Dynamic Nuclear Polarization Enhanced Nuclear Magnetic Resonance and Electron Spin Resonance Studies of Hydration and Local Water Dynamics in Micelle and Vesicle Assemblies

Abstract

We present a unique analysis tool for the selective detection of local water inside soft molecular assemblies (hydrophobic cores, vesicular bilayers, and micellar structures) suspended in bulk water. Through the use of dynamic nuclear polarization (DNP), the (1)H NMR signal of water is amplified, as it interacts with stable radicals that possess approximately 658 times higher spin polarization. We utilized stable nitroxide radicals covalently attached along the hydrophobic tail of stearic acid molecules that incorporate themselves into surfactant-based micelle or vesicle structures. Here, we present a study of local water content and fluid viscosity inside oleate micelles and vesicles and Triton X-100 micelles to serve as model systems for soft molecular assemblies. This approach is unique because the amplification of the NMR signal is performed in bulk solution and under ambient conditions with site-specific spin labels that only detect the water that is directly interacting with the localized spin labels. Continuous wave (cw) electron spin resonance (ESR) analysis provides rotational dynamics of the spin-labeled molecular chain segments and local polarity parameters that can be related to hydration properties, whereas we show that DNP-enhanced (1)H NMR analysis of fluid samples directly provides translational water dynamics and permeability of the local environment probed by the spin label. Our technique therefore has the potential to become a powerful analysis tool, complementary to cw ESR, to study hydration characteristics of surfactant assemblies, lipid bilayers, or protein aggregates, where water dynamics is a key parameter of their structure and function. In this study, we find that there is significant penetration of water inside the oleate micelles with a higher average local water viscosity (approximately 1.8 cP) than in bulk water, and Triton X-100 micelles and oleate vesicle bilayers mostly exclude water while allowing for considerable surfactant chain motion and measurable water permeation through the soft structure.

Dynamic nuclear polarization of 13C in aqueous solutions under ambient conditions

“…The coupling factor strongly depends on the translational correlation time, s t , between the radical and nucleus of interest. The coupling factor decreases with increasing correlation times [1,18], and previous reports have shown that the dipolar interaction for water [19] and small organic molecules [16] with organic radicals is modulated mainly by translational motion, which leads us to discuss our results in terms of translational instead of rotational correlation. The translational correlation time is a function of the distance of closest approach, d, between the nucleus and radical and their respective diffusion coefficients, D I and D S [1]:…”

On the surface properties of oleate micelles and oleic acid/oleate vesicles studied by spin labeling

“…At [oleic acid + oleate] = 25 mM and above pH 10, both lipophilic TEMPO-spin labels are fully incorporated into the micellar structures, because the uptake of free oleate by the micelles is significantly faster (with a correlation time <10 −9 s) as compared to the release of oleate from the micelles into the bulk solution (with a correlation time >10 −4 s) (Fukuda et al, 2001). Fast surfactant diffusion from water into micelles implies that the water exchange into and out of the micelles is even faster, with lipid to water association lifetimes of the order of <100 ps (McCarney et al, 2008).…”

“…ESR spectra of the spin-labeled oleic acid/sodium oleate system with lipophilic spin labels, characterized by anisotropic motions were simulated with the program "Chili" (Stoll and Schweiger, 2006). The spectra of ASL (the water soluble spin label) characterized by fast isotropic motion were simulated with "Garlic" of the EasySpin 3.1.1. software package (Stoll and Schweiger, 2006;McCarney et al, 2008). Best fit of the spectra was obtained by the least-square fitting method.…”

L-band Overhauser dynamic nuclear polarization

“…For 0.35 T, experimental enhancements of up to À140 have been reported by Höfer et al [21]. A novel assortment of experiments that uniquely and sensitively probe the internal water dynamics of lipids, proteins and coacervates in the 0.1-0.5 ns correlation timescale has been successfully carried out at 0.35 T [20,[27][28][29]. Further explanation of this time scale is provided in Section 2.…”