Modelling the reduced xanthine oxidase in active sulfo and inactive desulfo forms

Synthesis of two complexes, [NBu^<em>n</em>₄][Mo^IVO(mnt)(S-Tol)(N–N)] (<em>N</em>–<em>N</em> = 2,2′-bipyridine (1a) or 1,10-phenanthroline (1b); mnt = maleonitriledithiolate; S-Tol = toluenethiol) are reported. These on treatment with H₂S generate the corresponding [NBu<small>^<em>n</em></small>₄][Mo^IVO(mnt)(SH)(N–N)] (2a and 2b) complexes bearing the susceptible hydrosulfide coordination. 2a (and 2b) upon chemical oxidation show EPR spectra with the appearance of a Mo(V) signal at &lt;<em>g</em>&gt; = 1.976 (for 2a and 2b). Such signal changed to another slow Mo(V) signal at &lt;<em>g</em>&gt; = 1.949 at the expense of the initial signal. This conversion is accelerated in the presence of trace amount of moisture in the solvent. These data are similar to the Moco isolated from the xanthine oxidase (XO) as reported by Bray. 2a (and 2b) responds to one electron electrochemical oxidation and the generated pentavalent species responds to proton coupled electron transfer reaction. A predominantly metal centered HOMO is observed in 2a (and 2b) from DFT calculations. In the inhibited Moco of XO bearing a –SH moiety, the HOMO shows considerably less electron density residing on the Mo d_<em>xy</em> orbital and maximum electron density is distributed to the phospho-ester group rendering the Mo(IV) center incapable of participating in the electron transfer process manifesting inhibition