Monger T G, Cogdell R J, Parson W W
Biochim Biophys Acta. 1976 Oct 13;449(1):136-53. doi: 10.1016/0005-2728(76)90013-x.
Chromatophores from photosynthetic bacteria were excited with flashes lasting approx. 15 ns. Transient optical absorbance changes not associated with the photochemical electron-transfer reactions were interpreted as reflecting the conversion of bacteriochlorophyll or carotenoids into triplet states. Triplet states of various carotenoids were detected in five strains of bacteria; triplet states of bacteriochlorophyll, in two strains that lack carotenoids. Triplet states of antenna pigments could be distinguished from those of pigments specifically associated with the photochemical reaction centers. Antenna pigments were converted into their triplet states if the photochemical apparatus was oversaturated with light, if the primary photochemical reaction was blocked by prior chemical oxidation of P-870 or reduction of the primary electron acceptor, or if the bacteria were genetically devoid of reaction centers. Only the reduction of the electron acceptor appeared to lead to the formation of triplet states in the reaction centers. In the antenna bacteriochlorophyll, triplet states probably arise from excited singlet states by intersystem crossing. The antenna carotenoid triplets probably are formed by energy transfer from triplet antenna bacteriochlorophyll. The energy transfer process has a half time of approx. 20 ns, and is about 1 X 10(3) times more rapid than the reaction of the bacteriochlorophyll triplet states with O2. This is consistent with a role of carotenoids in preventing the formation of singlet O2 in vivo. In the absence of carotenoids and O2, they decay half times of the triplet states are 70 mus for the antenna bacteriochlorophyll and 6-10 mus for the reaction center bacteriochlorophyll. The carotenoid triplets decay with half times of 2-8 mus. With eak flashes, the quantum yields of the antenna triplet states are in the order of 0.02. The quantum yields decline severely after approximately one triplet state is formed per photosynthetic unit, so that even extremely strong flashes convert only a very small fraction of the antenna pigments into triplet states. The yield of fluorescence from the antenna bacteriochlorophyll declines similarly. These observations can be explained by the proposal that single-triplet fusion causes rapid quenching of excited single states in the antenna bacteriochlorophyll.
用持续时间约15纳秒的闪光激发光合细菌的载色体。与光化学电子转移反应无关的瞬态光吸收变化被解释为反映细菌叶绿素或类胡萝卜素向三重态的转化。在五种细菌菌株中检测到了各种类胡萝卜素的三重态；在两种缺乏类胡萝卜素的菌株中检测到了细菌叶绿素的三重态。天线色素的三重态可以与那些与光化学反应中心特异性相关的色素的三重态区分开来。如果光化学装置被光过度饱和，如果初级光化学反应被P - 870的预先化学氧化或初级电子受体的还原所阻断，或者如果细菌在基因上缺乏反应中心，天线色素就会转化为它们的三重态。只有电子受体的还原似乎会导致反应中心三重态的形成。在天线细菌叶绿素中，三重态可能通过系间窜越从激发单重态产生。天线类胡萝卜素三重态可能由三重态天线细菌叶绿素的能量转移形成。能量转移过程的半衰期约为20纳秒，比细菌叶绿素三重态与O2的反应快约1×10³倍。这与类胡萝卜素在体内防止单线态O2形成的作用是一致的。在没有类胡萝卜素和O2的情况下，天线细菌叶绿素三重态的半衰期为70微秒，反应中心细菌叶绿素的半衰期为6 - 10微秒；类胡萝卜素三重态的半衰期为2 - 8微秒。用弱闪光时，天线三重态的量子产率约为0.02。在每个光合单位形成大约一个三重态后，量子产率会严重下降，因此即使是极强的闪光也只能将极少量的天线色素转化为三重态。天线细菌叶绿素的荧光产率也有类似的下降。这些观察结果可以通过单重态 - 三重态融合导致天线细菌叶绿素中激发单重态快速猝灭的提议来解释。
