[Equilibrium of thiamine diphosphate complexation with divalent ions--formation constants].

The divalent metal ion-thiamine diphosphate (TDP) complexes are of potential interest because such complexes are directly involved in enzymatic reactions. The binding of Ni2+ to TDP has been studied in order to gain some insight in the TDP-metal interactions which could help to understand the role of the metal ions in general in the enzymatic processes. A technique of pH titration is developped which brings definitive proof of metal-TDP bonding in aqueous solutions. The results indicate that the metal ion interacts both with the pyrophosphate group and the pyrimidine ring. The complexation decreases the pK for the phosphate ionization. The ring nitrogen is a some 12 times weaker ligand of Ni2+ than is pyrophosphate. The stability constants obtained for an ionic strength 0.5 in tetramethylammonium bromide are reported; the maximum of the apparent formation constant, between pH 4.5 and pH 7.5, occurs for pH 6.9. The poor solubility of nickel hydroxyde is responsible for increasing interaction in alcaline solution.