Carbon-13 nuclear magnetic resonance studies of cobalamins

The coenzyme B12 derivative N1-methyl-5'-deoxyadenosylcobalamin: synthesis, characterization, stability towards dimroth rearrangement, and Co-C thermolysis product and kinetic studies

The mechanism of Co-C heterolysis of adenosylcobalamin (AdoCbl) is studied, specifically the effect of a positive charge produced by methylation (CH3+, a "sticky proton" analog of H+ addition) to the most basic adenine N1 nitrogen site in AdoCbl. This is accomplished by the synthesis, characterization and Co-C thermolysis in the presence of the radical trap TEMPO of the N1 methylated AdoCbl derivative, [N1-methyl-5'-deoxyadenosycobalamin]+, 5. A variety of needed additional syntheses and other control experiments are also reported, including effective purification methods using ultrafiltration membranes, procedures that may prove to be the more generally useful part of the synthetic work reported. The thermolysis results with 5 are unequivocal in showing that a positive charge at the N1 nitrogen in the adenosyl group of AdoCbl has no effect within +/- 8% on the products or rates of Co-C thermal cleavage. The data are most consistent with a recently suggested, new mechanism for AdoCbl Co-C heterolysis at less acidic pH values, one not involving the protonation of the beta-ribose oxygen that is known to occur under strongly acidic conditions. In addition, the present study illustrates some of the positive features, and also some of the pitfalls, in the approach where CH3+ is used as a "sticky proton" analog in mechanistic studies.

5'-Methylbenzimidazolyl-cobamides are the corrinoids from some sulfate-reducing and sulfur-metabolizing bacteria

The sulfate-reducing bacteria Desulfobacterium autotrophicum, Desulfobulbus propionicus and Archaeoglobus fulgidus (VC-16) and the sulfur-metabolizing archaebacteria Desulfurolobus ambivalens and Thermoplasma acidophilum were found to contain considerable amounts of corrinoids, that were isolated and crystallized in their Co beta-cyano form. In three other sulfur-metabolizing archaebacteria, Thermoproteus neutrophilus, Pyrodictium occultum and Staphylothermus marinus significant amounts of corrinoids were not detected under the isolation methods used. The samples from the three sulfate-reducers were identified with Co alpha-[alpha-(5'-methylbenzimidazolyl)]-Co beta-cyanocobamide. This corrinoid was also obtained from a 5-methylbenzimidazole-supplemented Propionibacterium fermentation and was structurally characterized by ultraviolet/visible, CD, fast-atom-bombardment MS, 1H-and 13C-NMR spectroscopy. Also the major corrinoid from T. acidophilum was (tentatively) analyzed as a 5'-methylbenzimidazolyl-cobamide, whereas the main corrinoid from D. ambivalens was indicated to be vitamin B12 (a 5',6'-dimethylbenzimidazolyl-cobamide). The 5'-methylbenzimidazolylcobamides are found here as the common corrins of some sulfate-reducing and sulfur-metabolizing bacteria. The structural diversity due to the differing nucleotide bases of the corrins examined here and in methanogenic and acetogenic bacteria appears not to correlate to the biological function(s) of the corrins, but rather to be determined by biosynthetic properties of these organisms under natural growth conditions.

The corrinoid from Methanobacterium thermoautotrophicum (Marburg strain). Spectroscopic structure analysis and identification as Co beta-cyano-5'-hydroxybenzimidazolyl-cobamide (factor III)

The corrinoids from Methanobacterium thermoautotrophicum were extracted as the Co-cyano derivative, which was isolated in crystalline form. A consistent set of spectroscopic data was acquired (ultraviolet/visible, circular dichroic, infrared, fast-atom-bombardment mass, 1H-NMR and 13C-NMR spectra), which allowed the structural analysis of this complete corrinoid. It was assigned the structure of the Co beta-cyano-5'-hydroxybenzimidazolyl-cobamide and was identified with Friedrich and Bernhauer's 'factor III' by comparison with an authentic sample.