Strong carotenoid-to-peptide interaction immediately after triplet excitation triggering conformational changes in photo-reaction center-bound 15-cis-spheroidene as revealed by submicrosecond time-resolved Raman spectroscopy

Intermolecular interactions and structural changes of 15-cis-spheroidene in the binding pocket of reaction center from Rhodobacter sphaeroides 2.4.1 have been studied by submicrosecond time-resolved Raman spectroscopy at physiological temperature. Immediately after triplet excitation, strong enhancement of the 13-methyl asymmetric deformation Raman line of the carotenoid (Car) at 1458 cm−1 has been observed. The intensity enhancement can be explained in terms of the severe steric interaction between the Car 13-methyl group and the peptide CO oxygen of Gly M178 caused by the isomerization of the bound 15-cis-spheroidene toward all-trans. Subsequent conformational changes of the Car have also been observed.