Kinetics of monensin complexation with sodium ions by 23NaNMR spectroscopy

Estimation of environmentally relevant chemical properties of veterinary ionophore antibiotics

Monensin (MON) and salinomycin (SAL), known as polyether ionophore antibiotics (IPAs), are extensively used in livestock industry and can enter the environment via animal manure and agricultural runoff. Although some studies have investigated the environmental fate and transformation of IPAs, the lack of information on IPAs' aqueous-phase chemical properties is a major hindrance for further in-depth research. This study was able to experimentally determine the acidity constants (pKa), metal-complex dissociation constants (Kdiss), and intrinsic aqueous solubility of MON species, and some of these properties of SAL. The pKa value of MON was found to be 4.5, close to other aliphatic carboxylic acids and the predicted value by the computer program ChemAxon. The metal-complex dissociation constants of MON were estimated to be 0.058 and 0.573 with sodium ion (Na(+)) and potassium ion (K(+)), respectively. The Kdiss value of SAL with sodium ion was found to be 1.31. Compared to the previous values determined in organic solvents, the Kdiss of MON in aqueous phase are several orders of magnitude higher but maintain the same relative selectivity toward metal ions (Na(+) versus K(+)). The determined pKa and Kdiss values were also used to assess the aqueous solubility limits of different IPA species under different pH and metal ion concentrations. Results from this study provide more accurate information for the properties of IPAs. The obtained constants can be applied to predict the speciation of IPAs in various aquatic systems and help shed light on the environmental fate of IPAs.

Circular Dichroism is Sensitive to Monovalent Cation Binding in Monensin Complexes

Monensin is a natural antibiotic that exhibits high affinity to certain metal ions. In order to explore its potential in coordination chemistry, circular dichroism (CD) spectra of monensic acid A (MonH) and its derivatives containing monovalent cations (Li(+) , Na(+) , K(+) , Rb(+) , Ag(+) , and Et4 N(+) ) in methanolic solutions were measured and compared to computational models. Whereas the conventional CD spectroscopy allowed recording of the transitions down to 192 nm, synchrotron radiation circular dichroism (SRCD) revealed other bands in the 178-192 nm wavelength range. CD signs and intensities significantly varied in the studied compounds, in spite of their similar crystal structure. Computational modeling based on the Density Functional Theory (DFT) and continuum solvent model suggests that the solid state monensin structure is largely conserved in the solutions as well. Time-dependent Density Functional Theory (TDDFT) simulations did not allow band-to-band comparison with experimental spectra due to their limited precision, but indicated that the spectral changes were caused by a combination of minor conformational changes upon the monovalent cation binding and a direct involvement of the metal electrons in monensin electronic transitions. Both the experiment and simulations thus show that the CD spectra of monensin complexes are very sensitive to the captured ions and can be used for their discrimination. Chirality 28:420-428, 2016. © 2016 Wiley Periodicals, Inc.

NMR kinetic studies of the ionophore X-537A-mediated transport of manganous ions across phospholipid bilayers

Nuclear magnetic resonance spectroscopy has been applied as a method for studying manganous ions transport across the membrane of phosphatidylcholine vesicles. The rates of the ionophore X-537A (lasalocid A)-mediated Mn2+ transport have been measured as a function of ionophore concentration, pH of the vesicle suspension, and temperature. The translocation was found to occur via a neutral complex composed of one manganous ion bound in two ionized X-537A molecules (Mn X2). The activation energy for the overall transport process was determined to be 22 +/- 5 kcal/mol. Also a pKa of 5.0 +/- 0.2 was determined for the ionophore acid dissociation equilibrium in the vesicle suspension.