Fast Dynamics of Difluprednate in Micelles or Swollen-Micelles Revealed by 19F Nuclear Magnetic Resonance Spin Relaxation Rates.

Knowledge of molecular rotational dynamics is critical to interpret solution nuclear magnetic resonance (NMR) spectroscopy. The observation of sharp solute NMR signals in micelles contradicted the surfactant viscosity effects noted in the Stokes-Einstein-Debye (SED) equation. Herein, the 19F spin relaxation rates of difluprednate (DFPN) drug dissolved in polysorbate-80 (PS-80) micelles and castor oil swollen micelles (s-micelle) were measured and adequately fit using an isotropic diffusion model based spectral density function. Despite the high viscosity of PS-80 and castor oil, the fitting results revealed fast 4 and 12 ns dynamics of DFPN in both micelle globules. The observation of the fast ns motion in the viscous surfactant/oil micelle phase demonstrated motion decoupling between solute molecules inside micelles and the micelle itself in an aqueous solution. These observations support the role of intermolecular interaction in governing the rotational dynamics of small molecules, versus the viscosity of the solvent molecules as defined in the SED equation.