Electromerism and linkage isomerism in biologically-relevant Fe-SO complexes.

Sulfur monoxide, SO, is a relatively unstable molecule whose metal-coordinating properties have received little attention in bioinorganic chemistry. Reported here is a density functional theory (DFT) examination of the four possible oxidation states for a heme-SO/OS adduct previously proposed to be a part of the catalytic cycle of sulfite reductases. The FeOS and FeSO isomers are found to be degenerate in energy in most cases, suggesting that they both may be observable; the FeSO isomers would be the ones more likely to occur during the catalytic cycle of sulfite reductases - a cycle which indeed is initiated with the sulfite bound to iron via the sulfur, not via the oxygen.