EVALUATION OF THE QUANTUM YIELD FOR E-]Z ISOMERIZATION OF BILIRUBIN BOUND TO HUMAN SERUM-ALBUMIN - EVIDENCE OF INTERNAL-CONVERSION PROCESSES COMPETING WITH CONFIGURATIONAL PHOTOISOMERIZATION

The quantum yield (phi(ZZ)) for the E --> Z photoisomerization of bilirubin (BR) bound to human serum albumin (HSA) under laser light irradiation has been evaluated with absorbance spectroscopy implemented by the high-performance liquid chromatography technique. The value at 458 nm is about 0.2, four times smaller than the value previously reported in the literature. As for the Z --> E configurational and the structural photoisomerization processes, the quantum yield varies with excitation wavelength, from 0.23 in the blue to 0.16 in the green. The sum of the quantum yields of the Z half arrow right over half arrow left E configurational photoisomerization reactions is 0.2-0.3 in the blue-green spectral region, thus giving direct evidence of the existence of internal conversion processes of the BR-HSA complex which compete with configurational photoisomerization. An evaluation of the quantum yields of the Z half arrow right over half arrow left E reactions for filtered broad-band light excitation in the 390-530 nm spectral region from data already published in the literature is also reported. Good agreement with our data of the quantum yields phi(ZZ) and phi(ZE) is found, despite the diversity of the experimental procedures in the two cases.