Photoisomerization of Bilirubins and the Role of Intramolecular Hydrogen Bonds

The quantum yields and the photostationary state isomer ratios in the photoisomerization of bilirubin are strongly governed by the properties of the reaction media. Among the reaction media examined, the quantum yields for ZZ→ZE isomerization (φZZ–ZE) and for cyclization (φLR) were the highest in buffered aqueous solution (potassium phosphate buffer, pH 7.4) containing human serum albumin (HSA) in a 1 : 1 molar ratio to ZZ-BR, 0.11 and 2 × 10−3, respectively, on 436 nm irradiation. The effects of the reaction media surrounding bilirubin are discussed.