Photophysics of the Variable Quantum Yield of Asymmetric Bilirubin

Abstract

Bilirubin (BR), responsible for neonatal jaundice, is a 'Siamese twin' type of molecule containing two pyrromethenone chromophores conjoined by a saturated carbon CH₂ group. Because neonatal jaundice is cured by phototheraphy, bilirubin has been extensively studied by laser means. When the chromophores in each half of the molecule are identical, we have symmetrical BR (SBR); when they are not, we have antisymmetric BR (ASBR). The quantum yield of the photoproducts from SBR is not wavelength-dependent, while that from ASBR is, in organic solvents. Because of the proximity of the two chromophores, both the ASBR and SBR systems are subject to Davidov (dynamic electric dipole) splitting of the chromophore excited states. A quantum mechanical calculation shows that when the two (asymmetric) chromophore states are not degenerate, the higher Davidov state is preferentially occupied by the chromophore with the 'original' higher energy, and the lower Davidov state by the chromophore of 'original' lower energy. This is just what is required for the quantum yield to vary with wavelength. If the variation of the quantum yield of asymmetric bilirubin in the presence of human serum albumin is approximated by a square-wave (narrow line approximation), a quantum mechanical calculation of the ratio of the short wavelength photoproduct yield with the long wavelength one is in agreement with accepted values for the 'original' energy difference of the chromophores, and the Davidov splitting parameter.