Heteronuclear NMR studies of cobalamins. 9. Temperature-dependent NMR of organocobalt corrins enriched in /sup 13/C in the organic ligand and the thermodynamics of the base-on/base-off reaction
The synthesis of alkylcobinamides (RCbi) enriched in /sup 13/C in the organic axial ligand by standard reductive alkylation procedures from /sup 13/C-enriched alkyl halides has been shown to lead essentially exclusively to ..beta..-alkylcobamides (i.e., the organic ligand in the upper axial ligand position) by uv-visible and NMR spectroscopy and HPLC coupled with direct synthesis of /sup 13/CH/sub 3/Cbi of known stereochemistry from /sup 13/C-enriched methylcobalamin (/sup 13/CH/sub 3/Cbl). The ionic strength dependence of the on/off pK/sub a/ of CH/sub 3/Cbl, CH/sub 3/CH/sub 2/Cbl, and ..cap alpha..-ribazole has also been investigated. Measurements of the relative /sup 13/C resonance frequency of /sup 13/CH/sub 3/Cbl and /sup 13/CH/sub 3/Cbi as a function of temperature at several ionic strengths have been used to investigate the thermodynamics of the base-on/base-off reaction of cobalamins. Additional evidence has thus been obtained for a previously detected species of base-off, but benzimidazole-deprotonated, cobalamin in which the benzimidazole B3 nitrogen atom is hydrogen-bonded to an e side chain N-H, and the thermodynamics of formation of this species have been evaluated. These results have been confirmed by similar measurements on mixtures of /sup -/OOC/sup 13/CH/sub 2/Cbl and /sup -/OOC/sup 13/CH/sub 2/Cbi. Although the new species is the predominant form (80-90%) of the base-off but benzimidazole-deprotonated cobalamins in solution at all ionic strengths, its presence has only a minor effect on the primary on/off equilibrium constants previously calculated for RCbl's. 45 references, 4 figures, 8 tables.
- Research Organization:
- Univ. of Texas, Arlington (USA)
- OSTI ID:
- 6334832
- Journal Information:
- Inorg. Chem.; (United States), Vol. 27:20
- Country of Publication:
- United States
- Language:
- English
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ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ALKANES
SYNTHESIS
AMINES
COBALT COMPLEXES
PHOSPHORUS COMPOUNDS
CARBON 13
LABELLED COMPOUNDS
NMR SPECTRA
TRACER TECHNIQUES
CARBON ISOTOPES
COMPLEXES
EVEN-ODD NUCLEI
HYDROCARBONS
ISOTOPE APPLICATIONS
ISOTOPES
LIGHT NUCLEI
NUCLEI
ORGANIC COMPOUNDS
SPECTRA
STABLE ISOTOPES
TRANSITION ELEMENT COMPLEXES
400201* - Chemical & Physicochemical Properties
400202 - Isotope Effects
Isotope Exchange
& Isotope Separation