Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA.
J Phys Chem B. 2010 Nov 25;114(46):15269-77. doi: 10.1021/jp106451q. Epub 2010 Oct 25.
The nature of the intermolecular vibrational modes between the redox-active chromophores and the protein medium in the photosynthetic reaction center is central to an understanding of the structural origin of the quantum efficiency of the light-driven charge-separation reactions that result in storage of solar energy. In recent work on this issue, we have characterized the low-frequency vibrational coherence of Zn(II) meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP) and compared it to that from bacteriochlorophyll a in polar solution and in the small light-harvesting subunits B820 and B777. The charge-transfer character of ZnTMPyP's π* excited states afford us the opportunity to characterize how the intermolecular vibrational modes and potential with the surrounding medium are affected by the charge on the porphyrin macrocycle. The excited-state vibrational coherence observed with Q-band (S(1) state) excitation at 625 nm of ZnTMPyP in methanol solution contains dominant contributions from a pair of rapidly damped (effective damping time γ < 400 fs) components that are assigned to the hindered translational and librational porphyrin-solvent intermolecular modes. The 256 cm(-1) mean frequency of the intermolecular modes is significantly higher than that observed previously in the ground state, 79 cm(-1), with Soret-band excitation at 420 nm [Dillman et al., J. Phys. Chem. B. 2009, 113, 6127-6139]. The increased mode frequency arises from the activation of the ion-dipole and ion-induced-dipole terms in the intermolecular potential. In the ground state, the π-electron density of ZnTMPyP is mostly confined to the region of the porphyrin macrocycle. In the excited state, the π-electron density is extensively delocalized from the porphyrin out to two of the peripheral N-methylpyridyl rings, each of which carries a single formal charge. The charge-dependent terms contribute to a significant stabilization of the equilibrium geometry of the porphyrin-solvent complex in the excited state. In the photosynthetic reaction center, these terms will play an important role in trapping the charged products of the forward, charge-separation reactions, and the location of the bacteriopheophytin acceptor in a nonpolar region of the structure enhances the rate of the secondary charge-separation reaction.
在光合作用反应中心中，氧化还原活性发色团与蛋白质介质之间的分子间振动模式的性质对于理解导致太阳能存储的光驱动电荷分离反应的量子效率的结构起源至关重要。在这个问题的最近工作中，我们已经对 Zn(II)meso-四（N-甲基吡啶基）卟啉（ZnTMPyP）的低频振动相干性进行了表征，并将其与极性溶液中的细菌叶绿素 a 以及小的光收集亚基 B820 和 B777 进行了比较。ZnTMPyP 的π*激发态的电荷转移特性使我们有机会描述分子间振动模式和与周围介质的势能如何受到卟啉大环电荷的影响。在甲醇溶液中用 Q 带（S(1)态）激发（625nm）观察到的激发态振动相干性包含一对快速衰减（有效阻尼时间γ<400fs）的主要贡献，这些贡献归因于受阻的平移和扭转卟啉-溶剂分子间模式。分子间模式的 256cm(-1)平均频率明显高于以前在基态观察到的频率，为 79cm(-1)，在 Soret 带激发（420nm）时[Dillman 等人，J.Phys.Chem.B.2009,113,6127-6139]。模式频率的增加来自于分子间势中的离子偶极和离子诱导偶极项的激活。在基态下，ZnTMPyP 的π 电子密度主要局限于卟啉大环的区域。在激发态下，π 电子密度从卟啉扩展到两个外围的 N-甲基吡啶环，每个环带有一个单形式电荷。电荷相关项对激发态卟啉-溶剂配合物平衡几何结构的显著稳定化有贡献。在光合作用反应中心，这些项将在捕获正向电荷分离反应的带电产物以及细菌叶绿素 a 受体在结构的非极性区域中的位置方面发挥重要作用，从而提高二级电荷分离反应的速率。
