High-speed magic angle spinning solid-state H-1 nuclear magnetic resonance study of the conformation of gramicidin a in lipid bilayers

One- and two-dimensional solid-state H-1 nuclear magnetic resonance spectra of gramicidin A incorporated in a dimyristoylphosphatidylcholine membrane have been obtained with use of high-speed magic angle spinning. By rotating the sample at 13 kHz, it is possible to observe signals in the H-1 spectra between 6.0 and 9.0 ppm attributable to the aromatic protons of the tryptophan residues and the formyl group proton of gramicidin A. Two-dimensional solid-state COSY spectra provided information for the peak assignments. Moreover, changes in the 1H spectra have been observed as a function of the co-solubilization solvent initially used to prepare the samples and therefore as a function of the conformation adopted by gramicidin A. Three organic solvents have been used: trifluoroethanol, a mixture of methanol/chloroform (1:1 v/v), and ethanol. The conformational interconversion of gramicidin A from the double helix conformation to the channel structure for the sample prepared from ethanol was confirmed by following the time evolution of the proton spectra.