Stretched Gelatin Gels as Chiral Alignment Media for the Discrimination of Enantiomers by NMR Spectroscopy

Abstract

NMR in gummi bears: What started as pure fun could open up interesting applications: stretched gelatin gels as chiral polymers can be used to distinguish enantiomeric solutes through their residual dipolar couplings. The method is demonstrated on a mixture of L‐ and D‐alanine (see picture).

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

“…Long-range 1 H, 13 C and onebond 13 C, 13 C RDCs, for example, are approximately one order of magnitude smaller than corresponding heteronuclear 1 H, 13 C one-bond couplings. With the stretching apparatus it should be possible to tune the size of RDCs in such a way that optimal conditions for their measurement are obtained.…”

“…After preparing the polymer gel samples in the stretching apparatus, we decided to measure heteronuclear 1 H, 13 C onebond couplings that contain the most easily accessible RDCs. For their measurement we used the so-called CLIP-HSQC pulse sequence, a t 2 -coupled HSQC with pure absorption lineshapes [16].…”

“…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].…”

Chiral recognition of ethers by NMR spectroscopy

“…87 Other approaches exploit guest-host interactions by accommodating one enantiomer better in a given supramolecular cavity than the other. Examples are cyclodextrins, 24,103-108 chiral crown ethers, and calixarenes but also liquid crystalline polypeptides [109][110][111][112] embedding the substrate in a partially ordered array (chiral hosting auxilia- ries, CHA). Here, it is the mere shape of the chiral host rather than its polarity or ability to form binding interactions which generates discrimination.…”

“…109,110 In Courtieu's experiments, 109 chiral substrate molecules experience a differential ordering effect by interacting with poly-g-benzyl-L-glutamate (PBLG), and the enantiomers respond by differences in the NMR observables, such as chemical shift anisotropies and residual dipolar couplings. This method can be applied successfully to low-polarity 111 In another report, the authors investigated partially deuterated ethers, oxiranes, alcohols, esters, tosylates, chlorides, bromides, and hydrocarbons; they found excellent signal dispersions in 1 H-decoupled 2 H NMR spectra.…”