Electron-spin resonance studies of carotenoids incorporated into reaction centers of Rhodobacter sphaeroides R26.1

Abstract

The carotenoids, hydroxyneurosporene, methoxyneurosporene, spheroidene, hydroxyspheroidene, spheroidenone, 3,4-dihydroanhydrorhodovibrin, and spirilloxanthin have been incorporated into reaction centers of the photosynthetic bacterium Rhodobacter sphaeroides (Rhodopseudomonas sphaeroides) R26.1 in a carotenoid-to-primary donor ratio of 1:1. These carotenoids have different functional groups and varying extents of π electron conjugation. The importance of these structural features on binding to the reaction center and quenching the primary donor triplet state was assessed. The carotenoid binding site on reaction centers of Rb. sphaeroides R26.1 exhibits a preference to binding carotenoids which contain polar functional groups. Electron-spin resonance spectroscopy was used to detect carotenoid triplet state formation. The zero-field splitting parameters of the reconstituted carotenoids revealed that carotenoids isomerize upon binding to the reaction center. The structure of the reaction center-bound carotenoids is one where the π electron conjugation is interrupted.