Structure of alamethicin in solution: nuclear magnetic resonance relaxation studies

An NMR relaxation study at 500 MHz of the icosapeptide antibiotic alamethicin is reported. This study lends further support to the partly helical, partly extended, amphiphilic, and dimeric structure recently proposed for this peptide in methanolic solutions [Banerjee, U., Tsui, F. P., Balasubramanian, T. N., Marshall, G. R., & Chan, S. I. (1983) J. Mol. Biol. 165, 757]. The N-acetyl methyl groups toward the N terminus of alamethicin in this solvent system were found to exhibit unusual NMR relaxation behavior. The decay of the transverse magnetization due to these protons was nonexponential, but the spin-lattice relaxation recovery of the longitudinal magnetization was exponential. In a solution saturated with urea, however, both decays were exponential. These observations are shown to be consistent with the proposed structure. Studies in water yielded qualitatively similar but more complex results. The transverse relaxation times suggest further aggregation in water and indicate that the larger aggregates in water may be made up of the smaller units observed in methanol.