ab initio Modeling of the Manganese centre of Photosystem II

               

Principal Investigator

Ronald J. Pace

Department of Chemistry

The Faculties

Co-Investigators

Damien A. Kuzek

Department of Chemistry

The Faculties

Projects

v64 - PC

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. In particular, the spectral modelling 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.
 
         
               

   
               

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

Encouraging progress has been made in modelling 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.


 

 

 

 

Appendix A -
 

       

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 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.

A small FORTRAN program has been written to extract the electron hyperfine coupling parameters from the Gaussian output. This has given us another tool in which to explore the possibilities as far as attempting to model the single ion results in terms of what is understood from the simulation of the multiline signal which has been done in this group.

All this work has suggested a number of "wet" chemistry experiments which are currently being perused in conjunction with this work.

In addition to the above we have just started to model water exchange rates on first row transition metal ions.

What computational techniques are used?

The computational technique being used currently is that of ab initio computational modelling by the use of the quantum chemistry program "Gaussian 98".

       
- Appendix A