Ionic liquid crystals as alignment medium to measure residual dipolar couplings for carbohydrates Murali Dama, Stefan Berger ⇑ Institute of Analytical Chemistry, University Leipzig, Johannisallee 29, D-04103 Leipzig, Germany article info Article history: Received 20 April 2013 Received in revised form 23 May 2013 Accepted 30 May 2013 Available online 10 June 2013 Keywords: Ionic liquid crystals Residual dipolar coupling NMR Glucose abstract Ionic liquids consisting of N-dodecyl-N-methyl pyrrolidinium bromide [C 12 MPB] in a mixture with D2O, decanol, and DMSO were for the first time found to give anisotropic molecular alignment in magnetic fields and are useful to measure residual dipolar couplings (RDCs) from polar analytes, for example, glu- cose. The system shows less quadrupolar splitting of the deuterated solvent signal compared with other liquid crystal systems and hence less undesired line broadening. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Residual dipolar couplings (RDCs) are essential parameters in structural determination of organic biomolecules through NMR Spectroscopy. These RDCs are proving to be very powerful for the constitutional, configurational, and conformational analysis of small molecules. 1 Before RDCs, in addition to the chemical shifts and scalar couplings, the determination of the three-dimensional structure of organic molecules by NMR spectroscopy usually in- volved the measurement of NOEs 2,3 and cross-correlated relaxa- tion 4 to obtain information about dihedral angles, distances, and projection angles, respectively. 5 However these NMR parameters provide only localized structural information and they fail to pro- vide the relative configuration of remotely located molecular fragments. 6 RDCs can enable the assignment of relative configuration in addition to the NOE due to their r À3 dependence. In solid state dipolar couplings are in the kHz range, in solution state they are averaged to zero. By partially orienting the corresponding analytes in the magnetic field residual dipolar couplings are observed. To force the molecules to adapt a minor degree of orientation an anisotropic medium is required. 7 The size of the dipolar couplings can be reduced by using an ori- enting medium that induces a low degree of order (order parame- ter S <10 À3 ). By inducing a low degree of order the anisotropic NMR parameters are significantly averaged, but not entirely. The size of the dipolar coupling is reduced tremendously, such that dipolar couplings are usually smaller than scalar couplings and the spec- tral quality of high-resolution NMR spectra is retained. For water soluble biomolecules, liquid crystals, 8 bicelles, 9 mi- celles, 10,11 bacteria phages, 12,13 and paramagnetic lanthanide tags 14,15 have been developed as anisotropic media to induce the magnetic orientation. Small organic molecules are usually dissolved in organic solvents and hence, for using the alignment approach, it is necessary to find a liquid crystalline alignment med- ium based on an organic solvent. 16 Stretched polymer gels, 17 liquid crystals such as Poly-c-benzyl-L/D-glutamate (PBLG/PBDG), 18,19 Poly-c-ethyl-L/D-glutamate (PELG/PEDG), 20,21 Polyisocyanides, 22 and Polyacetylenes 23 have been developed as alignment media for non polar organic molecules. For polar organic molecules, stretched polymer gels such as cross-linked poly(vinylacetate), 24 cross-linked poly(acrylamide), 25 cross-linked poly(acrylonitrile), 26 and lyotropic liquid crystalline phases of mixtures of C 12 E 5 / n-hexanol in mixtures of DMSO/D 2 O (1:1) 16 were developed as alignment media. Sample preparation to measure RDCs in stretched polymer gels will take 2–8 weeks depending on the suitable anisotropic med- ium. Polystyrene gels need up to 4 weeks, based on our experience with these gels. Cross linked PBLG gels need 1–2 months to equil- ibrate the polymer sticks and 7–10 days for sample diffusion. 27 Poly(dimethylsiloxane) gels are reported to need 12 days to equil- ibrate the polymer sticks. 28 In the system described below no incu- bation time is needed. Extraction of RDCs in liquid crystal alignment media happens to be complicated, due to the relatively large degree of orientation in these media and since the systems become unaligned below a cer- tain LC concentration. 29–31 Here we study another liquid crystal 0008-6215/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carres.2013.05.017 ⇑ Corresponding author. Tel.: +49 341 9736100; fax: +49 341 9736115. E-mail address: stberger@rz.uni-leipzig.de (S. Berger). Carbohydrate Research 377 (2013) 44–47 Contents lists available at SciVerse ScienceDirect Carbohydrate Research journal homepage: www.elsevier.com/locate/carres