A spectroscopic study of the wavelength-dependent photoisomerizations of bilirubins bound to human serum albumin

The wavelength-dependent photoisomerizations of the asymmetric bilirubin BR-IX? and of the symmetric bilirubin-III? (BR-III) and mesobilirubin-XIII? (MBR-XIII) bound to human serum albumin (HSA) in aqueous solution were analysed with the help of an exciton coupling model. The modelling was based on the absorption and circular dichroism (CD) spectra (bisignate Cotton effect). Time-dependent density functional theory (TD-DFT) of the free BR-IX molecule suggested the presence of two main bands of exciton coupling character in the blue region of the spectrum, and other weaker bands of charge transfer character at longer wavelengths. These peculiarities were taken into account to fit the photoisomerization quantum yields in the blue-green region as functions of the wavelength, obtaining the bandshape of the exciton coupling bands from the experimental CD spectra. The other excitons were extracted from the decomposition of the band resulting from the difference between the absorption spectrum and the sum (normalized-to-absorption) of the two CD excitons. We expressed photoisomerization quantum yields in terms of the sum of the contributions to photon absorption deriving from all the exciton states normalized to total absorption. For all the reversible photoprocesses of bilirubins and for the irreversible one of BR-IX? in HSA (i.e. lumirubin formation), we give reliable mean values of the individual state excitation probabilities and photoisomerization efficiencies in the pigment-protein complex.