通过傅里叶变换红外光谱研究嗜热绿菌假定的同二聚体反应中心中P840自由基阳离子的电子和振动结构。

Electronic and vibrational structure of the radical cation of P840 in the putative homodimeric reaction center from Chlorobium tepidum as studied by FTIR spectroscopy.

作者信息

Noguchi T, Kusumoto N, Inoue Y, Sakurai H

机构信息

Photosynthesis Research Laboratory, Institute of Physical and Chemical Research (RIKEN), Saitama, Japan.

出版信息

Biochemistry. 1996 Dec 3;35(48):15428-35. doi: 10.1021/bi9613638.

DOI:10.1021/bi9613638

PMID:8952495

Abstract

Light-induced FTIR difference spectra of P840 upon its oxidation (P840+/P840) have been measured with the reaction center complex from the green sulfur bacterium Chlorobium tepidum. A broad band centered near 2500 cm-1 was observed in P840+, which is comparable to the band near 2600 cm-1 previously observed in P870+ of purple bacteria and assigned to the electronic transition in the bacteriochlorophyll a (BChla) dimer (Breton et al. (1992) Biochemistry 31, 7503-7510]. The intensity of this electronic band found in P840+ was about the same as that in P870+. The P840+ spectrum also showed several intensified vibrational modes, which are characteristic of the P870+ spectrum as well. These similar features of the electronic transition and the intensified lines indicate that P840+ is a BChla dimer whose electronic structure is similar to P870+. Based on the previous theoretical works, the possibility that P840+ has an asymmetric structure as P870+ was suggested. Also, two strong positive bands at 1707 and 1694 cm-1 probably assigned to the keto C9 = O stretching modes of P840+ were observed in the P840+/P840 spectrum. Three different interpretations are possible for the presence of the two C9 = O bands: (i) P840+ is an asymmetric dimer cation. (ii) P840+ has a symmetric structure, and the time constant of positive charge exchange between the two BChla molecules coincides with that of IR spectroscopy (10-13 s). (iii) The electric field produced by the positive charge on P840+ affects the C9 = O frequency of the neutral BChla in P840+ itself (when the charge exchange time is slower than the time scale of 10-13 s) or of a BChla in the close proximity of P840+. The negative bands at 1734 and 1684 cm-1 were assigned to the ester C10 = O and the keto C9 = O of neutral P840, respectively, both of which are free from hydrogen bonding. These results and interpretations regarding the structural symmetry and the molecular interactions of P840 and P840+ are discussed in the framework of the "homodimeric" reaction center of green sulfur bacteria.

摘要

利用嗜热绿硫菌的反应中心复合物，测量了P840氧化时（P840⁺/P840）的光诱导傅里叶变换红外差谱。在P840⁺中观察到一个以2500 cm⁻¹附近为中心的宽带，这与先前在紫色细菌的P870⁺中观察到的2600 cm⁻¹附近的带相当，并归因于细菌叶绿素a（BChla）二聚体中的电子跃迁（布雷顿等人，《生物化学》，1992年，第31卷，7503 - 7510页）。在P840⁺中发现的这个电子带的强度与P870⁺中的大致相同。P840⁺光谱还显示出几种增强的振动模式，这些也是P870⁺光谱的特征。电子跃迁和增强谱线的这些相似特征表明，P840⁺是一个电子结构与P870⁺相似的BChla二聚体。基于先前的理论研究，有人提出P840⁺可能具有与P870⁺一样的不对称结构。此外，在P840⁺/P840光谱中观察到两个分别位于1707和1694 cm⁻¹的强正带，可能归因于P840⁺的酮基C9 = O伸缩模式。对于这两个C9 = O带的存在有三种不同的解释：（i）P840⁺是一个不对称二聚体阳离子。（ii）P840⁺具有对称结构，并且两个BChla分子之间的正电荷交换时间常数与红外光谱的时间常数一致（10⁻¹³ s）。（iii）P840⁺上的正电荷产生的电场影响P840⁺自身（当电荷交换时间慢于10⁻¹³ s的时间尺度时）或P840⁺附近的BChla的中性BChla的C9 = O频率。1734和1684 cm⁻¹处的负带分别归因于中性P840的酯基C10 = O和酮基C9 = O，它们都不存在氢键。关于P840和P840⁺的结构对称性和分子相互作用的这些结果和解释，在嗜热绿硫菌的“同二聚体”反应中心框架内进行了讨论。