Energy Transfer from Carotenoids to Bacteriochlorophylls

Abstract

The photosynthetic apparatus contains light-harvesting (LH) pigment-protein complexes that capture light energy from the sun and transfer it efficiently to the reaction center. In photosynthetic bacteria, carotenoids supplement the non-optimal LH capacity of bacteriochlorophyll (BChl) in the 400–500 nm region of the visible spectrum. Thus, carotenoid-to-BChl energy transfer provides an essential process for enhancing the ability of these systems to capture light energy and convert it into useful work. Carotenoids have at least two states involved in energy transfer to BChl. These are the S2 state into which absorption from the ground state, S0, is strongly allowed, and a low-lying, S1 state into which absorption is forbidden by symmetry. These two states represent the primary energy donors for carotenoid-to-BChl energy transfer. The S2 state transfers energy with an efficiency between 30 and 70%, the value of which is only slightly dependent on the structure of the carotenoid. The S1-mediated energy transfer pathway depends strongly on the π-electron conjugation length of the carotenoid. This route is essentially closed for carotenoids with eleven or more conjugated carbon-carbon double bonds because in these cases the S1 energy of the carotenoid lies too low to enable transfer to BChl. Besides the main S2 and S1 pathways, the past few years of investigations have raised the prospect of other carotenoid excited states participating in energy transfer. The possibilities include vibrationally hot S1 states, a state denoted S* thought to be formed by a branched deactivation pathway from S2, and a state with symmetry representation 1B u − predicted on the basis of theoretical computations to lie between S1 and S2. This chapter reviews the evidence for these states and discusses their possible involvement as energy donors in the process of light-harvesting in photosynthetic bacteria.