Diffusion coefficients of small molecules as guests in various phases of pluronic L64 measured by one-dimensional electron spin resonance imaging

The diffusion coefficients of small nitroxide probes as guests in aqueous s olutions of the triblock copolymer poly(ethylene oxide)-b-poly(propylene ox ide)-b-poly(ethylene oxide) EO13PO30EO13 (Pluronic L64) were measured at 30 0 K by one-dimensional electron spin resonance imaging (1D ESRI). The metho d 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 gradient s and simulation of these spectra in order to extract the diffusion coeffic ient, D. The rate of transport of each probe as a function of polymer conte nt in the various phases (micellar, hexagonal, lamellar, and reverse micell ar) of aqueous L64 depends on the probe location in the self-assembled syst em. The probe site was deduced from the analysis of the ESR spectra; the is otropic hyperfine splitting, a(N), from the N-14 nucleus of the >NO fragmen t in the probes was the polarity sensitive parameter. D values for the cati onic probe 4-(N,N,N-trimethyl)ammonium-2,2,6,6-tetramethyl-piperidine-1-oxy l iodide that is known to reside in the water domains follow the expression D = D-0 exp(-aw(2)), where Do is the diffusion coefficient in the neat sol vent and wt is the weight fraction of the polymer. For the hydrophobic prob e 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 alm ost constant for Lot 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 betwe en water and the EO domains, D decreases with increase in the polymer conte nt, but the decrease is more prominent for wt in the range 0.10-0.30; at w( 2) = 0.90, D is similar to that of the polymer chains. The method of 1D ESR I enables the measurement of D values for guests present in small concentra tions, typically less than or equal to 2 x 10(-8) mol/L. This method and th e results obtained in this study are relevant for assessing both the rate o f transport of drugs in drug delivery systems and the partitioning of vario us guests in complex systems, for instance in biological membranes.