Efficient light harvesting through carotenoids

We review the factors that control the efficiency of carotenoid-chlorophyll excitation transfer in photosynthetic light harvesting. For this we summarize first the recently developed theory that describes electronic couplings between carotenoids and chlorophylls and we outline in particular the influence of length of conjugated system and of symmetry breaking on the couplings. We focus hereby on the structurally solved lycopene-BChl system of LH 2 from Rhodospirillum molischianum and the peridinin-Chl a system of PCP from Amphidinium carterae. In addition, we review recent spectroscopic data for neurosporene, spheroidene and lycopene, three carotenoids with different lengths of conjugated systems. On the basis of the measured energies, emission lineshapes, solution and protein environment lifetimes for their 2A(g)(-) and 1B(u)(+) states as well as of the theoretically determined couplings, we conclude that the transfer efficiencies from the 2A(g)(-) state are controlled by the Car(2A(g)(-))-BChl(Q(g)) electronic couplings and the 2A(g)(-) --> 1A(g)(-) internal conversion rates. We suggest that symmetry breaking and geometry rather than length of conjugated system dominate couplings involving the 2A(g)(-) state. Differences in transfer efficiencies from the 1B(u)(+) state in LH 2 and PCP are found to be dominated by the differences in spectral overlap. The role of the 1B(u)(+) state is likely to be influenced by a lower-lying (in longer polyenes), optically forbidden 1B(u)(-) state.