This paper discusses the photophysical processes that determine the regularities of Z,Z bilirubin IXα photoisomerization - the mechanism that forms the basis of the widely used optical technology for treating hyperbilirubinemia (jaundice) of newborns. Based on a study of the spectrofluorescence and polarization characteristics of complexes of Z,Z bilirubin IXα with human serum albumin (HSA) and bovine serum albumin (BSA), it is established that one cause of the predominant formation of Z,E bilirubin IXα among the cis-trans photoisomerization products of Z,Z bilirubin IXα is the bichromophore character of the structure of the pigment, the nonidentity of the chromophores that form it, and the presence of interchromophore intramolecular energy transfer. The highest efficiency of nonradiative transfer is found for the bilirubin-HSA complex, and this correlates with the literature data concerning the higher selectivity of the bilirubin photoisomerization process in the indicated complex by comparison with BSA. The factor that promotes the process of intermolecular energy transport when the fluorescence lifetime of bilirubin is anomalously short (the picosecond or subpicosecond range) is the presence of excitonic interactions between the chromophores.
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