Alternative Fast and Slow Primary Charge‐Separation Pathways in Photosystem II

Abstract

Photosystem-II (PSII) is a multi-subunit protein complex that harvests sunlight to perform oxygenic photosynthesis. Initial light-activated charge separation takes place at a reaction centre consisting of four chlorophylls and two pheophytins. Understanding the processes following light excitation remains elusive due to spectral congestion, the ultrafast nature, and multicomponent behaviour of the charge-separation process.Here, using advanced computational multiscale approaches which take into account the large-scale configurational flexibility of the system, we identify two possible primary pathways to radical-pair formation that differ by three orders of magnitude in their kinetics. The fast (short-range) pathway is dominant, but the existence of an alternative slow (long-range) charge-separation pathway hints at the evolution of redundancy that may serve other purposes, adaptive or protective, related to formation of the unique oxidative species that drives water oxidation in PSII.

Unravelling Mn4Ca cluster vibrations in the S1, S2 and S3 states of the Kok–Joliot cycle of photosystem II

“…These four consecutive oxidation events, occurring along five steps of the so-called Kok-Joliot cycle (namely, S 0 -S 4 ), 5 result in conformational and redox changes of the cluster and the neighbouring residues. Both theoretical [6][7][8][9][10][11][12] and experimental [13][14][15][16][17][18] studies have been conducted to characterize the structures of the cluster and the neighboring residues in the different steps of the catalytic cycle. In the last decades, crucial geometrical information has come from extended X-ray absorption fine structure, 19,20 and in the last years from X-ray crystallography [21][22][23] that has been able to explore states different from the dark-adapted S 1 .…”

Evidence of a Distinctive Enantioselective Binding Mode for the Photoinduced Radical Cyclization of α-Chloroamides in Ene-Reductases

“…To this aim, we employ a multiscale methodology based on quantum-mechanical (QM) calculations, molecular dynamics (MD) simulation, and the perturbed matrix method (PMM) . The combined MD-QM-PMM approach has proven its effectiveness in various applications, such as computing redox potentials, − simulating photoinduced charge transfer processes, − and time-resolved spectroscopic observable . By means of this approach, the dynamical effects of the complex protein environment can be taken into account in the calculation of the free energy variations associated with the cyclization process.…”

Stabilization of Charge-Transfer Excited States in Biological Systems: A Computational Focus on the Special Pair in Photosystem II Reaction Centers

“…13 Moreover, atomistic first-principles and semiempirical electronic structure calculations have contributed to identifying the nature of exciton transfer of LH complexes in PSII, 14−16 the emergence of nonconservative circular dichroism in the RC by calculating model parameters of exciton and charge-transfer coupling, 17,18 stimulating nonradiative processes after photoexcitation with nonadiabatic dynamics, 19,20 the origin of low-lying excitations in PSI 21 and PSII, 22 and the nature of charge-transfer states in the RC of PSII. 23,24 The RC of the cyanobacterium Thermosynechococcus vulcanus, on which we focus in the present study, contains six pigments, i.e., two Chl molecules denoted as the special pair P D1 −P D2 as well as two Chl−pheophytin (Pheo) pairs termed Chl D1 −Pheo D1 and Chl D2 −Pheo D2 adjacent to the special pair as illustrated in Figure 1. 6 In the case of oxygenic photosynthesis, the actual mechanism of charge separation, i.e., the precise pigments involved in converting the exciton into separated charges, is still a topic of debate.…”