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South African Journal of Chemistry

On-line version ISSN 1996-840X
Print version ISSN 0379-4350

S.Afr.j.chem. (Online) vol.61  Durban  2008

 

RESEARCH ARTICLE

 

The ligand substitution reactions of hydrophobic vitamin B12 derivatives. Reaction of cobyric acid heptapropyl ester with heterocyclic N-donor ligands

 

 

Mohamed S.A. HamzaI, *; Mohamed A. ElawadyII; Helder M. MarquesIII

IDepartment of Chemistry, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt
IIDepartment of Radiation Protection, Nuclear Power Plants Authority, Cairo, Egypt
IIIMolecular Sciences Institute, School of Chemistry, University of the Witwatersrand, P.O. WITS, Johannesburg 2050, South Africa

 

 


ABSTRACT

The hydrophobic cobyrinic acid heptapropyl ester corrinoids XCbs-Pr (axial ligandX = CN-,SO32-,CH3- and CH3CH2-) have been prepared from vitamin B12 by hydrolysis of the amide side chains and their conversion to propyl esters. Both the position of the y-band and the general shape of the UV-visible spectra of these complexes show significant solvent dependence as the polarity of the solvent is varied. The equilibrium constants, K, for the reaction of five-membered heterocyclic nitrogenous bases (the azoles imidazole, pyrazole and 1,2,4-triazole) with displacement of coordinated H2O in aquacyanocobyrinic acid heptapropyl ester, and coordination by the predominantly five-coordinate complexes sulphitocobyrinic acid heptapropyl ester, ethylcobyrinic acid heptapropyl ester and methylcobyrinic acid heptapropyl ester, have been determined spectrophotometrically at 25 °C in water, methanol, acetonitrile, ethyl acetate and toluene. Values of K are dependent on the identity of the trans ligand (X = CN- >SO32- > CH3- >CH3CH2-); they increase with the basicity of the azole (pyrazole < 1,2,4-triazole < imidazole); and they increase as the solvent polarity increases (toluene < ethyl acetate < acetonitrile < methanol < H2O). Molecular mechanics calculations suggest that these effects are largely electronic in origin.

Keywords: Hydrophobic vitamin B12, cobalt corrinoids, equilibrium constants, solvent polarity, trans influence


 

 

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Received 17 October 2007
Revised 22 April 2008
Accepted 18 May 2008

 

 

* To whom correspondence should be addressed. E-mail: hamzamsa@yahoo.com

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