The detailed analysis of conformational space of A-conotoxin GI in aqueous solution has been performed on the basis of two-dimensional NMR spectroscopy data using multiconformational approach. As the result, two topologically distinct interconvertible sets of GI conformations (populations of 78% and 22%) have been found. A common feature of the two sets is the Asn4-Cys7 β-turn. The Gly8 to Tyr11 region has a structure of right-handed helical turn in the major set and two sequential bends in the minor one. N-terminus and C-terminus also have different orientations, anti-parallel in the major conformational set and parallel in the minor one. An average pairwise rmsd for backbone heavy atoms is 0.56 Å in the major set, 0.23 Å in the minor, and 1.85 Å between the structures of the two sets. The X-ray structure of GI [Guddat, L. W., Martin, J. A., Shan, L., Edmundson, A. B. & Gray, W. R. (1996) Biochemistry 35, 11 329Ϫ11 335] has the same folding pattern as the major NMR set, the average backbone rmsd between the two structures being 0.77 Å .Keywords : conotoxin ; nicotinic acetylcholine receptor; NMR; conformational analysis.The venom of marine snails of the Conus genus contains structure, dynamics, and their relationship with the biological function of peptides. However, in practice high-resolution X-ray oligopeptide neurotoxins which are sophisticated tools for studying a wide range of receptors and ion channels : A-conotoxins structures of small flexible peptides are often biased by crystal packing forces and may differ from a biologically active conforblock nicotinic acetylcholine receptors (nAChR), µ-conotoxins and ω-conotoxins act on voltage-gated sodium and calcium mation. In turn, NMR spectroscopy faces technical difficulties when dealing with flexible molecules. Quite often it is stated channels, respectively, while the targets of the contatokins are the N-methyl-D-aspartate receptors (see review [1]). The most that a number of different conformations in fast exchange are present in solution, among which one can expect a biologically numerous family is that of A-conotoxins consisting of over 10 members whose primary structure has been established and bio-active conformation. Several techniques were proposed for analysis of NMR data on flexible molecules [11Ϫ18], but the logical activity analysed. The efficiency of the interaction between nAChR and A-conotoxins depends on the species and sub-problem is still far from being solved. Here we introduce a new approach for the analysis of NMR data of flexible peptides and type of the nAChR. Most of A-conotoxins block the neuromuscular nAChR. However, several recently discovered A-conotox-show that A-conotoxin GI in aqueous solution is represented by two topologically distinct sets of interconvertible conformations. ins act on neuronal nAChR [2]. Besides the above-mentioned species-selectivities and subtype-selectivities, recent data describe the affinity and binding selectivity to the Torpedo membranes or to AChR in the mouse cell line for the naturally occur-MA...