The binding affinity
between antibiotic ionophores and alkali ions
within supported lipid bilayers was evaluated using affinity chromatography.
We used zonal elution and frontal analysis methods in nanovolume liquid
chromatography to characterize the binding selectivity of the carrier
and channel ionophores valinomycin and gramicidin A within different
phosphatidylcholine bilayers. Distinct binding sensitivity to the
lipid phase, both in affinity and selectivity, is observed for valinomycin,
whereas gramicidin is less sensitive to changes in a membrane environment,
behavior that is consistent with ion binding occurring within the
interior of an established channel. There is good agreement between
the chromatographic retention and the reported binding selectivity
measured by other techniques. Surface potential near the binding site
affects ion retention and the apparent association binding constants,
but not the binding selectivity or enthalpy measurements. A model
accounting for the surface potential contributions of retained ions
during frontal analyses yields values close to intrinsic binding constants
for gramicidin A (K
A for K+ between 70 and 120 M–1) using reasonable estimates
of the initial potential that is postulated to arise from the underlying
silica.
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