DISTINCT NMR for Sign Determination of C–H Dipolar Couplings in Liquid-Crystalline Lipids

A recently introduced DISTINCT technique for the determinator and the B0 field. Comparison of the two equations shows tion of the signs of C–H dipolar couplings is described in detail. that the coupling constant of the averaged dipolar interaction, It is a switching-angle spinning experiment that employs scalar dV d , is the product of the rigid-limit coupling constant dd and couplings to make sign-dependent heteronuclear coherence obthe C–H bond order parameter, which is the averaged term servable. The pulse sequence and the time evolution of the density in Eq. [2] , operator are described, both for proton-detected local field evolution and the carbon-detected separated local field evolution. The dU d Å 12dd »3 cos U 0 1 ... . [3] original 1D DISTINCT experiment has also been developed into a 2D technique to allow the direct observation of antiphase dipolar powder spectra, characteristic of the signs of the dipolar couplings. In a dipolar Pake spectrum observed for isotropically oriThese techniques are demonstrated on a liquid-crystalline lipid, ented domains, this averaged coupling constant corresponds lecithin. q 1995 Academic Press, Inc. to the splitting between the two maxima. A conventional C–H dipolar spectrum is symmetric, since each orientation contributes a pair of lines at /v V d and 0vV d . Thus, for ÉdU dÉ INTRODUCTION £ 12 dd , one cannot distinguish spectra arising from couplings with equal magnitude but opposite signs of dU d . This lack of C– H dipolar couplings can be powerful probes of the knowledge concerning the sign severely restricts the inforstructure, order, and dynamics of lipids, polymers, and other mation about segmental orientation that can be drawn from organic materials. Without the need for isotopic labeling, dU d . For instance, a value of ÉdU dÉ Å 0.45 can come about in these couplings provide information similar to that obtained many ways, while dU d Å 00.45 shows that internuclear vecby H NMR (1–3) . In rigid solids, the dipolar frequency tors are nearly perpendicular to the director at most times. In for a C–H bond at an angle uCH with respect to the B0 field addition to providing the average orientations of internuclear is vectors, the sign of the dipolar coupling constant dU d is crucial for an order-tensor analysis of liquid-crystalline systems. vd Å 12dd(3 cos uCH 0 1), dd Å 0 m0 4p \ gCgH r CH . [1] Traditionally, the signs of dipolar couplings have only been determined in macroscopically oriented liquid-crystalline systems (4–7) , mostly through scaling of the dipolar In uniaxial liquid crystals, motional averaging results in a couplings relative to the scalar J coupling. Recently, we dipolar frequency have presented two switching-angle spinning (SAS) techniques that allow the determination of the sign of the C–H v V d Å 12dd »3 cos U 0 1 ... 1 2 (3 cos 2 b 0 1), [2] dipolar coupling relative to that of the J coupling in an unoriented system (8) . Both techniques involve proton dewhere U denotes the angle between the C–H vector and the tection of local fields (PDLF) in the evolution period, a director ( the unique motional axis) of the domain in which magnetization-transfer step, and C MAS detection. The the segment is located, and b is the angle between the direcfirst relies on comparing the splittings found in 2D spectra with different spinning scaling factors. The second technique, called DISTINCT for dipolar sine term by indirect* To whom correspondence should be addressed. coupling transformation, uses the J coupling to make the † Present address: Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003. sign-dependent sine term of the dipolar coherence observ-