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

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 M2+=Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+, 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 (NaNO3) and 25 °C. Application of previously determined straight-line plots of versus pKH(R-PO3)H for simple phosph(on)ate ligands, R-PO3 2−, where R represents a residue that does not affect complex formation, proves that the (CH3)3N+ unit of PMCh− somewhat inhibits, due to charge repulsion, complex formation with the PO3 2− group. In fact, a comparison of previous results obtained for RCH2CH2OCH2PO3 2− ( PME-R2−) ligands (R is non-interacting) with the data obtained now for PMCh−, i.e. (CH3)3N+CH2CH2OCH2PO3 2−, shows that for all the divalent metal ions studied (except Cu2+) the inhibition amounts to log ΔM/PMCh=0.46±0.05; in the case of Cu2+ 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 Cu2+. 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 M2+ 1:1 complexes formed with phosphocholine; to facilitate such estimations we also measured the acidity constant of monoprotonated phosphocholine.