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 C = O 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. (C) 2008 Elsevier B.V. All rights reserved.