Resonance CARS and molecular orbital studies of the binding of bilirubin to human serum albumin

Abstract

Abstract Resonance CARS (coherent anti-Stokes Raman scattering) spectra reveal that the double bond nature of the lactam CO bond of bilirubin decreases significantly on formation of a complex with human serum albumin (HSA). A mechanism is proposed in which the electrostatic attraction between the positively charged amino acid residues in HSA, most probably the protonated amino groups in the side chains of lysine or arginine residues, and the negative charges induced on the lactam oxygen atoms of bilirubin by the positive charges on the protonated amino groups, makes an important contribution to the binding of bilirubin to HSA. Semi-empirical molecular orbital calculations of the structure, stabilization energy and vertical transition energy of a model compound of bilirubin in the field of external positive unit charge provide firm support for the proposed mechanism of the binding of bilirubin to HSA.