INDUCTION OF WAVELENGTH-DEPENDENT PHOTOCHEMISTRY IN BILIRUBINS BY SERUM-ALBUMIN

Quantum yields for Z reversible arrow E photoisomerization of the natu ral bi-chromophore bilirubin IX alpha in ammoniacal methanol solution were found to vary with excitation wavelength, whereas no variation (w ithin experimental error) was observed for xanthobilirubic acid and fo r symmetrically substituted bilirubins (bilirubin III alpha, bilirubin XIII alpha, and mesobilirubin XIII alpha) in the same solvent. The qu antum yield for Z reversible arrow?E photoisomerization of xanthobilir ubic acid bound to human serum albumin was also invariant with excitat ion wavelength. In contrast, quantum yields for Z reversible arrow E p hotoisomerization of the symmetrically-substituted bilirubins did show marked excitation wavelength dependency when they were bound to human serum albumin. These results show that quantum yields for Z reversibl e arrow E photoisomerization of bilirubins are markedly sensitive to p rotein binding and to the nature of pyrrole ring beta-substituents. Th ey also demonstrate that wavelength-dependent photochemistry is charac teristic of bilirubins with non-identical pyrromethenone chromophores, as expected from exciton coupling theory, and that complexation with albumin induces wavelength-dependent photochemistry in symmetrically s ubstituted bilirubins.