QUANTUM YIELD and EQUILIBRIUM POSITION OF THE CONFIGURATIONAL PHOTOISOMERIZATION OF BILIRUBIN BOUND TO HUMAN SERUM ALBUMIN*

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Abstract
Abstract— At low fluence rates it is possible to observe a photoequilibrium between Z,Z‐bilirubin(BR) and its E isomers(collectively called PBR) bound to human serum albumin(HSA).For excitation centered at 465 nm, the fraction of PBR/HSA in the photoequilibrium mixture was observed to be 0.22 + 0.02. The quantum yield for conversion of BR/HSA to PBR/HSA was found to be 0.20 0.02. The equality of the quantum yield value and the fraction of PBR/HSA in the photoequilibrium mixture is consistent with a simple mechanism for the photoisomerization in which optically excited singlet states of BR and PBR convert rapidly and with virtually total efficiency to a common excited intermediate with twisted geometry that subsequently decays to BR and PBR in the ratio 4:1, respectively. Quantum yields for other photoprocesses of BR bound to HSA are much lower than that for configur‐ational isomerization. The central role suggested for configurational isomerization in the phototherapy for neonatal hyperbilirubinemia is supported by these observations.