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 between1H-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.

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

Mixtures of long-chain and short-chain phosphatidylcholine (PC) were characterized by multinuclear (13C, 31P, 2H) solid-state nuclear magnetic resonance. This work complements and extends previous characterization of such mixtures by focusing on concentrated mixtures at temperatures above the gel to liquid crystalline phase transition temperature (Tm) of the long-chain PC component. Above Tm it was observed that highly oriented, bilayer-like assemblies could be formed of mixtures of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC) in molar ratios ranging from approximately 1:3.5 to 1:2 (DHPC:DMPC) over a considerable range of lipid concentrations (at least 3-40% w/v total lipid, for a 1:2.5 sample). Orientation was observed to occur only in an L alpha-like phase. The NMR data can be accounted for by a general model of the DHPC-DMPC aggregates in which DHPC can be found in two distinct populations (one highly ordered, one not). The averaged conformations of the glycerol backbone/headgroup regions of the long- and short-chain PC composing the assemblies were judged by solid-state 13C NMR to be similar to each other. The information gleaned about these mixtures and the quality of the oriented NMR spectra obtained suggest that DHPC-DMPC mixtures may prove to be useful as model membrane media in solid-state NMR studies of biomembranes.

http://www.cs.duke.edu/brd/Teaching/Bio/asmb/Papers/NMR/Residual-dipolar-coupling/Spinning/bi00152a029.pdf

Characterization of magnetically orientable bilayers in mixtures of dihexanoylphosphatidylcholine and dimyristoylphosphatidylcholine by solid-state NMR.

2. Theoretical treatment of dipolar interactions 376 2.1 Anisotropic interactions as probes of macromolecular structure and dynamics 376 2.1.1 The dipolar interaction 376 2.1.2 Averaging in the solution state 377 2.2 Ordering of a rigid body 377 2.2.1 The Saupe order tensor 378 2.2.2 Orientational probability distribution function 380 2.2.3 The generalized degree of order 380 2.3 Molecular structure and internal dynamics 381

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0033583500003656

NMR structures of biomolecules using field oriented media and residual dipolar couplings.

Magnetically-oriented phospholipid micelles as a tool for the study of membrane-associated molecules

A molecular mechanical force field is described for the conformational analysis of oligosaccharides. This force field has been derived by the addition of new parameters to the AMBER force field and is compatible with simulations of proteins. This new parametrization is assessed by comparison of the theoretically predicted conformations of Man alpha 1-3Man beta 1-4GlcNAc with the corresponding crystal structure. Molecular dynamics simulation data are presented for this structure both in vacuo and with the explicit inclusion of water molecules. While the former demonstrate significant torsional oscillations about glycosidic linkages at physiological temperature, in the latter these oscillations are highly damped due to the stabilizing influence of a "cage" of solvent-solvent and solvent-solute hydrogen bonds.

A molecular mechanical force field for the conformational analysis of oligosaccharides: comparison of theoretical and crystal structures of Man alpha 1-3Man beta 1-4GlcNAc.

Magnetic field induced ordering of bile salt/phospholipid micelles: new media for NMR structural investigations

We present here a protocol for the determination of oligosaccharide solution conformation from a combination of molecular mechanics calculations and NMR distance constraints treated as pseudoenergies. As an illustration of our methodology we have chosen the determination of the solution conformation of the tetrasaccharide headgroup of the glycolipid globoside. In order to test the ability of our methodology to avoid becoming trapped in local minima, we have chosen three starting structures, well displaced from one another in conformational space. The structures obtained upon convergence of the calculations with distance constraint pseudoenergies were quite similar to one another. For two of the three glycosidic linkages in globoside, the results from the calculations were virtually identical for each of the three starting structures. We also apply our protocol to a model which allows for the existence of multiple conformers in an effort to explore the possibility of conformational flexibility in the oligosaccharide headgroup of globoside.

Preetha Ram

Papers

Magnetic field induced ordering of bile salt/phospholipid micelles: new media for NMR structural investigations

A molecular mechanics‐NMR pseudoenergy approach to the solution conformation of glycolipids

Molecular Mechanics NMR Pseudoenergy Protocol To Determine Solution Conformation of Complex Oligosaccharides