Abstract
This article discusses the photoinduced charge transfer (CT) kinetics within the reaction center complex of photosystem II (PSII RC). The PSII RC exhibits a structural symmetry in its arrangement of pigments forming two prominent branches, D1 and D2. Despite this symmetry, the CT has been observed to occur exclusively in the D1 branch. The mechanism to realize such functional asymmetry is yet to be understood. To approach this matter, we applied the theoretical tight-binding model of pigment excitations and simulated CT dynamics based upon the framework of an open quantum system. This simulation used a recently developed method of computation based on the quasi-adiabatic propagator path integral. A quantum CT state is found to be dynamically active when its site energy is resonant with the exciton energies of the PSII RC, regardless of the excitonic landscape we utilized. Through our investigation, it was found that the relative displacement between the local molecular energy levels of pigments can play a crucial role in realizing this resonance and therefore greatly affects the CT asymmetry in the PSII RC. Using this mechanism phenomenologically, we demonstrate that a near 100-to-1 ratio of reduction between the pheophytins in the D1 and D2 branches can be realized at both 77 K and 300 K. Our results indicate that the chlorophyll Chl D 1 is the most active precursor of the primary charge separation in the D1 branch and that the reduction of the pheophytins can occur within pico-seconds. Additionally, a broad resonance of the active CT state implies that a large static disorder observed in the CT state originates in the fluctuations of the relative displacements between the local molecular energy levels of the pigments in the PSII RC.
Highlights
Photosystem II (PSII) is the only biological system that has the unique ability to oxidize H2 O toO2 [1]
We refer to the excited state for m 6= n as the charge transfer (CT) state and that for m = n as the Frenkel exciton (FE) state which is denoted by |m∗ i = |m+ m− i
Our results show that the activities of the CT states in the photosystem II reaction center (PSII RC) depend highly on the level shifts representing the relative displacements in the molecular energy levels of the pigments
Summary
Photosystem II (PSII) is the only biological system that has the unique ability to oxidize H2 O toO2 [1]. Photosystem II (PSII) is the only biological system that has the unique ability to oxidize H2 O to. The oxygen produced in the photosystem II reaction center (PSII RC) provides the source of oxygen in Earth’s atmosphere providing the essential basis of life. Similar to reaction centers found in purple bacteria, the D1 and D2 proteins comprise the core of the PSII RC along with a number of noncovalently associated cofactors, +. The entire structure of the PSII RC includes two closely related proteins which form one large unit dimeric in structure, resulting in a symmetrical complex. This symmetry in the PSII RC can Catalysts 2020, 10, 718; doi:10.3390/catal10060718 www.mdpi.com/journal/catalysts
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