title: Dipyridamole Interacts with the Polar Part of Cationic Reversed Micelles in Chloroform: 1H NMR and ESR Evidence
abstract: The interaction of dipyridamole (DIP) with reversed micelles (RM) of cetyltrimethylammonium chloride (CTAC) in CDCl3 at different water contents was studied. The position and T1 relaxation of the water peak upon addition of extra water revealed three concentration ranges of CTAC: <10 mM (impurity water is mainly dispersed in CDCl3), >50-100 mM (water mainly inside the RM), and intermediate range. The resonances of CTAC protons in the polar layer broadened and displaced by up to 0.07 ppm as a function of CTAC concentration and extra water. At 10 mM CTAC, the addition of 40 mM DIP shifted the head group signals to high field by about 0.1 ppm. At high and intermediate CTAC concentrations, four nitroxide spin probes, hydrophobic 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), hydrophilic 4-amine-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPAMINE), and lipophilic 5- and 16-doxyl stearic acids (5- and 16-DSA), underwent partial immobilization. The rotational correlation time of TEMPAMINE (rather than TEMPO, 5-, and 16-DSA) in RM moderately increased upon addition of 1.5-2.0 mM DIP. At an excess of CTAC, only one DIP peak at 3.88 ppm remained measurable, and its selective T1 fell from 0.34 to 0.12 s. The association constant for DIP and CTAC was between 10 and 35 M-1. Thus, DIP incorporates into the polar region of RM influencing packing and dynamics of surfactant head groups. In contrast, in aqueous CTAC micelles, the preferential localization of DIP substituents is inside the nonpolar micelle core, and the binding constant is two orders of magnitude above that for RM.
authors.affiliation: Instituto de Quimica de Sao Carlos, Universidade de Sao Paulo, Sao Carlos, SP, 13560-970, Brazil
authors.initials: VE
authors.lastname: Yushmanov
authors.initials: M
authors.lastname: Tabak