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Metal‐Ion‐Coordinating Properties of a Viral Inhibitor, a pyrophosphate analogue, and a herbicide metabolite, a glycinate analogue: The solution properties of the potentially five‐membered chelates derived from phosphonoformic acid and (aminomethyl)phosphonic acid
Author(s) -
Song Bin,
Chen Dong,
Bastian Matthias,
Sigel Helmut,
Bruce Martin R.
Publication year - 1994
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19940770706
Subject(s) - chemistry , deprotonation , chelation , phosphonate , potentiometric titration , inorganic chemistry , metal ions in aqueous solution , stereochemistry , metal , medicinal chemistry , aqueous solution , equilibrium constant , pyrophosphate , ion , organic chemistry , enzyme
The pyrophosphoric‐acid‐analogue phosphonoformic acid (pfa) and the amino‐acid‐analogue (aminomethyl)phosphonic acid (ampa) both form, in the deprotonated state, i.e. , as – OOC–PO 3 2−and H 2 NCH 2 PO 3 2− , respectively, five‐membered chelate rings with metal ions. pfa inhibits both phosphate transport and virus replication, while ampa is a metabolic product of the common herbicide glyphosate ( N ‐(phosphonomethyl)glycine). The acidity constants of H 2 pfa – and H 2 ampa± as well as the stability constants of the [M(Hpfa)], [M(pfa)] – , [M(Hampa)] + , and [M(ampa)] complexes, where M 2+ Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Cu(2,2′‐bipyridyl) 2+ , Cu(1,10‐phenanthroline) 2+ , Zn 2+ , or Cd 2+ , have been determined by potentiometric pH titrations in aqueous solution at 25° and I 0.1 M (NaNO 3 ). The structures of isomeric complexes and the connected intramolecular equilibria are deduced and evaluated based on the equilibrium constants measured and those calculated via the p K a values of the above mentioned ligands and previously established log K vs. p K a straight‐line plots ( H. Sigel et al., Helv. Chim. Acta 1992 , 75 , 2634) for a simple phosphonate‐M 2+ coordination. pfa forms stronger complexes than ampa with all the above mentioned metal ions, with the single exception of [Cu(ampa)] which is slightly more stable than [Cu(pfa)] – . In neutral solutions, more precisely at pH of ca. 6, pfa complexes of alkaline‐earth‐metal ions retain one phosphonate‐bound proton, [M(Hpfa)], while those of the transition‐metal ions chelate with the trianionic ligand, pfa 3– . In accord with increasing ligand‐basicity, the stability‐constant order for all metal‐ion complexes is oxalate > pfa > pyrophosphate but, owing to proton competition in pyrophosphate, in neutral solutions metal‐ion complexation of pfa 3– competes with P 2 O 7 4− . With ampa alkaline‐earth‐metal ions interact only with the phosphonate group of even the dianionic ligand (though Mg 2+ appears to form a low fraction of a [Mg(ampa)] chelate) while transition‐metal ions form chelates which are comparable in stability to those of glycinate.