Changes in the Structure of Reaction Center-Bound 15-cis-Spheroidene in the T₁ State and Its Intermolecular Interaction with the Binding Pocket as Revealed by Submicrosecond Time-Resolved Raman Spectroscopy at Room Temperature

Abstract

   Changes in the structure of 15-cis-spheroidene in the T₁ state and intermolecular interaction with peptide binding pocket in the reaction center have been examined by time-resolved Raman spectroscopy at room temperature as a follow-up study of time-resolved EPR spectroscopy [Kakitani et al. Biochemistry 45 (2006) 2053].
   Immediately after triplet excitation, a strong enhancement of the 13-methyl asymmetric-deformation Raman line was seen at 1458 cm^–1, which was ascribed to severe steric interaction between the Car 13-methyl carbon and the peptide C=O oxygen of Gly M178. This steric interaction seems to perturb the initial rotational motion around the cis C15=C15' bond, and to trigger the subsequent rotational motion around the neighboring trans C13=C14 bond. Careful consideration lead us to the idea that the dynamic mechanism through the rotational motion around the central double bond(s) plays the most important role, rather than the static mechanism through the spin–orbit coupling, in the triplet-energy dissipation.