Bacteriochlorophyll e monomers, but not aggregates, sensitize singlet oxygen: implications for a self-photoprotection mechanism in chlorosomes.

Abstract

Sensitization of singlet delta oxygen (O2(1Δg)) by bacteriochlorophyll e (BChle) has been investigated to gain a better understanding of the photoprotection mechanism(s) operating in chlorosomes of green photosynthetic bacteria. The sensitization process has been studied in media where BChle forms monomers (acetone and aqueous solutions containing 0.5% Triton X-100 [TX]) and in systems where BChle aggregates, namely, aqueous solutions containing 0.003% monogalactosyl diglyceride (MGDG) and chlorosomes (control as well as hexanol perturbed) from Chlorobium phaeobacteroides strain CL1401. In Ar-purged acetone, BChle triplets (BChle†) have a lifetime of a few tens of microseconds; however, in air-saturated acetone, quenching of BChle† by ground-state oxygen (O2(3Σ−g)) and formation of O2(1Δg) take place. The O2(1Δg) so formed is susceptible to quenching by BChle0, a ground-state BChle molecule. A Stern–Volmer analysis reveals a linear fit between the decay rate of O2(1Δg) and the BChle concentration. The rate constants for the quenching of O2(1Δg) by BChle0 and for the deactivation of O2(1Δg) by the solvent come out to be kq= (1.4 ± 0.1) × 10^9 M−1 s−1 and k0= (18.5 ± 0.7) × 10^3 s−1, respectively. The absolute quantum yield of O2(1Δg) sensitization by BChle monomers is 0.65 ± 0.15 in air-saturated acetone. In aqueous phase, the triplet lifetime of BChle aggregates in native or hexanol-perturbed chlorosomes shortens by more than two orders of magnitude when compared with the triplet lifetime of BChle monomers in 0.5% TX solution (a few hundreds of microseconds). Quenching by carotenoids (Car) makes only a minor contribution to the decay of BChle† in aggregates. Because O2(1Δg) sensitization by BChle† could be detected neither in MGDG aggregates nor in chlorosomes (control as well as hexanol perturbed), it is concluded that (1) this process is highly likely when BChle is present as a monomer but not when it is tightly packed in artificial aggregates or in chlorosomes; and (2) Car, though vital for the baseplate BChla, are dispensable for BChle.