laser flash photolysis (265-nm) of these pigments led to monophotonic photoionization with quantum yields of 0.08 for BR and 0.03 for BV. No evidence for triplet formation or for photoisomerization was found after 265-nm laser excitation. However, 347-nm excitation of BR in chloroform led to simultaneous photoisomerization and radical formation, but the radicals are thought to have originated from a pathway other than photoionization. The relevance of these observations to BR photoreactivity is discussed. BR radical ions in alkaline solution did not react with tryptophan (TrpH), but the semioxidized TrpH radical oxidized BR with k = 4.3 X 10(8) dm3 mole-1 sec-1. When human serum albumin (HSA) was oxidized using radiolytically generated azide radicals, a radical transformation involving TrpH and TyrOH residues occurred with k = 3.8 X 10(3) sec-1. When BR was complexed with the protein the transformation rate was reduced to 1.6 X 10(3) sec-1. This was interpreted in terms of a conformational change in the protein. Identification of the probable residues involved provided information about the primary BR binding site which was consistent with an earlier report.