The Mechanism of Triplet-Energy Dissipation by 15-cis-Spheroidene Bound to the Reaction Center from Rhodobacter sphaeroides 2.4.1 As Determined by Time-Resolved EPR Spectroscopy at Low Temperatures

Abstract

Time-resolved EPR spectra of 15-cis-spheroidene bound to the reaction center (RC) from Rhodobacter sphaeroides 2.4.1 were recorded. A 4-component analysis of the spectral data by singular-value decomposition followed by global fitting identified the triplet transformations of Car, i.e., ³Car(I) → ³Car(R) → ³Car(II) →, in which ³Car(R) functions as a leaking channel of triplet population. The sequential transformations were ascribed, by simulation of the zero-field splitting parameters using a polyene model, to the conformational changes of (0°, 0°, 0°) → (+20°, –20°, +20°) → (+45°, –40°, +40°), concerning the rotational angles around the cis C15=C15', trans C13=C14 and trans C11=C12 bonds. Inhomogeneous orientation of the Car–RC assembly was identified by spectral simulation, a fact which suggests the generation of a magnetic dipole moment during the conformational changes. Thus, the rotational motions around the central double bonds play a key role in the triplet-energy dissipation.