Local ordering and dynamics in anisotropic media by magnetic resonance: from liquid crystals to proteins

ABSTRACT

Magnetic resonance methods have been used extensively for over 50 years to elucidate molecular structure and dynamics of liquid crystals (LCs), providing information quite unique in its rigour and extent. The ESR- or NMR-active probe is often a solute molecule reporting on characteristics associated with the surrounding (LC) medium, which exerts the spatial restrictions on the probe. The theoretical approaches developed for LCs are applicable to anisotropic media in general. Of particular interest is the interior space of a globular protein labelled, e.g. with a nitroxide moiety or a ¹⁵N–¹H bond. The ESR or NMR label plays the role of the probe and the internal protein surroundings the role of the anisotropic medium. A general feature of the restricted motions is the local ordering, i.e. the nature, magnitude and symmetry of the spatial restraints exerted at the site of the moving probe. This property is the main theme of the present review article. We outline its treatment in our work from both the theoretical and the experimental points of view, highlighting the new physical insights gained. Our illustrations include studies on thermotropic (nematic and smectic) and lyotropic liquid crystals formed by phospholipids, in addition to studies of proteins 71.

Graphical Abstract

KEYWORDS:

• Anisotropic media
• local ordering
• stochastic liouville equation
• slowly relaxing local structure
• microscopic-order-macroscopic-disorder

Acknowledgments

It is with great sadness that we acknowledge the passing away of Prof. Zeev Luz of the Weizmann Institute, Rehovot, Israel. Zeev Luz made significant contributions to the study of the dynamic structure of molecules in anisotropic media by NMR and ESR. He pioneered, and uniquely promoted, the application of ²H NMR to liquid crystals. His death deprives us of a highly esteemed scientist and much-valued mentor, teacher and friend. The entire Magnetic Resonance community mourns his loss.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the NIH under Grants P41GM103521 and R01GM123779 to J.H.F.; the ISF under Grant 369/15 to E.M.; and the BSF under Grant 2016097 to E.M. and J.H.F; the National Institutes of Health [P41GM103521, R01GM123779]; Israel Science Foundation [369/15, Israel-USA Binational Science Foundation]; United States- Israel Binational Science Foundation [2016097].