Nitroxide spin probes for magnetic resonance characterization of ordered systems

Abstract

Stable free radicals belonging to different classes have been synthesized in view of potential applications in studies of organized systems. Among them a new pyrroline nitroxide with a high fluorine content, 2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy-3-[(1H,1H-perfluoro-1-octoxy)carbonyl] (3), apt to probe the hydrophobic perfluoro-rich domains inside macroaggregates, has been prepared by a rarely encountered approach namely the spin-labelling of the very cosurfactant participating in the self-assembling process. Emphasis has then been laid upon the synthesis of further stable free radicals from the group of substituted nitronyl nitroxides. A general experimental procedure has been adapted according to the nature of the 2-substituent, resulting in a series of nitronyl nitroxides with a wide range of hydrophobicity and showing spectral parameters in good agreement with literature values. Using the spin probe-electron spin resonance technique, 3 and the 1-H-imidazol-1-yloxy-4,5-dihydro-4,4,5,5-tetramethyl-2-ethyl-3-oxide (7, R=C2H5) or the 1-H-imidazol-1-yloxy-4,5-dihydro-4,4,5,5-tetramethyl-2-undecyl-3-oxide (7, R=C11H23) have been applied in ESR investigations on vesicle phases of a zwitterionic surfactant (TDMAO)/perfluorocosurfactant (PFC) system and on Triton X-100 reverse micelles, respectively. While spin probe 3 proved to be insensitive to differences between the charged and uncharged phases, probes 7 were able to detect changes in the micelle micropolarity induced by hydration. The results corroborate well with previous UV measurements in the Triton X-100 system using the same probes 7.