Hydrophobic vitamin B12. Part 9. An artificial holoenzyme composed of hydrophobic vitamin B12 and synthetic bilayer membrane for carbon-skeleton rearrangements

Abstract

The incorporation of hydrophobic vitamin B₁₂ derivatives, which have ester groups in place of the peripheral amide moieties of the naturally occurring vitamin B₁₂, into single-compartment vesicles composed of synthetic lipids having an alanyl residue in the single-chain segment, is primarily controlled by the hydrophobicity of the peripheral ester groups. Such incorporation into vesicles of another synthetic lipid, having a histidyl residue in place of an alanyl residue, was much enhanced when coordination was allowed to take place between the nuclear cobalt of the hydrophobic vitamin B₁₂ and the imidazolyl moiety of the lipid. Microenvironmental properties around heptapropyl cobyrinate derivatives placed in single-compartment vesicles of the former lipid were examined by electronic, fluorescence and fluorescence polarization measurements as well as by differential scanning calorimetry. The hydrophobic vitamin B₁₂ was incorporated into the intramembrane domain composed of assembly of the single-chain segment of each lipid molecule, and its molecular motion was markedly suppressed under such microenvironmental conditions. Carbon-skeleton rearrange-ment reactions of alkyl ligands bound to heptapropyl cobyrinate were markedly favoured in the single-compartment vesicle, relative to the reactions in methanol and benzene, under anaerobic photolysis conditions at ordinary temperatures. The 1,2-migration of electron-withdrawing groups, such as acetyl, cyano, carboxylic ester and thioester, apparently arises both from suppression of molecular motion and desolvation effects operating on the alkylated hydrophobic vitamin B₁₂ in the vesicle. Finally, the catalytic mediator, constituted with heptapropyl cobyrinate perchlorate and the single-compartment vesicle in aqueous media, was coupled with a substrate-activation system, composed of atmospheric oxygen and vanadium(III) ions, to establish a real artifical holoenzyme. 2-Acetyl-2-ethoxycarbonylpropane, 2-cyano-2-ethoxycarbonylpropane and 1-acetyl-1-ethoxy-carbonylethane were converted catalytically into the corresponding rearrangement products under aerobic photolysis conditions at 20 °C. A plausible reaction mechanism for the catalytic reaction is discussed.