Precise Measurement of RDCs in Water and DMSO Based Gels Using a Silicone Rubber Tube for Tunable Stretching

Abstract

Abstract:Residual dipolar couplings (RDCs) are of great interest for the structure determination of biomacromolecules as well as of organic molecules like synthetic or natural products. Their accurate measurement requires a proper degree of alignment for the molecule under investigation. As has been shown recently, a stretching device based on a flexible silicone rubber tube provides an easy, rapid, and reversible variation of alignment strength. We show in this article that such an apparatus is not limited to gelatin, but can also be used with covalently cross-linked hydrogels and even gels with polar organic solvents like DMSO. Using sucrose and a cyclic hexapeptide, we were able to demonstrate that the approximately linear relation of alignment with the extension factor of the stretched gel allows the measurement of RDCs with high precision if corresponding spectra are acquired at various alignment strengths. The method seems to be widely applicable to the structure determination of molecules of arbitrary size using standard 5 mm NMR equipment.

Structures of Storage‐Induced Transformation Products of the Beer’s Bitter Principles, Revealed by Sophisticated NMR Spectroscopic and LC–MS Techniques

“…To determine the relative chirality of these centers, a number of experiments were performed to measure residual dipolar couplings (RDCs), which recently enabled the successful prochiral assignment [30][31][32][33] and configurational analysis [34][35][36][37][38][39][40] of several organic compounds. Two samples of tricyclocohumol (4 a) were prepared; one was dissolved in [D 6 ]DMSO, and the other one was diffused into a poly(acrylonitrile)/ [D 6 ]DMSO gel, [41][42][43] which was then stretched using the method reported recently [44,45] to obtain partial alignment of the solute. Scalar H)) coupling constants were measured for the two samples using the clean inphase (CLIP)-HSQC experiment [46] ( Figure 4C).…”

Tunable Alignment for All Polymer Gel/Solvent Combinations for the Measurement of Anisotropic NMR Parameters

“…Since the prediction of the orientational order for small organic molecules is still not easily achieved, this frequently leads to unnecessary and time-consuming sample preparations and eventually to a corresponding loss of valuable material. Only with the introduction of a stretching apparatus by the Kuchel group [4,5] the situation changed and it is now possible to rapidly and reversibly adjust the degree of orientation within a single sample using standard liquid state NMR equipment.…”

“…While the original stretching apparatus can be used with gelatin [4,6] and other gels containing polar solvents such as D 2 O and DMSO, [5] it is incompatible with more apolar organic solvents such as chloroform, dichloromethane, or tetrahydrofuran. The silicone tube as the main part of the device also allows water and humidity to penetrate into the gel sample [5] and leads to an intense, broad signal around 0.1 ppm in corresponding 1 H NMR spectra.…”

“…The silicone tube as the main part of the device also allows water and humidity to penetrate into the gel sample [5] and leads to an intense, broad signal around 0.1 ppm in corresponding 1 H NMR spectra. It is therefore of very limited use for the analysis of most organic compounds.…”

“…Following our design of the stretching device equipped with a silicone tube, [5] the flexible perfluorinated elastomer tube is placed inside a cut-open 5 mm NMR tube and fixed at the bottom with a specially designed screw clamp made out of PCTFE (Figure 1). The elastomer tube works as an expandable container for the swollen gel with the solute molecule inside.…”