Ab Initio Modeling of the Manganese Centre of Photosystem II

Principal Investigator

Ronald J. Pace

Department of Chemistry

The Faculties

Computational studies carried out in this group have shown that the electron paramagnetic resonance spectra (EPR) observed from the manganese containing water oxidising catalytic site in plant Photosystem II are consistent with an unusual ligand geometry for the metals in this centre (1). In particular, the spectral modeling suggests that the EPR signals arise from an oxo bridged Mn dimer with low symmetry. Formally the oxidation states of the Mn are III and IV. The magnetic hyperfine interactions inferred for the Mn III are basically consistent with those expected for a d4 ion with extreme axial distortion (one ligand missing), but those for the Mn IV (d3) are unprecedented, suggesting a highly rhombic hyperfine pattern not seen in conventional Mn IV compounds. UV-Vis absorption data suggests the possibility that a protein derived Mn ligand, probably the imidazole ring of a histidine, is oxidised in the EPR visible state of the centre, thus contributing to this unusual hyperfine pattern. The project aims to computationally explore the possibility of this in the metal centre of Photosystem II.

Co-Investigators

Damien A. Kuzek

Department of Chemistry

The Faculties

Projects

v64 - PC

What are the results to date and the future of the work?

Encouraging progress has been made in modeling the manganese centre of Photosystem II. A number of preliminary investigations have been conducted so far with "idealised" structures to represent the manganese cluster. The main emphasis to date has been on investigating a monomeric system which has 4 or 5 Oxygen donors and an imidazole donor to the Mn metal centre.

We have examined a large number of different computational approaches for the determination of the ab initio wavefunction and as a result two different trends have been observed from this data. The classically expected result for the electron distribution has not been observed, except at a very low level of computational theory. Rather an electron transfer is observed to occur onto the manganese centre from the imidazole ring. This observation is very promising as it helps to explain the observed complexity in the experimental hyperfine structure of one of the EPR signals associated with the manganese complex in the enzyme.

Further work has been conducted in the hope of determining the energetics of the two conditions where electron transfer as indicated above is occurring. A manual restricted geometry


- Appendix A



optimisation for each of the two cases has been conducted with somewhat unusual results. However, a great deal more work is required to understand the results that are being generated from this.

Finally work has been commenced in attempting to model a dimeric Mn centre. As yet very little advancement has been achieved, though this work is still very much at the initial stages with only one structure so far yielding any results.

What computational techniques are used?

The computational technique being used currently is that of ab initio computational modeling by the use of the quantum chemistry program "Gaussian 94".