Raman and Infrared Spectra for All-trans-astaxanthin in Dimethyl Sulfoxide Solvent

The Raman and infrared spectra of all- trans -astaxanthin (AXT) in dimethyl sulfoxide (DMSO) solvent were investigated experimentally and theoretically. Density functional calculations of the Raman spectra predict the splitting of the ν 1 band into ν 1-1 and ν 1-2 components. The absence of splitting in Raman experimental spectra is ascribed to the competition between the two symmetric C=C stretching vibrations of the backbone chain. The ν 1 band is very sensitive to the excitation wavelength: resonance excitation stimulates the higher-frequency ν 1-2 mode, and off-resonance excitation corresponds to the lower-frequency ν 1-1 mode. Analyses of the intramolecular hydrogen bonding between C=O and O−H in the AXT/DMSO system reveal that the C4=O1⋯ H1−O3 and C4′=O2⋯ H2−O4 bonds are strengthened and weakened, respectively, in the electronically excited state compared with those in the ground state. This result reveals significant variations of the AXT molecular structure in different electronic states.