The chemistry of vitamin B12. Part 20. Diaquocobinamide : pK values and evidence for conformational isomers
An improved method for preparing aqueous solutions of diaquocobinamide without hydrolysis products is described. The pKa values have been determined in 0.2 mol dm–3 NaClO4 at 25 °C as pK1= 5.9 ± 0.1 and pK2= 10.3 ± 0.2 and are shown to involve one proton each. Evidence is presented that diaquo-, aquohydroxo-, and dihydroxo-cobinamide exist in solution as isomers depending on hydrogen bonding between the axial ligands and different amide side-chains.