The conformation of gramicidin A in dimethylsulphoxide solution. A full analysis of the one- and two-dimensional 1H, 13C, and 15N nuclear-magnetic-resonance spectra.

The combined application of one- and two-dimensional high-field NMR techniques has led to the first assignment of the 1H, 13C, and 15N spectra of the pentadecapeptide gramicidin A in dimethylsulphoxide solution. The 62.9-MHz and 100.6-MHz 13C spin-lattice relaxation times and 13C-{1H} NOE factors for the backbone α carbons have been analysed in the ‘model-free’ approach to give a single correlation time (τm) for isotropic overall molecular motion and an order parameter and internal correlation time for each CαH 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. The average values of the vicinal HNCαH couplings are 7.4 Hz and 8.4 Hz respectively for the alternate l- and d-amino acid residues. The values are not consistent with either a ribbon conformation for the backbone or a right-handed β6.3 helix; they are consistent with the model proposed by Glickson et al. [Glickson, J. D., Mayers, D. F., Settine, J. M. & Urry, D. W. (1972) Biochemistry 11, 477–486] in which there is a rapid conformational order ⇌ disorder equilibrium, the ordered structure being the left-handed β6.3 helix and the disordered state having local random-coil character.