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

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.