Gobets Bas, Valkunas Leonas, van Grondelle Rienk
Division of Physics and Astronomy of the Faculty of Exact Sciences and Institute of Molecular Biological Sciences, Vrije Universiteit, Amsterdam, The Netherlands.
Biophys J. 2003 Dec;85(6):3872-82. doi: 10.1016/S0006-3495(03)74802-4.
In the absence of an accurate structural model, the excited state dynamics of energy-transferring systems are often modeled using lattice models. To demonstrate the validity and other potential merits of such an approach we present the results of the modeling of the energy transfer and trapping in Photosystem I based upon the 2.5 A structural model, and show that these results can be reproduced in terms of a lattice model with only a few parameters. It has recently been shown that at room temperature the dynamics of a hypothetical Photosystem I particle, not containing any red chlorophylls (chls), are characterized by a longest (trapping) lifetime of 18 ps. The structure-based modeling of the dynamics of this particle yields an almost linear relationship between the possible values of the intrinsic charge-separation time at P700, 1/gamma, and the average single-site lifetime in the antenna, tauss. Lattice-based modeling, using the approach of a perturbed two-level model, reproduces this linear relation between tauss and 1/gamma. Moreover, this approach results in a value of the (modified) structure-function corresponding to a structure exhibiting a mixture of the characteristics of both a square and a cubic lattice, consistent with the structural model. These findings demonstrate that the lattice model describes the dynamics of the system appropriately. In the lattice model, the total trapping time is the sum of the delivery time to the reaction center and the time needed to quench the excitation after delivery. For the literature value of tauss=150 fs, both these times contribute almost equally to the total trapping time of 18 ps, indicating that the system is neither transfer- nor trap-limited. The value of approximately 9 ps for the delivery time is basically equal to the excitation-transfer time from the bulk chls to the red chls in Synechococcus elongatus, indicating that energy transfer from the bulk to the reaction center and to the red chls are competing processes. These results are consistent with low-temperature time-resolved and steady-state fluorescence measurements. We conclude that lattice models can be used to describe the global energy-transfer properties in complex chromophore networks, with the advantage that such models deal with only a few global, intuitive parameters rather than the many microscopic parameters obtained in structure-based modeling.
在缺乏精确结构模型的情况下，能量转移系统的激发态动力学通常使用晶格模型进行建模。为了证明这种方法的有效性和其他潜在优点，我们展示了基于2.5埃结构模型对光系统I中的能量转移和俘获进行建模的结果，并表明这些结果可以用仅包含几个参数的晶格模型来重现。最近的研究表明，在室温下，一个假设的不包含任何红色叶绿素（chl）的光系统I颗粒的动力学特征是最长（俘获）寿命为18皮秒。对该颗粒动力学的基于结构的建模得出，P700处的本征电荷分离时间的可能值1/γ与天线中平均单位点寿命tauss之间几乎呈线性关系。使用微扰双能级模型方法的基于晶格的建模重现了tauss和1/γ之间的这种线性关系。此外，这种方法得出的（修正）结构函数值对应于一种呈现正方形和立方晶格特征混合的结构，这与结构模型一致。这些发现表明晶格模型能够恰当地描述系统的动力学。在晶格模型中，总俘获时间是到达反应中心的传递时间与传递后淬灭激发所需时间之和。对于文献中tauss = 150飞秒的值，这两个时间对18皮秒的总俘获时间贡献几乎相等，表明该系统既不是转移限制也不是俘获限制。传递时间约为9皮秒，这基本上等于聚球藻中从大量chl到红色chl的激发转移时间，表明从大量到反应中心以及到红色chl的能量转移是相互竞争的过程。这些结果与低温时间分辨和稳态荧光测量结果一致。我们得出结论，晶格模型可用于描述复杂发色团网络中的全局能量转移特性，其优点是此类模型仅处理几个全局、直观的参数，而不是基于结构的建模中获得的许多微观参数。
