The electronic structure of protein chains L and M in photosynthetic reaction center (PRC) of Rhodobacter sphaeroides (Van Niel) Imhoff, Truper et Pfennig) was studied by using the Overlapping Dimer Approximation method and the Extended Negative Factor Counter method at ab initio level. The result indicated that: (1) Amino acid residues, the molecular orbitals of which composed the main components of frontier orbitals of protein chain L (M ), are located at the random coil areas of chain L (α helix areas of chain M ). Since the random coil is flexible and more easy to change its conformation in the electron transfer process and to reduce the energy of the system, and the structure of the α helix is reletively stable, this difference might be one of the causes for the electron transfer in photosynthetic reaction center (PRC) only takes place along the L branch. (2) The His residues which axially coordinated to the “special pair” P and accessory chlorophyll molecules （ABChls） are essentially important for the ELUMO levels of P and ABChl. But, the corresponding molecular orbitals of these His residues do not appear in the composition of frontier orbitals of protein chains. It means that the interaction between pigment molecules and protein chains do not influence the contribution to the frontier orbitals of protein chains explicitly, but influences the corresponding ELUMO levels significantly.
球形红假单胞菌反应中心中蛋白的量子化学研究
徐红 马淑华 沈玲玲* 张晓东 张兴康 张启元**
（中国科学院化学研究所分子科学中心分子动态与稳态国家重点实验室，北京100080）
摘要：采用重叠二体近似方法和建立在从头算水平上的扩展负本征值因数计算方法 (extendednegativefactorcountermethod)研究了球形红假单胞菌 (Rhodobactersphaeroides (VanNiel)Imhoff,TruperetPfennig)中光合反应中心中蛋白链L及M的电子结构。结果表明 :(1)对组成蛋白链L(M)的前线轨道有重要贡献的氨基酸残基分布在L链的自由螺旋区 (M链的α螺旋区 )。由于自由螺旋是有柔曲性的 ,它易于在电子转移的过程中改变其构象并降低体系的能量 ,而α螺旋结构却相对稳定。这种差别有可能是光合反应中心中电子转移只沿L支进行的原因之一。(2 )与特殊对分子及辅助叶绿素分子形成轴向配位的组氨酸残基对于特殊对P和辅助叶绿素分子的ELUMO有重要影响 ,但此组氨酸的相应分子轨道的贡献并没有出现在蛋白链的前线轨道组成中。这意味着色素分子与蛋白链之间的相互作用对蛋白链前线轨道的贡献没有影响 ,但却能影响相应色素分子的ELUMO能级。
关键词： 光合反应中心；球形红假单胞菌；蛋白的电子结构；从头算；扩展负本征值因数计算方法
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