Direct Measurement of Distances and Angles in Biomolecules by NMR in a Dilute Liquid Crystalline Medium

TLDR

In isotropic solution, internuclear dipolar couplings average to zero due to rotational diffusion. The study aims to improve the accuracy of NMR-determined structures and extend the size limit of proteins that can be analyzed. By dissolving macromolecules in a dilute aqueous nematic discotic liquid‑crystalline medium, the authors create a tunable alignment of the solute with the magnetic field while preserving high‑resolution NMR, allowing dipolar couplings between 1H–1H, 1H–13C, 1H–15N, and 13C–13C to be measured. Distances and angles obtained from these dipolar couplings in human ubiquitin match its crystal structure with excellent agreement.

Abstract

In isotropic solution, internuclear dipolar couplings average to zero as a result of rotational diffusion. By dissolving macromolecules in a dilute aqueous nematic discotic liquid-crystalline medium containing widely spaced magnetically oriented particles, a tunable degree of solute alignment with the magnetic field can be created while retaining the high resolution and sensitivity of the regular isotropic nuclear magnetic resonance (NMR) spectrum. Dipolar couplings between 1H-1H, 1H-13C, 1H-15N, and 13C-13C pairs in such an oriented macromolecule no longer average to zero, and are readily measured. Distances and angles derived from dipolar couplings in human ubiquitin are in excellent agreement with its crystal structure. The approach promises to improve the accuracy of structures determined by NMR, and extend the size limit.