COMPUTER MODELING OF ELECTRON-PARAMAGNETIC RESONANCE-ACTIVE MOLYBDENUM(V) SPECIES IN XANTHINE-OXIDASE

Density Functional Theory calculations have been used to investigate s tructural models for the 'Very Rapid' and 'Inhibited' EPR signals ascr ibed to xanthine oxidase Mo-V species. Analysis of the observed hyperf ine coupling tensors has suggested close Mo-C contacts in both cases a nd a side-on interaction between the substrate's carbonyl group and th e Mo centre has been proposed. Attempts to confirm this for several mo del '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 m odel system or the interpretation of the experimental data requires re vision. In contrast, a good model can be developed for the 'Inhibited' species, which arises during reaction with formaldehyde, which is con sistent with the EPR and other experimental data. However, rather than involving side-on co-ordination of the formaldehyde carbonyl group, t he 'Inhibited' species forms a C-S bond between the formaldehyde and t he sulfide ligand.