Carotenoid Radical Cation Formation in LH2 of Purple Bacteria: A Quantum Chemical Study

Abstract

In LH2 complexes of Rhodobacter sphaeroides the formation of a carotenoid radical cation has recently been observed upon photoexcitation of the carotenoid S₂ state. To shed more light onto the yet unknown molecular mechanism leading to carotenoid radical formation in LH2, the interactions between carotenoid and bacteriochlorophyll in LH2 are investigated by means of quantum chemical calculations for three different carotenoidsneurosporene, spheroidene, and spheroidenoneusing time-dependent density functional theory. Crossings of the calculated potential energy curve of the electron transfer state with the bacteriochlorophyll Q_x state and the carotenoid S₁ and S₂ states occur along an intermolecular distance coordinate for neurosporene and spheroidene, but for spheroidenone no crossing of the electron transfer state with the carotenoid S₁ state could be found. By comparison with recent experiments where no formation of a spheroidenone radical cation has been observed, a molecular mechanism for carotenoid radical cation formation is proposed in which it is formed via a vibrationally excited carotenoid S₁ or S* state. Arguments are given why the formation of the carotenoid radical cation does not proceed via the Q_x, S₂, or higher excited electron transfer states.