title: Calcium-phosphonate interactions: solution behavior and Ca2+ binding by 2-hydroxyethylimino-bis(methylenephosphonate) studied by multinuclear NMR spectroscopy.
abstract: The tetra-acid 2-hydroxyethylimino-bis(methylenephosphonic acid) (HEIBPH, 1) and its ring condensation product, the triacid 2-hydroxy-2-oxo-4-phosphonemethyl-1,4,2-oxazaphosphorinane (2), were investigated for determination of protonation constants using (31)P, (1)H, and (13)C NMR spectroscopy in a wide pH range. As for other alpha-amino-phosphonic 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 Ca(2+) 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 LCaH(2), LCaH, LCaNaH, LCa, and LCa(2) (L = 1). Only the phosphonate groups are involved in Ca(2+) binding at pH > 3, while the phosphonate, hydroxyl, and amine functionalities coordinate to Ca(2+) 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.
authors.affiliation: Crystal Growth and Design Laboratory, Department of Chemistry, University of Crete, P.O. Box 2208, Voutes Campus, Heraklion GR-71003, Crete, Greece. demadis@chemistry.uoc.gr
authors.forename: Konstantinos D
authors.initials: KD
authors.lastname: Demadis
authors.forename: Nikoleta
authors.initials: N
authors.lastname: Stavgianoudaki
authors.forename: Gisbert
authors.initials: G
authors.lastname: Grossmann
authors.forename: Margit
authors.initials: M
authors.lastname: Gruner
authors.forename: Joseph L
authors.initials: JL
authors.lastname: Schwartz