The wavelength dependent configurational photoprocesses in the asymmetric bilirubin BR-IX alpha (BR-IX) and symmetrically-substituted bilirubin-III alpha (BR-III) and mesobilirubin-XIII alpha (MBR-XIII) in aqueous solution bound to human serum albumin (HSA) was analysed with the aid of absorption spectra and exciton bands deduced from circular dichroism spectra (bisignate Cotton effect). Assigned excited states by TD-DFT of the free molecule validates the exciton coupling model for the description of the interaction between the main bands of BR-IX in the blue region of the spectrum, and the presence of other two bands of charge transfer character at longer wavelengths. To fit the photoisomerisation quantum yields as a function of wavelength in the blue-green region, we took into account the absolute values of the two CD excitons deduced by fitting experimental spectra, and the “difference” bands from absorption spectrum and the sum, normalised to the absorption spectrum, of the two CD excitons. These “difference” bands contain the convolution of the two charge transfer bands, modified like the CD ones by the complexation with HSA. We fitted also the photoisomerisation quantum yields of lumirubin, the irreversible photoisomer of BR-IX in HSA. For this photoisomer like for the other reversible ones, we found reliable values of chromophore efficiencies in the pigment-protein complex.
A spectroscopic study of wavelength dependent photoisomerizations of bilirubins in aqueous solution bound to HSA
PERSICO, MAURIZIO;
2005-01-01
Abstract
The wavelength dependent configurational photoprocesses in the asymmetric bilirubin BR-IX alpha (BR-IX) and symmetrically-substituted bilirubin-III alpha (BR-III) and mesobilirubin-XIII alpha (MBR-XIII) in aqueous solution bound to human serum albumin (HSA) was analysed with the aid of absorption spectra and exciton bands deduced from circular dichroism spectra (bisignate Cotton effect). Assigned excited states by TD-DFT of the free molecule validates the exciton coupling model for the description of the interaction between the main bands of BR-IX in the blue region of the spectrum, and the presence of other two bands of charge transfer character at longer wavelengths. To fit the photoisomerisation quantum yields as a function of wavelength in the blue-green region, we took into account the absolute values of the two CD excitons deduced by fitting experimental spectra, and the “difference” bands from absorption spectrum and the sum, normalised to the absorption spectrum, of the two CD excitons. These “difference” bands contain the convolution of the two charge transfer bands, modified like the CD ones by the complexation with HSA. We fitted also the photoisomerisation quantum yields of lumirubin, the irreversible photoisomer of BR-IX in HSA. For this photoisomer like for the other reversible ones, we found reliable values of chromophore efficiencies in the pigment-protein complex.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
