NEW INSIGHTS INTO NATURE'S WATER OXIDISING COMPLEX

Abstract

The structural intricacies by which nature's water oxidising complex (WOC) catalyses the vital water oxidation and oxygen formation reactions is still largely unknown. This thesis provides an in-depth computational analysis of the key S2 and S3 states of this catalytic cycle. The S2 X-band EPR spectrum shows a S = 1/2 multi-line signal and a broad S = 5/2 signal at g = 4.1 previously attributed to an open and closed cubane respectively. In Chapter 4 BS-DFT calculations found that an open cubane form of the WOC alone transitions from a low spin (LS), S = 1/2, to a high spin (HS), S = 5/2, form on protonation of the bridging O4 oxo and is therefore responsible for both EPR signals. A key intermediate between the S2 and S3 states is a deprotonated HS form, previously assigned as S = 5/2 by EPR. Chapter 5 provides a thorough reassessment of this spin-state. Here, BS-DFT calculations on the S2 HS form with W1 deprotonated along with EPR spectral simulations proved that this signal is more accurately described by an S = 7/2 spin state, providing further evidence for O4 protonation in S2. Structural elucidations of the S3 state have confirmed the insertion of a new oxygen, O6, to the WOC but disagreed in its location, with observed O5-O6 bond distances ranging from 1.5 to 2.1 Ã. An oxo-hydroxo form (O-O >2.4 Ã) has previously been assigned through agreement with the observed S = 3 EPR signal and 55Mn hyperfine coupling (hfc). In Chapter 6, BS-DFT analysis of a peroxo form (1.5 Ã) shows strong agreement with structural determinations of the S3 state and yields the correct S = 3 spin state, whilst the observed hfcs agreed with an oxo-hydroxo. In Chapter 7, a proposed equilibrium between a peroxo and an oxo-oxo form is examined BS-DFT and intrinsic bond orbital analysis. This study showed in detail the electronic-level movements that facilitate the dynamic O-O bond forming equilibrium in the S3 state, allowing rapid evolution of O2 in the transient S4 state. The new insights provided within this thesis have led to a reassessment of the proposed mechanism by which nature catalyses this vital reaction.

Date of Award	31 Dec 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robin Pritchard (Supervisor), Michael Anderson (Supervisor) & Patrick O'Malley (Supervisor)