Resonance Raman analysis of astaxanthin–protein complexes

A survey of resonance Raman spectra of carotenoproteins, including astaxanthin, is presented. The data are discussed and compared with absorption measurements in order to establish the mode of binding. Three mechanisms seem to be dominant in explaining the changes which occur in the resonance Raman and absorption spectra of astaxanthin. The polarization of the π‐electron system is responsible for the large red shift in the absorption λ max and the decrease in the CC stretching wavenumber. A distortion of the backbone plane involving double bonds can produce an increase of the CC stretching wavenumber and a red shift of the absorption band. Effects of the medium on excited states can produce red or blue shifts of the absorption band without perturbation of the ground electronic state properties. The interference of these mechanisms can explain the observed changes in the resonance Raman spectrum of astaxanthin bound to proteins and the great diversity of the results.