Kinetics of monensin complexation with sodium ions by 23NaNMR spectroscopy

The kinetics of the sodium binding to the ionophore monensin (Mon) in methanol has been studied by 23NaNMR spectroscopy. Fast quadrupole relaxation of the bound sodium affected the relaxation rate of the free sodium through an exchange process between these two species. The exchange was found to be dominated by the reaction: Na+ + Mon-in equilibrium MonNa. The dissociation rate constant at 25 degrees C is 63 s(-1), with an activation enthalpy of 10.3 kcal/mol and activation centropy of -15.8 cal/mol deg. These results indicate that the specificity of the binding of sodium ions to monensin is reflectied in the relatively slow dissociation process. The entropy changes indicate that the activated monensin-sodium complex undergoes a conformational change, but the existence of a conformational change in monensin anion prior to complexation is excluded.