Arellano Juan B, Melø Thor Bernt, Borrego Carles M, Naqvi K Razi
Department of Physics, Norwegian University of Science and Technology, Trondheim.
Photochem Photobiol. 2002 Oct;76(4):373-80. doi: 10.1562/0031-8655(2002)076<0373:bembna>2.0.co;2.
Sensitization of singlet delta oxygen (O2(1delta(g))) 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 triplets) have a lifetime of a few tens of microseconds; however, in air-saturated acetone, quenching of BChle triplets by ground-state oxygen (O2(3sigma(-)g)) and formation of O2(1delta(g)) take place. The O2(1delta(g)) 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(1delta(g)) and the BChle concentration. The rate constants for the quenching of O2(1delta(g)) by BChle0 and for the deactivation of O2(1delta(g)) by the solvent come out to be kq = (1.4 +/- 0.1) x 10(9) M(-1) s(-1) and k0 = (18.5 +/- 0.7) x 10(3) s(-1), respectively. The absolute quantum yield of O2(1delta(g)) 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 triplets in aggregates. Because O2(1delta(g)) sensitization by BChle triplets 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.
为了更好地理解绿色光合细菌叶绿体中运行的光保护机制，对细菌叶绿素e（BChle）敏化单线态δ氧（O2(1δ(g))）进行了研究。在BChle形成单体的介质（丙酮和含有0.5% Triton X - 100 [TX]的水溶液）以及BChle聚集的体系（即含有0.003%单半乳糖基二甘油酯（MGDG）的水溶液和来自嗜盐绿菌属菌株CL1401的叶绿体（对照以及己醇扰动的））中研究了敏化过程。在氩气吹扫的丙酮中，BChle三重态（BChle三重态）的寿命为几十微秒；然而，在空气饱和的丙酮中，基态氧（O2(3σ - g)）对BChle三重态的猝灭以及O2(1δ(g))的形成发生。如此形成的O2(1δ(g))易被基态BChle分子BChle0猝灭。Stern - Volmer分析揭示了O2(1δ(g))的衰减速率与BChle浓度之间的线性拟合。BChle0对O2(1δ(g))的猝灭速率常数以及溶剂对O2(1δ(g))的失活速率常数分别为kq = (1.4 ± 0.1)×10(9) M(-1) s(-1)和k0 = (18.5 ± 0.7)×10(3) s(-1)。在空气饱和的丙酮中，BChle单体对O2(1δ(g))敏化的绝对量子产率为0.65 ± 0.15。在水相中，与0.5% TX溶液中BChle单体的三重态寿命（几百微秒）相比，天然或己醇扰动的叶绿体中BChle聚集体的三重态寿命缩短了两个多数量级。类胡萝卜素（Car）的猝灭对聚集体中BChle三重态的衰减贡献很小。因为在MGDG聚集体或叶绿体（对照以及己醇扰动的）中均未检测到BChle三重态对O2(1δ(g))的敏化，所以得出结论：（1）当BChle以单体形式存在时此过程极有可能发生，而当它紧密堆积在人工聚集体或叶绿体中时则不然；（2）Car虽然对底板BChla至关重要，但对BChle却是可有可无的。
