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Antiamoebin I in Methanol Solution: Rapid Exchange between Right‐Handed and Left‐Handed 310‐Helical Conformations

Antiamoebin I (Aam‐I) is a membrane‐active peptaibol antibiotic isolated from fungal species belonging to the genera Cephalosporium, Emericellopsis, Gliocladium, and Stilbella. Antiamoebin I has the amino acid sequence: Ac‐Phe1‐Aib‐Aib‐Aib‐Iva‐Gly‐Leu‐Aib8‐Aib‐Hyp‐Gln‐Iva‐Hyp‐Aib‐Pro‐Phl16. By using the uniformly 13C,15N‐labeled sample of Aam‐I, the set of conformationally dependent J couplings and 3hJNC couplings through H‐bonds were measured. Analysis of these data along with the data on magnetic nonequivalence of the 13Cβ nuclei (Δδ(13Cβ)) in Aib and Iva residues allowed us to draw the univocal conclusion that the N‐terminal part (Phe1–Gly6) of Aam‐I in MeOH solution is in fast exchange between the right‐handed and left‐handed 310‐helical conformations, with an approximately equal population of both states. An additional conformational exchange process was found at the Aib8 residue. The 15N‐NMR‐relaxation and CD‐spectroscopy measurements confirmed these findings. Molecular modeling and Monte Carlo simulations revealed that both exchange processes are correlated and coupled with significant hinge‐bending motions around the Aib8 residue. Our results explain relatively low activity of Aam‐I with respect to other 15‐amino acid residue peptaibols (for example, zervamicin) in functional and biological tests. The high dynamic ‘propensity’ possibly prevents both initial binding of the antiamoebin to the membrane and subsequent formation of stable ionic channels according to the barrel‐stave mechanism.