Studies of the complexation reactions of polyether ionophore antibiotics monensin A and lasalocid A with lead(II), zinc, calcium and sodium cations.
Abstract
Lead-ionophore complexes of the form PbLX and PbL2 (L = monensin A or lasalocid A; X- = anion) were isolated and characterized by elemental analysis (X = Cl--) and ESI-MS (X = Cl --, NO3, CH3COO--). Changes in the UV-Vis spectra indicated that lead(II) also forms stable 1:1 complexes (PbL) with both ionophores in 80% methanol-water (w/w). The reactions of polyether ionophore antibiotics monensin A and lasalocid A with lead(II), sodium, calcium and zinc cations were studied by several methods. The work was undertaken to provide the chemical basis for the effectiveness and selectivity found for the ionophore-mediated lead(II) transport in model membrane studies. Lead complexes that could participate in transport were isolated, and their stoichiometry and composition were determined. The protonation and complex formation equilibria with lead(II) and several biologically important cations were studied in solution. One- and two-dimensional NMR methods were employed to obtain information about the solution structures and cation binding sites in the lead-ionophore complexes. The protonation constants and complex formation constants of monensin A and lasalocid A have been determined in 80% methanol-water (w/w) at 25° C and I = 0.05 M (Et4NClO4). Potentiometric titrations for monensin, and both potentiometric and spectrophotometric titrations for lasalocid gave the following values for the concentration protonation constants: log KcH* = 6.83 +/- 0.05 and log Kc H* = 4.84 +/- 0.01, respectively. Potentiometric titration data was consistent with the presence of PbL, PbL2, and PbLOH species. The values of the formation constants obtained pointed to the significance of ternary PbLOH species at the physiological pH in solution. The magnitudes of the 1:1 complex formation constants decreased in the following order for lasalocid A: Pb2+ > Zn2+ > Ca2+, Na+; and for monensin A: Pb2+ > Na+ > Zn2+ > Ca2+. These trends mirrored those for the rates of lead transport. The results are discussed with respect to the reaction orders for ionophore-mediated lead(II) transport, transport selectivity, and possible transport modes for monensin and lasalocid.
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