Bridging the gap between structural and lattice models: a parameterization of energy transfer and trapping in Photosystem I.

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.