Abstract
Time-resolved EPR spectra of 15-cis-spheroidene bound to the reaction center from Rhodobacter sphaeroides 2.4.1 were recorded at low temperatures. (1) A three-component analysis of the spectral-data matrices by singular-value decomposition followed by global fitting identified the transformation of the triplet carotenoid, (3)Car(I) --> (3)Car(II); during this process, the leak of the triplet population was suggested. A four-component analysis suggested the presence of a representative intermediate, (3)Car(R), that forms a leak channel of the triplet population. (2) A theoretical calculation of the zero-field splitting parameters, |D| and |E|, by the use of a polyene model, showed that the transformation, (3)Car(I) --> (3)Car(R) --> (3)Car(II), accompanies the conformational changes of (0 degrees , 0 degrees , 0 degrees ) --> (+20 degrees , -20 degrees , +20 degrees ) --> (+45 degrees , -40 degrees , +40 degrees ) around the central cis C15=C15', trans C13=C14, and trans C11=C12 bonds, respectively. (3) The initial, rapid decrease followed by the inversion of spin polarization along the z axis of (3)Car was observed, which was correlated with a change in the spin angular momentum. (4) In reference to the binding pocket of the Car, determined by X-ray crystallography, the conformational changes were ascribed to the intrinsic isomerization property of 15-cis (3)Car as well as the Car-peptide intermolecular interaction; a detailed picture was proposed. All of the above results support the mechanism of triplet-energy dissipation proposed previously: the rotational motions around the central double bonds cause a change in the orbital angular momentum and, through the spin-orbit coupling, a change in the spin angular momentum, which enhances the T(1) --> S(0) intersystem crossing dissipating the triplet energy.
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