Calcium-Phosphonate Interactions: Solution Behavior and Ca2+ Binding by 2-Hydroxyethylimino-bis(methylenephosphonate) Studied by Multinuclear NMR Spectroscopy

The tetra-acid 2-hidroxyethylimino-bis(methylenephosphonic acid) (HESPH, 1) and its ring condensation product, the triacid 2-hydroxy-2-oxo-4-phosphonemethyl-1, 4,2-oxazaphosphorinan e (2), were investigated for determination of protonation constants using P-31, H-1, and C-13 NMR spectroscopy in a wide pH range. As for other alpha-aminophosphonic acids, the first protonation of 1 is straightforward and occurs at the nitrogen, while for 2 the first protonation occurs simultaneously at the exo phosphonate group, allowing estimation of the microscopic protonation constants. The complexation of Ca2+ with 1 in a 1:1 molar ratio in aqueous solutions and in the presence of a 5-fold excess Na+ is rationalized by the products LCaH2, LCaH, LCaNaH, LCa, and LCa2 (L = 1). Only the phosphonate groups are involved in Ca2+ binding at pH > 3, while the phosphonate, hydroxyl, and amine functionalities coordinate to Ca2+ at pH > 6-7, as soon as the proton at N is lost. Probable conformation states of ions of 1 and 2 are estimated by means of the dependence of vicinal coupling constants (3)J(HH) and (3)J(PC) from dihedral angles.