Diffusion Coefficients of Small Molecules as Guests in Various Phases of Pluronic L64 Measured by One-Dimensional Electron Spin Resonance Imaging

Abstract

The diffusion coefficients of small nitroxide probes as guests in aqueous solutions of the triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) EO13PO30EO13 (Pluronic L64) were measured at 300 K by one-dimensional electron spin resonance imaging (1D ESRI). The method is based on encoding the spatial distribution of the probes as a function of time in ESR spectra recorded in the presence of magnetic field gradients and simulation of these spectra in order to extract the diffusion coefficient, D. The rate of transport of each probe as a function of polymer content in the various phases (micellar, hexagonal, lamellar, and reverse micellar) of aqueous L64 depends on the probe location in the self-assembled system. The probe site was deduced from the analysis of the ESR spectra; the isotropic hyperfine splitting, aN, from the 14N nucleus of the >NO fragment in the probes was the polarity sensitive parameter. D values for the cationic probe 4-(N,N,N-trimethyl)ammonium-2,2,6,6-tetramethyl-piperidine-1-oxyl iodide that is known to reside in the water domains follow the expression D = D0 exp(−aw2), where D0 is the diffusion coefficient in the neat solvent and w2 is the weight fraction of the polymer. For the hydrophobic probe 5DSE, the methyl ester of doxylstearic acid where 5 indicates the carbon atom to which the doxyl group is attached, D is significantly lower and almost constant for w2 in the range 0.20−0.80. For the probe perdeuterio-2,2‘,6,6‘-tetramethyl-piperidone-N-oxide that is located at the interface between water and the EO domains, D decreases with increase in the polymer content, but the decrease is more prominent for w2 in the range 0.10−0.30; at w2 = 0.90, D is similar to that of the polymer chains. The method of 1D ESRI enables the measurement of D values for guests present in small concentrations, typically ≤2 × 10-3 mol/L. This method and the results obtained in this study are relevant for assessing both the rate of transport of drugs in drug delivery systems and the partitioning of various guests in complex systems, for instance in biological membranes.