Studies of hyperfine shifts of lipid 31P resonances due to hydrated phospholipid inverted micelles in benzene are presented. Systems with distinct resonances from micelles containing no paramagnetic ions, and from micelles containing a single praseodymium(III) or a single europium(III) ion (and three nitrate counterions) have been generated. The addition of an ionophoric antibiotic from Streptomyces lasaliensis, lasalocid-A (X537A). causes both resonances to broaden and. with further additions, coalesce and eventually resharpen as a single line. Dilution of only the ionophore reverses these spectral changes. This is interpreted as a manifestation of dynamic NMR (DNMR. exchange broadening): i.e., that the ionophore catalyzes the equilibrium exchange of metal ions from micelle to micelle to the point where it becomes fast on the NMR time scale. This exchange is inhibited by protons or other competitive metal ions. We have simulated the spectra with a total lineshape analysis program and have thus extracted the average preexchange lifetimes for various concentrations of the antibiotic. We find a reasonably good first-order dependence on lasalocid-A concentration in each of several different experiments. This is in contrast to the higher order concentration dependences often observed by others using different techniques employing bilayer membranes. We favor a diffusional carrier mechanism involving surface aggregates of lasalocid-A for our process. This leads to the implication that a higher order concentration dependence found for a bilayer system arises from a different mechanism. The ionophore valinomycin does not catalyze rapid exchange in our system.
ASJC Scopus subject areas
- Organic Chemistry