Changes in Molecular Structure upon Triplet Excitation of All-trans-Spheroidene in n-Hexane Solution and 15-cis-Spheroidene Bound to the Photo-Reaction Center from Rhodobacter sphaeroides As Revealed by Resonance-Raman Spectroscopy and Normal-Coordin

摘要

The S_0 and T_1 Raman spectra of all-trans-spheroidene and its deuterio derivatives (10-d, 12-d, 14-d, 15-d, 15'-d, 14'-d, and 15,15'-d_2) were recorded in n-hexane solution. The T_1 state was generated by the use of a sensitizer, anthracene. An empirical normal-coordinate analysis of the spectral data was performed by using Urey-Bradley-Shimanouchi force field; non-UBS cross terms were also introduced. By the use of each carbon-carbon stretching force constant as a scale of bond order, large changes in bond order upon triplet excitation were identified in the central part of the conjugated chain: decrease in bond order was in the order, C13=C14 > C11=C12 > C9=C10 > C15=15', whereas increase in bond order was in the order, C12-C13 > C14-C15 ≈ C14'-C15'. In other words, the triplet-excited region where the largest changes in bond order take place was located, not at the center of the entire carbon skeleton, but at the center of the conjugated chain. The S_0 and T_1 Raman spectra of 15-cis-spheroidene and its deuterio derivatives (12-d, 14-d, 15-d, 15'-d, 14'-d and 15,15'-d_2) incorporated into the photoreaction center from Rhodobacter sphaeroides R26 were also recorded. The T_1 state of spheroidene was generated by excitation of bacteriochlorophyll at the Q_x absorption and subsequent triplet-energy transfer to the carotenoid. The S_0- and T_1-state carbon-carbon stretching force constants showed changes in bond order similar to those of all-trans-spheroidene in solution. However, empirical and normal-coordinate analysis of the T_1 Raman spectra showed twistings around the C15=C15', C13=C14 and C11=C12 bonds, the values of which were temporarily estimated to be +45°, -30° and +30°, respectively. A hypothetical mechanism of triplet-energy dissipation triggered by rotational motion around those double bonds has been proposed.