Effects of the environment on microperoxidase-11 and on its catalytic activity in oxidation of organic sulfides to sulfoxides.

Microperoxidase-11 (MP-11, also known as heme undecapeptide of cytochrome c) was immobilized by encapsulation into sol-gel silica glass and by physisorption, chemisorption, and covalent attachment to silica gel. We then compared these species with one another and with dissolved microperoxidase-11 as catalysts for the sulfoxidation of methyl phenyl sulfide by hydrogen peroxide. MP-11 is prone to oligomerization in solution, both via axial ligation and via intermolecular interactions. When the ligation oligomerization is suppressed upon immobilization, heme becomes more accessible, and the sulfoxide yield increases 4-6 times, from 15% up to 95%. When the ligation oligomerization of dissolved MP-11 is suppressed by protonation and acetylation of amino groups and by addition of methanol, sodium dodecyl sulfate (SDS), or trifluoroethanol, the sulfoxide yield increases 3-5 times (up to 76%). The oligomerization via intermolecular interactions is important for preserving enantioselectivity in immobilized and dissolved MP-11. For MP-11 in amine-rich and especially alcohol-rich environments, the enantioselectivity is vanishingly low, presumably because amino and hydroxyl groups cause a conformation change in the catalyst. In other environments, the MP-11 species are aggregated via intermolecular interactions in micellar (SDS) solution and on the surface of the silica gel, or via axial ligation in aqueous buffer at pH 6.0. Under these conditions, the enantioselectivity is enhanced; the enantiomeric excess (ee) becomes as high as 46%. An understanding of the effects of the aggregation state and consequent properties on the catalytic activity of MP-11 allowed us to control the yield and enantioselectivity of sulfoxidation reaction.