Wavelength dependent configurational photoisomerisation of bilirubins interpreted by spectral absorptions

We measured the 4Z,15Z→4Z,15E configurational photoisomerization quantum yields of bilirubin-IXα (BR-IX), the intrinsically asymmetric bile pigment associated with jaundice, when it is irradiated in ammoniacal methanol solution, and of BR-IX and symmetrically-substituted bilirubins (bilirubin-IIIα and mesobilirubin-XIIIα) bound to human serum albumin (HSA) in aqueous solution. The quantum yield wavelength dependence was attributed to fast changes induced in the molecule conformation before photoisomerization.