Metal-ion binding properties of O-phosphonatomethylcholine (PMCh −). : Effect of the positive charge of a distant trimethylammonium group on the coordinating qualities of a phosph(on)ate group

• Published in: Inorganica Chimica Acta Vol. 331; no. 1; pp. 109 - 116
• Main Authors: Fernández-Botello, Alfonso, Gómez-Coca, Raquel B., Holý, Antonı́n, Moreno, Virtudes, Sigel, Helmut
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 28.03.2002
• Subjects: Acidity constants; Charge repulsion; Choline derivatives; Phosphocholine; Phosphonate complexes; Stability constants; Ternary complexes; Charge repulsion; Choline derivatives; Acidity constants; Phosphonate complexes; Stability constants; Ternary complexes; Phosphocholine
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(01)00763-0

The acidity constant of monoprotonated O-phosphonatomethylcholine, H(PMCh) ±, an analogue of phosphocholine, as well as the stability constants of the M(PMCh) + complexes with the metal ions M 2+=Mg 2+, Ca 2+, Sr 2+, Ba 2+, Mn 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+ or Cd 2+, and also of the mixed ligand complexes, Cu(Bpy)(PMCh) + and Cu(Phen)(PMCh) + where Bpy=2,2′-bipyridine and Phen=1,10-phenanthroline, have been determined by potentiometric pH titrations in aqueous solution at I=0.1 M (NaNO 3) and 25 °C. Application of previously determined straight-line plots of log K M( R- PO 3) M versus p K H(R-PO 3) H for simple phosph(on)ate ligands, R-PO 3 2− , where R represents a residue that does not affect complex formation, proves that the (CH 3) 3N + unit of PMCh − somewhat inhibits, due to charge repulsion, complex formation with the PO 3 2− group. In fact, a comparison of previous results obtained for RCH 2CH 2OCH 2PO 3 2− ( PME-R 2−) ligands (R is non-interacting) with the data obtained now for PMCh −, i.e. (CH 3) 3N +CH 2CH 2OCH 2PO 3 2− , shows that for all the divalent metal ions studied (except Cu 2+) the inhibition amounts to log Δ M/PMCh=0.46±0.05; in the case of Cu 2+ and its mixed ligand complexes the inhibition of this relatively distant positive charge appears to be slightly more pronounced, which is probably due to the distorted coordination sphere of Cu 2+. The mentioned ‘constant’ repulsive effect provides evidence that in the M(PMCh) + complexes the ether oxygen does not participate in complex formation; this is different in the M(PME-R) complexes, where five-membered chelates form in equilibrium, the extent being dependent on the kind of metal ion involved. The present results also allow an estimate of the stability constants of the M 2+ 1:1 complexes formed with phosphocholine; to facilitate such estimations we also measured the acidity constant of monoprotonated phosphocholine. The acidity constant of monoprotonated O-phosphonatomethylcholine, H(PMCh) ±, an analogue of phosphocholine (PCh −), and the stability constants of M(PMCh) + complexes [M 2+=Mg 2+, Ca 2+, Sr 2+, Ba 2+, Mn 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+ or Cd 2+, and also Cu(2,2′-bipyridine) 2+ or Cu(1,10-phenanthroline) 2+] were determined in aqueous solution. Comparison with previous results for PME-R 2− (R is non-interacting) shows that for all M 2+ studied (except Cu 2+) the inhibition due to charge repulsion amounts to log Δ M/PMCh =0.46±0.05; for Cu 2+ and its mixed ligand complexes the inhibition is slightly more pronounced. The present results also allow an estimate of the stability constants for the M(PCh) + complexes.