Abstract:

The mechanism of the proton transfer coupled electron transfer (PT-ET) reactions between the menaquinone QA (MQ1) and ubiquinone QB (UQ1) in the bacterial photosynthetic reaction center of Rhodopseudomona viridis was studied by using the B3LYP/6-31G(d) method. The changes of standard Gibbs free energy ΔG0 of all possible reactions followed the ET reaction (1) were calculated. The results indicated that: (1) according to the ΔG0 values of corresponding reactions, UQ1 could not accept two electrons from MQ-1 continually without the coupled proton transfer reactions. Because of ΔG0 2b0, ΔG0 3b0 and ΔG04b0, the corresponding PT ET reactions could take place along with reactions (2b), (3b) and (4b) equentially; (2) on the gaseous condition, the first and second transferred protons (H+(1) and H+(2)) from the surrounding amino acid residues or water molecules will combine with the oxygen No.7 and oxygen No.8 of UQ1, respectively. On the condition of protein surroundings (by SCRF model,ε=4.0), the results are converse but the energy difference between the combination of H+(1) and H+(2) with UQ-1 is quite small. The difference of ΔG0 values between the corresponding reactions in gaseous surroundings and the SCRF model is not significant; (3) the PT ET reactions between MQ1-and UQ1-should be as follows: MQ1-+UQ1→MQ1+UQ1- (1) UQ1-(O(7)+H+(HisL190) →UQ1H (2b)(Gas) or UQ1－(O(8))+H+(H2O)→UQ1H (2b') (SCRF) or UQ1－(O(8))+H+ (ArgL217)→UQ1H (2b')(SCRF) MQ－+UQ1H→MQ1+UQ1H- (3b)(Gas) MQ1－+UQ1H→Q1+UQ1H- (3b') (SCRF) UQ1H-+H+(H2O)→UQ1H2 (4b)(Gas) or UQ1H－+H+ (ArgL217)→UQ1H2 (4b) (Gas) or UQ1H－+H+ (HisL190)→UQ1H2 (4b') (SCRF)

Rhodopseudomena viridis 细菌光合作用反应中心质体醌Q_A与泛醌Q_B间电子转移耦合质子转移机理的理论研究
马淑华 徐 红 张汝波 屈正旺 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结构国家重点实验室，北京!100080)

摘要：用量子化学B3LYP/ 6 - 31G(d)方法 ,对Rhodopseudomenaviridis细菌光合反应中心质体醌Q_A(MQ₁)与泛醌Q_B(UQ₁)间的电子转移耦合质子转移 (PT－ET)机理进行了研究。对反应 (1)之后的Gibbs自由能变化进行了计算。结果表明 ,(1)按照各反应的数值 ,在无耦合质子转移的情况下 ,UQ₁不可能由MQ_1^－连续接受两个电子。由于ΔG⁰₂b 0 ,ΔG03b 0和ΔG04b 0 ,相应的PT ET反应将依序沿 (2b)、(3b)及 (4b)进行。 (2 )在气相情况下 ,由于周围的氨基酸残基或水分子转移到UQ₁的第一个及第二个质子 (H+ (1)和H+ (2 ) )将先后分别与UQ₁的No.7和No .8氧原子结合 ;在蛋白环境情况下 (SCRF方法 ,ε =4 .0 ) ,质子耦合的顺序正相反 ,但H+ (1)和H+ (2 )与UQ-1结合的能量传递差很小。在气相及SCRF模拟环境中 ,相同反应间的ΔG0 无显著差别。 (3)MQ-1间UQ-1的PT ET反应如下 :　　MQ1-+UQ1→MQ1+UQ1-(1)　　UQ1-(O( 7) ) +H+ (HisL190 )→UQ1H (2b) (Gas)or　UQ1-(O( 8) ) +H+ (H2 O)→UQ1H (2b’) (SCRF)or　UQ1-(O( 8) ) +H+ (ArgL2 17)→UQ1H (2b’) (SCRF)　　MQ1-+UQ1H→MQ1+UQ1H-(3b) (Gas)　　MQ1-+UQ1H→MQ1+UQ1H-(3b’) (SCRF)　　UQ1H-+H+ (H2 O)→UQ1H2 (4b) (Gas)or　UQ1H-+H+ (ArgL2 17)→UQ1H2 (4b ) (Gas)or　UQ1H-+H+ (HisL 190) →UQ1H2(4b ’)(SCRF)
关键词： Rhodopseudomena viridis ；电子转移；质子转移；QA和QB；DFT

通讯作者。Fax:+86 (0) 10 62588930;E-mail: zhangqy @ infoc3.icas.ac.cn.

Key words: Rhodopseudomena viridis, electron transfer, proton transfer, QA and QB;