Computer modelling of electron paramagnetic resonance-active molybdenum(V) species in xanthine oxidase †

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Abstract
Density Functional Theory calculations have been used to investigate structural models for the ‘Very Rapid’ and ‘Inhibited’ EPR signals ascribed to xanthine oxidase MoV species. Analysis of the observed hyperfine coupling tensors has suggested close Mo–C contacts in both cases and a side-on interaction between the substrate’s carbonyl group and the Mo centre has been proposed. Attempts to confirm this for several model ‘Very Rapid’ species, based on a previous structure for the active site, either give short Mo–C contacts but too small a spin density on Mo or long Mo–C distances and a more reasonable Mo spin. Either the model system or the interpretation of the experimental data requires revision. In contrast, a good model can be developed for the ‘Inhibited’ species, which arises during reaction with formaldehyde, which is consistent with the EPR and other experimental data. However, rather than involving side-on co-ordination of the formaldehyde carbonyl group, the ‘Inhibited’ species forms a C–S bond between the formaldehyde and the sulfido ligand.