Characterization of Aqueous Lower Polarity Solvation Shells Around Amphiphilic TEMPO Radicals in Water

Solvation of the stable nitroxide radicals 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) and 4-Oxo-TEMPO (TEMPONE) in water and THF is studied. With electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band as well as spectral simulations, the existence of pure water shells enclosing TEMPO in aqueous solution that lead to significantly reduced local polarity at the nitroxide is shown. These aqueous lower polarity solvation shells (ALPSS) offer TEMPO a local polarity that is similar to that in organic solvents like THF. Furthermore, using double electron-electron resonance (DEER) spectroscopy, local enrichment and inhomogenous distribution without collisions of dissolved TEMPO in water is found that can be correlated with potentially attractive interactions mediated through ALPSS. However, no local enrichment of TEMPO is found in organic solvents such as THF. These results are substantiated by MD and metadynamics simulations and physical methods like DLS and MS.