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

Yukito Murakami, Yoshio Hisaeda, Teruhisa Ohno

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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 rearrangement 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(in) ions, to establish a real artifical holoenzyme. 2-Acetyl-2- ethoxycarbonylpropane, 2-cyano-2-ethoxycarbonylpropane and 1-acetyl-1- ethoxycarbonylethane 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.

Original languageEnglish
Pages (from-to)405-416
Number of pages12
JournalJournal of the Chemical Society, Perkin Transactions 2
Issue number3
Publication statusPublished - Dec 1 1991

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Holoenzymes
Vitamin B 12
Carbon
Membranes
Lipids
Esters
Photolysis
Fluorescence
Derivatives
Vanadium
Hydrophobicity
Cobalt
Benzene
Amides
Methanol
Differential scanning calorimetry
Chemical activation
Ions
Polarization
Oxygen

All Science Journal Classification (ASJC) codes

  • Chemistry(all)

Cite this

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title = "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 rearrangement 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(in) ions, to establish a real artifical holoenzyme. 2-Acetyl-2- ethoxycarbonylpropane, 2-cyano-2-ethoxycarbonylpropane and 1-acetyl-1- ethoxycarbonylethane 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.",
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AU - Murakami, Yukito

AU - Hisaeda, Yoshio

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