Factors controlling the efficiency of energy transfer from carotenoids to bacteriochlorophyll in purple photosynthetic bacteria

Abstract

Efficiencies of energy transfer from carotenoids to bacteriochlorophyll in purple photosynthetic bacteria have been studied with chromatophores, isolated pigment-protein complexes, and pigment-protein complexes reconstituted with a variety of carotenoids. Based on the efficiencies of energy transfer and the chemical structure of major carotenoids, photosynthetic bacteria used in this study are classified into two groups. (1) Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodocyclus gelatinosus show relatively high efficiencies (>70%) and contain spheroidene-series carotenoids which have nine or ten conjugated C=C bonds. (2) Rhodopseudomonas palustris, Rhodospirillum rubrum, and Chromatium vinosum show relatively low efficiencies (< 50%) and contain spirilloxanthin-series carotenoids which have 11 or 13 conjugated C=C bonds. Resonance Raman studies have shown that carotenoids in the former group of bacteria take planar polyene-chain structure, while carotenoids in the latter group take relatively distorted polyene-chain structure. Studies on carotenoids incorporated into carotenoid-less and carotenoid-deficient light-harvesting bacteriochlorophyll-protein complexes have led to a more decisive conclusion that the efficiency depends on not only carotenoids species but also apoproteins specific to bacterial species. As the number of conjugated C=C bonds in incorporated carotenoids increases, the efficiency of energy transfer decreases. This result gives strong support to the view that the energy transfer occurs from a forbidden excited state of carotenoids (2 1A g). Carotenoids, when incorporated into the pigment-protein complex from Ch. vinosum, are more distorted and show lower efficiencies than when they are incorporated into the pigment-protein complex from Rb. sphaeroides. Thus, the energy-transfer efficiency is also associated with apoproteins which determine the molecular structure of the bound carotenoids and the interaction between carotenoids and bacteriochlorophyll.