Study Of The Structural Reorganization Of A Synthetic Mimic Of The Oxygen-evolving Center In Photosystem Ⅱ By Electrochemical FTIR Spectroscopy

Photosynthesis is the largest-scale conversion of solar energy into chemically-stored energy in the earth.The emergence of the oxygen-evolving photosynthetic species leads to the production and accumulation of oxygen from zero up to 21% of the current atmosphere,which accelerates the emergence and evolution of the biosphere.Light-driven water oxidation in nature is accomplished by photosystem II,where it is catalyzed by the oxygen evolving center(OEC),a Mn4 Ca O5 cluster to accumulate four oxidizing equivalents and catalyze the formation of oxygen.The catalytic process is called Kok cycle,which has five metastable storage states Si(i = 0-4).Si turns to Si+1 during each photon-induced electron abstraction from the OEC.Oxygen is released when the S3 state returns to the S0 state via the hypothesized S4 state.The lability of the protein-bound OEC prevents it from electrochemical characterization and controlled modification of its structure for spectral assignments.Thus,IR spectra of manganese model compounds are indispensable for the interpretation of the IR spectra of the OEC.There are several multi-nuclear Mn complexes that have been prepared to understand the spectral properties of the Mn complex in the OEC.Along this trend of efforts,in 2015 Zhang et al.have synthesized an artificial Mn4 Ca O4 cluster with a dangling Mn4 atom,showing many structural similitude with the OEC.In contrast to the lability of the protein-bound OEC,this molecular mimic can facilitate electrochemical preparation of various intermediates by directly controlling the applied redox potentials,thus it can possibly provide spectroscopic signatures for the OEC as well as the reaction mechanism for the oxygen formation.In this thesis,we have carried out the electrochemical-FTIR spectroscopic study of the synthetic Mn cluster.1.We have home-built an electrochemical-FTIR spectroscopic sample cell of air-tight,with artificial diamond plates as the two optical windows,which allows the IR spectroscopic measurement of the low frequency region down to about 370 cm-1,suitable for investigation of Mn—O bonds,which generally have their fingerprints below 1000 cm-1.2.By investigation of the FTIR difference spectra of electrochemically prepared multiple oxidation states of the synthetic Mn cluster,we find that the electrochemical redox processes are almost reversible,and there are pairs of redox potentials differing about 0.2 V observed for S1 to S2 and S2 to S3 transitions respectively.This suggests that there are two different conformers in S2 state,similar as those in the OEC.In the low frequency region on going from S1 to S2 state,we observed an intensity decrease at 604 cm-1 band which is correlated to the intensity increase at 621 cm-1 and 460 cm-1band respectively,and this tendency continues to the S2 to S3 transition.With reference to the DFT calculated vibrational modes for the synthetic Mn cluster,604cm-1 band is assigned to the Mn1(III)-O1 bond in S1 state,and 621 cm-1 to Mn1(IV)-O1 bond in S2 state,and 460 cm-1 band to a newly formed Mn1(IV)-O5 bond in the closed conformer of S2 state.The continued increase of 460 cm-1 band intensity in S2 to S3 transition indicates that both the open and closed cubane conformers in S2 state are converted to the closed cubane form in S3 state.3.We further conform the formation of the closed cubane form of the synthetic Mn cluster by inspecting the CO2 IR rotation-vibration spectra during the redox process.When CO2 is trapped in the closed cubane structure,the spectral feature of rotation disappears,giving an additional probe for the formation of the closed cubane form of the synthetic Mn cluster.