METAL-ION BINDING-PROPERTIES OF DIHYDROXYACETONE PHOSPHATE AND GLYCEROL 1-PHOSPHATE

The acidity constants of H(R-MP)-, where R-MP2- = dihydroxyacetone phosphate (DHAP2-) and glycerol 1-phosphate (G1P2-), and the stability constants of the binary M(R-MP) complexes (M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+) were determined by potentiometric pH titrations in aqueous solution (I = 0.1 M, NaNO3; 25-degrees-C). The stability of the ternary Cu(Arm)(R-MP) complexes (Arm = 2,2'-bipyridyl or 1,10-phenanthroline) were also measured. On the basis of recent results for simple phosphate monoesters, R-MP2-, where R is a strictly noncoordinating residue (Massoud, S. S.; Sigel, H. Inorg. Chem. 1988, 27, 1447-1453), it is established that the stability of all the M(DHAP) and M(GIP) complexes is governed by the basicity of the phosphate group of DHAP2- and G1P2-. There are no indications in aqueous solution for the participation of the oxygen atom of the carbonyl or hydroxy groups at C-2 of these ligands in complex formation, which would on steric grounds bc possible. However, measurements with Cu2+ and DHAP2- or G1P2- in water containing 30 or 50% (v/v) 1,4-dioxane (I = 0.1 M, NaNO3; 25-degrees-C) prove that to some extent seven-membered chelates involving the mentioned oxygen atoms may be formed. This may also be surmised for the other mentioned divalent metal ions under appropriate conditions, because it is well-known that they all can interact with the oxygen of carbonyl or hydroxy groups, especially when the solvent has poorer solvating properties than water. This condition exists in active-site cavities of enzymes; therefore, the indicated type of metal ion interaction could play a role in certain metabolic processes involving DHAP (or GAP; see below) and G1P. It is further concluded that the proton and metal ion affinities of glyceraldehyde 3-phosphate (GAP2-) correspond in a first approximation to those of G1P2- because both ligands contain the same structural unit, i.e., -CH(OH)CH2OPO32-, which is responsible for the proton and metal ion binding properties, as shown now for G1P2-. This conclusion regarding GAP is meaningful because this ligand may hardly be studied directly due to its conversion into DHAP.