Heller B A, Holten D, Kirmaier C
Department of Chemistry, Washington University, St. Louis, Missouri 63130, USA.
Biochemistry. 1996 Dec 3;35(48):15418-27. doi: 10.1021/bi961362f.
The primary photochemistry in Rhodobacter capsulatus reaction centers (RCs) containing the Phe to Asp mutation at L polypeptide residue 121 near the photoactive bacteriopheophytin (BPhL) is characterized using ultrafast transient absorption spectroscopy. At 285 K, initial charge separation from P* proceeds with essentially unity quantum yield in approximately 6 ps to form a transient denoted P+I-. This transient is proposed to involve P+BPhL- and probably P+BChlL- as well (BChlL is the L-side bacteriochlorophyll molecule). P+I- decays in approximately 150 ps both by electron transfer to give P+QA- (approximately 78% yield) and by charge recombination to the ground state (approximately 22% yield). These results indicate that the F(L121)D mutant is closely related, in terms of its electron transfer properties, to previously reported RCs in which BPhL is replaced with a bacteriochlorophyll (beta-type RCs) or a pheophytin. However, the native BPhL pigment is retained in the F(L121)D mutant. We propose that the Asp at L121 raises the free energy of P+BPhL-, thereby giving rise to the altered photochemistry. At 77 K, the P+I- lifetime is shortened slightly to approximately 120 ps and the yield of P+QA- is increased to approximately 88%. This result is somewhat different from that obtained for beta-type RCs at low temperature, where the P+I- lifetime lengthens and the yield of P+QA- diminishes or stays about the same compared to the values near room temperature. We exploit these differences in developing a model for the charge separation process in the F(L121)D mutant. The effects of introducing an Asp near BPhL are compared to those obtained previously in two mutants in which an Asp is introduced near BChlL.
利用超快瞬态吸收光谱对红假单胞菌反应中心（RCs）进行了表征，该反应中心在靠近光活性细菌叶绿素（BPhL）的L多肽残基121处含有苯丙氨酸到天冬氨酸的突变。在285K时，从P*开始的初始电荷分离以接近1的量子产率在大约6皮秒内进行，形成一个瞬态，记为P+I-。该瞬态被认为涉及P+BPhL-，可能还涉及P+BChlL-（BChlL是L侧细菌叶绿素分子）。P+I-在大约150皮秒内通过电子转移产生P+QA-（产率约78%）和通过电荷复合回到基态（产率约22%）而衰减。这些结果表明，就其电子转移性质而言，F(L121)D突变体与先前报道的用细菌叶绿素（β型RCs）或叶绿素卟啉取代BPhL的RCs密切相关。然而，F(L121)D突变体中保留了天然的BPhL色素。我们认为L121处的天冬氨酸提高了P+BPhL-的自由能，从而导致光化学变化。在77K时，P+I-寿命略有缩短至约120皮秒，P+QA-的产率增加至约88%。这一结果与低温下β型RCs的结果有所不同，在低温下，P+I-寿命延长，P+QA-的产率与室温附近的值相比降低或基本不变。我们利用这些差异建立了F(L121)D突变体中电荷分离过程的模型。将在BPhL附近引入天冬氨酸的效果与之前在两个在BChlL附近引入天冬氨酸的突变体中获得的效果进行了比较。
