Dynamic Nuclear Polarization Studies of Local Water Dynamics in Soft Molecular Assemblies at 9.8 GHz

Abstract

We demonstrate that the application of dynamic nuclear polarization (DNP) at X-band on 1H nuclear magnetic resonance (NMR) signal of water under ambient condition, in combination with continuous-wave (cw) electron spin resonance (ESR), is a new and potentially powerful approach to study hydration and fluid dynamics of soft macromolecular assemblies. The source to amplify 1H NMR signal is stable nitroxide radicals, also known as spin labels, that serve as reporter species to illuminate the interaction between the spin-label-containing molecular site and water. The efficiency of DNP-induced 1H NMR signal amplification through the spin label reporter is directly related to the micro-viscosity of the water that interacts with the spin label. This information is complementary to the ESR measurement of rotational dynamics of the spin label or the spin-labeled molecular segment. Our combined 1H DNP and ESR approach is particularly useful for the analysis of soft macromolecular assemblies including micelles, vesicles, membrane materials or protein aggregates, where the study of internal hydration dynamics cannot be easily performed with other techniques. 1H NMR signal amplification factors of more than −130-fold at 0.35 T were predicted and experimentally achieved when employing sufficient microwave power at the resonant ESR frequency of 9.8 GHz. The X-band DNP tool can be relatively easily implemented by any interested researcher who has access to a standard cw ESR spectrometer and a single NMR channel at about 15 MHz frequencies.