Stretched Poly(dimethylsiloxane) Gels as NMR Alignment Media for Apolar and Weakly Polar Organic Solvents:  An Ideal Tool for Measuring RDCs at Low Molecular Concentrations

Abstract

The measurement of residual dipolar couplings (RDCs), meanwhile a standard method for obtaining structural information in biomolecular NMR, requires partial alignment of the sample. Special demands on alignment media so far limit the applicability of this approach to small molecules in organic solvents. Major limitations are the free scalability of alignment and the suppression of residual signals of the alignment medium to allow effective measurement of low-concentration samples. Here, we present stretched poly(dimethylsiloxane) (PDMS) cross-linked by beta-rays as an alignment medium with no visible impurities in 1H NMR spectra but a single signal at approximately 0.1 ppm that can easily be removed by slightly modified water suppression methods. Besides the free scalability, its applicability to the measurement of RDCs in small molecules at low concentration is demonstrated on a approximately 12 mM sample of spiroindene. The induced alignment tensor in this case can be predicted reasonably well by a simplified model on the basis of steric interactions only.

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

“…Since silicone rubber swells in most apolar organic solvents (see e.g. [8,24], where silicone is used for gel preparation), it cannot be used inside the stretching apparatus for solvents like chloroform, benzene, or dichloromethane. Tubing made out of fluorinated elastomer materials as e.g.…”

“…Next to classical liquid crystalline phases, mechanically stretched polymer gels form a second class of alignment media [4] which are applicable to a wide range of solutes and solvents [5][6][7][8][9][10][11][12]. Based on gelatin as the polymer gel [13], Kuchel et al recently developed a remarkable apparatus for rapid and reversible gel stretching [14] and demonstrated its usefulness by the distinction of enantiomers using RDCs and other anisotropic parameters [15].…”

Abstracts of oral presentations

“…Yet the requirement for cysteine at the site of ligation in standard NCL has limited its flexibility. Recently, different types of auxiliary groups [2,3] have been developed to extend the application of NCL to other ligation sites. However, the generally slower ligation rates especially with large fragments and the additional step required to cleave the auxiliary post-ligation have reduced their utility.…”

“…Here we present a novel strategy to synthesize proteins through a chemical ligation using unprotected peptide segments. Our scheme does not make use of auxiliary groups [2,3], instead originally exploits the features of some side chain removable functionalities. Ligation rates are high, comparable to NCL and the residues available for ligation are more frequent than cysteine.…”

“…Furthermore, the synthesis of a crossed alkene-bridged mimic of the nisin Z bisthioether ring system will be presented. This ring system could be synthesized in a single metathesis reaction step and the correct side chain connectivities may imply a considerable degree of preorganization of the linear metathesis precursor [2]. In addition, recent results using ring-closing alkyne metathesis on pentapeptides followed by stereoselective reduction of the triple bond will be discussed.…”