Characterization of Aqueous Lower-Polarity Solvation Shells Around Amphiphilic 2,2,6,6-Tetramethylpiperidine-1-oxyl Radicals in Water.

Abstract

Solvation of the amphiphilic nitroxide radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and hydrophilic 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPONE) in water and tetrahydrofuran (THF) is studied in detail. The existence of pure water shells enclosing TEMPO in an aqueous solution that leads to significantly reduced local polarity at the nitroxide moiety is shown with multifrequency electron paramagnetic resonance (EPR) spectroscopy at X- and Q-bands as well as spectral simulations. 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 spectroscopy, local enrichment and inhomogeneous distribution without direct molecular encounters of dissolved TEMPO in water are 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. In contrast to TEMPO, the structurally very similar nitroxide radical TEMPONE shows no ALPSS encapsulation behavior with water molecules in aqueous solutions. Ensemble-averaging methods such as dynamic light scattering and electrospray ionization mass spectrometry substantiate the EPR spectroscopically obtained results of ALPSS-encased TEMPO and attractive interactions between them, leading to a higher local concentration. Furthermore, force field molecular dynamics simulations and metadynamics deliver support for our conclusions.