QM/MM computational studies of substrate water binding to the oxygen-evolving centre of photosystem II

Abstract

This paper reports computational studies of substrate water binding to the oxygen-evolving centre (OEC) of photosystem II (PSII ), completely ligated by amino acid residues, water, hydroxide and chloride. The calculations are based on quantum mechanics/molecular mechanics hybrid models of the OEC of PSII, recently developed in conjunction with the X-ray crystal structure of PSII from the cyanobacterium Thermosynechococcus elongatus. The model OEC involves a cuboidal Mn 3 CaO 4 Mn metal cluster with three closely associated manganese ions linked to a single m 4 -oxo-ligated Mn ion, often called the 'dangling manganese'. Two water molecules bound to calcium and the dangling manganese are postulated to be substrate molecules, responsible for dioxygen formation. It is found that the energy barriers for the Mn(4)-bound water agree nicely with those of model complexes. However, the barriers for Ca-bound waters are substantially larger. Water binding is not simply correlated to the formal oxidation states of the metal centres but rather to their corresponding electrostatic potential atomic charges as modulated by charge-transfer interactions. The calculations of structural rearrangements during water exchange provide support for the experimental finding that the exchange rates with bulk 18 O-labelled water should be smaller for water molecules coordinated to calcium than for water molecules attached to the dangling manganese. The models also predict that the S 1 /S 2 transition should produce opposite effects on the two waterexchange rates.

Substrate–water exchange in photosystem II is arrested before dioxygen formation

“…2b)2632333435 or coupling of a Ca-bound oxyl radical with a high valent Mn-oxo group (Fig. 2c)14 may take place, if conditions are present in PSII that slow down the exchange of the Ca-bound substrate water seven to eight orders of magnitude over the exchange rates reported for water ligated to Ca 2+ in aqueous solutions37394058. Alternatively, both substrate –waters may be Mn-ligated and form the O–O bond via radical coupling (Fig.…”

Substrate water binding and oxidation in photosystem II

“…In addition to the channels, QM/MM modelling of water binding to the OEC makes it appear that the environment containing the Mn 4 Ca cluster has evolved to promote water-metal centre interaction and reduce substrate water's involvement with the Mn 4 Ca cluster's immediate environment (Sproviero et al 2008).…”

First Principle Modelling of Materials and Processes in Dye-Sensitized Photoanodes for Solar Energy and Solar Fuels

“…Most of the published computational modeling works so far are, indeed, focused on the characterization of the water From a computational point of view, modern first principles computational methodologies, essentially relying on density functional theory (DFT) and its time dependent extension (TDDFT), are able to describe most of the target characteristics of the individual systems (dyes, catalysts, semiconductors, etc.) [16,[43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] and of their active interfaces [16,41,43,47,[60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79]. The information extracted from these calculations serve as the basis for the explicit simulation of the photo-induced electron transfer by means of quantum or non-adiabatic dynamics.…”

“…While, however, computational modelling has played a prominent role in the development of the DSC technology, the understanding of the interfacial processes in DSPEC is still at its infancy, especially for what concerns the electron and hole transfer phenomena, which are central to efficient device functioning. Most of the published computational modeling works so far are, indeed, focused on the characterization of the water oxidation/proton reduction reaction mechanisms, [57,[101][102][103][104] with only a few studies reporting the interaction between the catalysts, dyes, and semiconductors [67,68,71,72].…”