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The combined application of one-and two-dimensional high-field NMR techniques has led to the first assignment of the 'H, 13C, and I5N spectra of the pentadecapeptide gramicidin A in dimethylsulphoxide solution. The 62.9-MHz and 100.6-MHz '."C spin-lattice relaxation times and 13C-{1H} NOE factors for the backbone a carbons have been analysed in the 'model-free' approach to give a single correlation time (7,) for isotropic overall molecular motion and an order parameter and internal correlation time for each C"H group in the backbone. The relatively high and constant values for the order parameter along the backbone indicate a degree of ordering of the structure, while the internal correlation times show that internal motions are progressively more rapid towards the N terminus. Veatch and Blout [21] relied largely upon CD spectra in proposing that gramicidin A in organic solvents probably exists as interconverting intertwined helical dimers, although these CD studies did not include (CH&SO as solvent due to its absorption in the wavelength region of interest. '."C-NMR relaxation rates were interpreted by Fossel et al. [8] as indicating that gramicidin A is a structured dimer in methanol solution, but a random-coil monomer in (CH3)2S0 solution. Rondelez and Litster [12] used light-scattering studies to show that the peptide exists in a dimeric state in methanol and dioxane solutions and as a monomer in (CH3)2S0 solution at concentrations in the range 18 -50 mg/ml. Recent work by Arseniev et al. [7] used the inter-proton nuclear Overhauser enhancement (NOE) effect in the 'H-NMR spectra of gramicidin A to show an anti-parallel n,z?$ double-helical conformation for the dimeric species in dioxane solution.

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