Spectroscopic and Computational Studies on the Water Oxidising Complex and Redox-Active Tyrosines of Photosystem II

Abstract

This thesis presents a two-part study of Photosystem II. The first, is a spectroscopic study primarily based on a novel low temperature FTIR system. The second is a series of computational studies on carboxylate-ligated transition metal complexes, culminating in a direct study of the vibrational frequencies associated with the first-shell carboxylate ligands of theWater Oxidising Complex. The findings of this thesis are summarised in the following:

Low temperature (10 K) illuminations on ZnSe cryostat windows induces transient and stable FTIR difference signals. The kinetic properties of these signals are highly reminiscent of the TyrZ “split signals” observed in EPR studies.

- The FTIR analogue to the visible illumination-induced, transient S1 TyrZ “split signal” was observed. Some characteristic features of the QA- /QA signal was observed, but features of the donor(s) could not be extracted from the data.

- The 10 K and 80 K TyrD./TyrD difference signals were obtained using multiple signal subtractions. Small spectral differences, possibly caused by electrostatic effects, were found between these signals. The possibility of TyrD. forming an imidazolium intermediate at 10 K was found to be unlikely, however.

- Deprotonation was found to suppress carboxylate ligand vibrational frequency shifts associated with manganese-centred oxidation. This appears to be a general feature of redoxactive transition metals, as the same effect was also observed in vanadium, chromium, iron and cobalt complexes.

- Charge conservation via simultaneous anionic ligand substitution with metal centre oxidation also suppresses carboxylate ligand frequency shifts. Solvent polarisation also has a similar effect. It was concluded that carboxylate ligand frequency shifts are primarily driven by electrostatic effects, i.e. the increased polarisation between the carboxylate ligand and metal centre, as the metal centre undergoes oxidation.

- Water or hydroxyl ligand deprotonation is most likely the reason for the sparsity of frequency shifts observed in mutagenic studies of the PSII Water Oxidising Complex (WOC). Frequency shift suppression via deprotonation appears to be most effective in clusters that are neutrally charged, possess a mean Mn oxidation of 3 in the S1 state, and have water and hydroxyl ligands in the O5 and W2 positions, respectively. Further, it is shown that Mn2 is likely to be oxidised in the S1/S2 transition, on the basis of the IR frequency shifts assigned to D1-Ala344. Show more