Synthesis and the micellar characteristics of poly(ethylene oxide)-deoxycholic acid conjugates

Novel polymeric amphiphiles were synthesized on the basis of poly(ethylene oxide) (PEO) as a hydrophilic segment and deoxycholic acid (DC) as a hydrophobic segment. Their micellar formation and characteristics in an aqueous phase were investigated by using H-1 NMR analysis, fluorescence techniques, light scattering, and transmission electron microscopy (TEM). The PEG-DC conjugates formed micelles in the aqueous phase with critical micelle concentrations (cmc's) in the range of 36-43 mg/L at 25 degrees C. The cmc values became higher at higher temperatures. The cmc's of the conjugates were much lower than that of the deoxycholic acid sodium salts. The mean diameters of the micelles were in the range of 120-180 nm, with a narrow size distribution. The TEM image showed micelles with a spherical shape. The partition equilibrium constants, K-v, of pyrene in the micellar solutions of the PEG-DC conjugate were 1.8 x 10(4) to 7.4 x 10(4) at 25 degrees C. The K-v value decreased as the temperature increased. The steady-state fluorescence anisotropy values (r) of 1,6-diphenyl-1,3,5-hexatriene (DPH) were 0.196-0.214 in PEG-DC solutions. The anisotropy values were not significantly dependent on the length of the hydrophilic PEO block. The fluorescence lifetime values for DPH in the PEG-DC conjugate solutions were in the range of 6.05-7.03 ns, and indicated that the core region of the micelles consisted of two regions with different hydrophobicity.