Abstract
Thermodynamic and equilibria properties of two vitamin B12 coenzyme analogues, i.e. 1-methyl-5-deoxy-β-d-(−)ribofuranosylcobalamin (RibCbl) and 1-methyl-5-deoxy-2,3-isopropylidene-β-d-(−)ribofuranosylcobal- amin (Rib*Cbl) were studied by u.v.-vis. spectroscopy. Their temperature-dependent equilibria between 6-coordinate base-on species and 5-coordinate unprotonated base-off species, and pH-dependent equilibria for the protonation and displacement of 5,6-dimethylbenzimidazole at 25°C have been examined. Values of Δ H and Δ S in neutral solution and at pH 4.00, and the axial base pKa were determined. Comparing the results with those for coenzyme B12 and other cobalamins, it has been found that Δ H and Δ S values for two ribosylcobalamins fall in the range of organocobalamins, but for RibCbl*, a ca. 1 unit difference in pKa values exists. The effect of the ribofuranose structure on the Co-N(dbzm) strength is considered.
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