Spatial structures of two polymyxin antibiotics are compared by means of one- and two-dimensional 1H NMR spectroscopy. Cyclic parts of polymyxins B and M contain a system of hydrogen bonds including two beta-turns, however, the analysis of coupling constants 3JHN-C alpha H demonstrated that torsional angles phi of peptide bonds of the residues forming beta-turns in polymyxin M depend on the type of the anion. An increase in lability of the polymyxin M cyclic part in comparison with polymyxin B correlated with the selective cleavage of the peptide bond Dab8-Dab9 of this antibiotic with subtilisin. A similar correlation was found for a short analogue of polymyxin B, a cycloheptapeptide.