Radical-pair energetics and decay mechanisms in reaction centers containing anthraquinones, naphthoquinones or benzoquinones in place of ubiquinone.
Radical-pair energetics and decay mechanisms in reaction centers containing anthraquinones, naphthoquinones or benzoquinones in place of ubiquinone.
复制标题
含有蒽醌、萘醌或苯醌代替泛醌的反应中心的自由基对能量学和衰变机制。
DOI：
10.1016/0005-2728(86)90243-4
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发表时间：
1986
期刊：
影响因子：
--
通讯作者：
Dutton,PL
中科院分区：
文献类型：
--
作者：
Woodbury,NW;Parson,WW;Gunner,MR;Prince,RC;Dutton,PL
在球形红细菌（以前称为球形红假单胞菌）的反应中心，光引起电子转移反应，从细菌叶绿素二聚体（P）的初始激发单重态（P *）形成自由基对态（P+ I-，或P F）。随后的电子转移到醌（Q）产生状态P+ Q−。反向电子转移可以从P+ Q−中再生出P，产生“延迟”荧光，其衰减寿命与P+ Q−大致相同。P+ Q−和P + Q −之间的自由能差可以从延迟荧光的初始振幅确定。在目前的工作中，我们提取的天然醌（泛醌）从RPS。sphaeroides反应中心，并取代它的各种蒽醌，萘醌和苯醌。我们发现了一个粗略的半波还原电位（E 1 2）的醌用于重建（在二甲基甲酰胺中测定极谱）和状态P+ Q−相对于P的表观自由能之间的相关性。随着醌的E1 2变得更负，P+ Q−和P之间的标准自由能隙减小。然而，这种相关性在数量上很弱。显然，醌在原位的有效中点电位（EM）微妙地依赖于与反应中心的蛋白质环境的相互作用。利用在天然反应中心测定的泛醌的Em值作为参考，以及在与其它醌类化合物重构的反应中心测定的P+ Q−的标准自由能，估算了12种不同醌类化合物的有效Em值.在天然反应中心，或在用醌重建的反应中心中，在P+ Q−和P + Q −之间的标准自由能隙大于约0.8 eV，从P+ Q−到基态（PQ）的电荷复合几乎完全通过温度不敏感的机制发生，可能是电子隧穿。当反应中心被醌类化合物重组，使P+ Q−和P + Q −之间的自由能隙小于0.8 eV时，部分或全部衰变通过热可接近的中间体进行。P+ Q−通过中间态衰变的速率常数的对数与P+ Q−的标准自由能之间存在线性关系。自由能越高，衰变越快。该中间体的动力学和热力学性质似乎并不强烈依赖于用于重构的醌，这表明该中间体可能不仅仅是P+ Q−的活化形式。以前有人提出，中间体是P F;然而，基于这一假设的动力学模型中P+ Q−的衰变动力学计算与荧光测量确定的热力学参数不一致。此外，不能检测到通常作为PF的衰变产物形成的三重态（PR）。这些观察认为，热可及的中间体是不相同的P-F状态，其特征在于以前，虽然它可能是一个强烈的放松形式的这个自由基对。
In reaction centers from Rhodobacter sphaeroides (formerly called Rhodopseudomonas sphaeroides), light causes an electron-transfer reaction that forms the radical pair state (P+ I−, or P F) from the initial excited singlet state (P∗) of a bacteriochlorophyll dimer (P). Subsequent electron transfer to a quinone (Q) produces the state P+ Q−. Back electron transfer can regenerate P∗ from P+ Q−, giving rise to ‘delayed’fluorescence that decays with approximately the same lifetime as P+ Q−. The free-energy difference between P+ Q− and P∗ can be determined from the initial amplitude of the delayed fluorescence. In the present work, we extracted the native quinone (ubiquinone) from Rps. sphaeroides reaction centers, and replaced it by various anthraquinones, naphthoquinones, and benzoquinones. We found a rough correlation between the halfwave reduction potential (E 1 2) of the quinone used for reconstitution (as measured polarographically in dimethylformamide) and the apparent free energy of the state P+ Q− relatively to P∗. As the E 1 2 of the quinone becomes more negative, the standard free-energy gap between P+ Q− and P∗ decreases. However, the correlation is quantitatively weak. Apparently, the effective midpoint potentials (E m) of the quinones in situ depend subtly on interactions with the protein environment in the reaction center. Using the value of the E m for ubiquinone determined in native reaction centers as a reference, and the standard free energies determined for P+ Q− in reaction centers reconstituted with other quinones, the effective E m values of 12 different quinones in situ are estimated. In native reaction centers, or in reaction centers reconstituted with quinones that give a standard free-energy gap of more than about 0.8 eV between P+ Q− and P∗, charge recombination from P+ Q− to the ground state (PQ) occurs almost exclusively by a temperature-insensitive mechanism, presumably electron tunneling. When reaction centers are reconstituted with quinones that give a free-energy gap between P+ Q− and P∗ of less than 0.8 eV, part or all of the decay proceeds through a thermally accessible intermediate. There is a linear relationship between the log of the rate constant for the decay of P+ Q− via the intermediate state and the standard free energy of P+ Q−. The higher the free energy, the faster the decay. The kinetic and thermodynamic properties of the intermediate appear not to depend strongly on the quinone used for reconstitution, indicating that the intermediate is probably not simply an activated form of P+ Q−. It has been suggested previously that the intermediate is P F; however, calculations of the decay kinetics of P+ Q− in kinetic models based on this assumption are inconsistent with the thermodynamic parameters determined from fluorescence measurements. In addition, the triplet state (P R) normally formed as a decay product of P F could not be detected. These observations argue that the thermally accessible intermediate is not identical with the P F state that has been characterized previously, though it could be a strongly relaxed form of this radical pair.