2H nuclear magnetic resonance of exchange-labeled gramicidin in an oriented lyotropic nematic phase

Lyotropic nematic liquid-crystalline phases, such as that formed by potassium laurate/decanol/KCl/water, are found to accept readily large amphiphilic solute molecules. Since these phases spontaneously orient in high magnetic fields, it becomes possible to obtain NMR spectra of biologically interesting solutes in an oriented axially symmetric environment. The amide hydrogens of the peptide backbone of gramicidin D (Dubos) were exchanged for deuterium, and the gramicidin was incorporated into a lyotropic nematic phase made with deuteriated buffer in place of water. 2H NMR spectra of oriented, exchange-labeled gramicidin were then obtained. The strong water signal from the deuteriated buffer was eliminated by using selective excitation and a polynomial subtraction procedure. The 2H NMR spectra at high temperature consist of twelve major quadrupolar doublets. The splittings observed are largely independent of temperature, suggesting a highly rigid backbone structure. Two of the doublets, which are chemically shifted relative to the others, show stronger temperature dependence. These two probably arise from the exchangeable amino hydrogens on the tryptophan indole moieties of the peptide. While we cannot yet assign all of the doublets, the spectra and nuclear magnetic relaxation data are consistent with a rigid slightly distorted beta LD6.3 helix undergoing axially symmetric reorientation about the director of the liquid-crystalline phase. The correlation time for the axially symmetric reorientation is determined by relaxation measurements to be about 10(-7) s.