Conjugation-length dependence of the T1 lifetimes of carotenoids free in solution and incorporated into the LH2, LH1, RC, and RC-LH1 complexes: possible mechanisms of triplet-energy dissipation

In addition to the roles of antioxidant and spacer, carotenoids (Cars) in purple photosynthetic bacteria pursue two physiological functions, i.e., light harvesting and photoprotection. To reveal the mechanisms of the photoprotective function, i.e., quenching triplet bacteriochlorophyll to prevent the sensitized generation of singlet oxygen, the triplet absorption spectra were recorded for Cars, where the number of conjugated double bonds (n) is in the region of 9-13, to determine the dependence on n of the triplet lifetime. The Cars examined include those in (a) solution; (b) the reconstituted LH1 complexes; (c) the native LH2 complexes from Rba. sphaeroides G1C, Rba. sphaeroides 2.4.1, Rsp. molischianum, and Rps. acidophila 10050; (d) the RCs from Rba. sphaeroides G1C, Rba. sphaeroides 2.4.1, and Rsp. rubrum S1; and (e) the RC-LH1 complexes from Rba. sphaeroides G1C, Rba. sphaeroides 2.4.1, Rsp. molischianum, Rps. acidophila 10050, and Rsp. rubrum S1. The results lead us to propose the following mechanisms: (i) A substantial shift of the linear dependence to shorter lifetimes on going from solution to the LH2 complex was ascribed to the twisting of the Car conjugated chain. (ii) A substantial decrease in the slope of the linear dependence on going from the reconstituted LH1 to the LH1 component of the RC-LH1 complex was ascribed to the minor-component Car forming a leak channel of triplet energy. (iii) The loss of conjugation-length dependence on going from the isolated RC to the RC component of the RC-LH1 complex was ascribed to the presence of a triplet-energy reservoir consisting of bacteriochlorophylls in the RC component.