THE CONFORMATION OF AN ALAMETHICIN IN METHANOL BY MULTINUCLEAR NMR-SPECTROSCOPY AND DISTANCE GEOMETRY SIMULATED ANNEALING

The solution conformation of the antibiotic peptide alamethicin was investigated using multinuclear spectroscopy and the distance geometry/simulated annealing algorithms from the program DSPACE. H-1-, C-13-, and N-15-nmr chemical shifts and homonuclear H-1 coupling constants suggest that the molecule is flexible in the vicinity of Gly-11 and Leu-12. The temperature dependence of the amide proton chemical shifts indicates that there is flexibility in the middle of the 20 residue peptide and provides evidence that, at the very N-terminus, the molecule adopts a 3(10)-helical conformation. The large differences in the C-13 chemical shifts of the pro-R and pro-S methyls of the alpha-aminoisobutyric acid residues were used to constrain those residues to the right-handed helical conformation in the distance geometry/simulated annealing algorithms. A family of 24 structures was generated but did nor converge to a common conformation when superimposed over the entire polypeptide sequence. The molecules did converge to a helical conformation over residues 1-10 and residues 13-18. The lack of convergence when the entire lengths of the molecules are superimposed is explained by the flexibility of the peptide near Gly-11/Leu-12. The results suggest that the protein consists of two helices connected by a flexible ''hinge.'' The flexibility of the molecule is discussed with respect to the macrodipole model of voltage gating. (C) John Wiley & Sons, Inc.