alpha-Conotoxin MII, isolated from Conus magus, is a potent peptidic toxin which specifically targets the mammalian neuronal nicotinic acetylcholine receptor, alpha3beta2 subtype. The three-dimensional structure of alpha-conotoxin MII in aqueous solution has been determined by two-dimensional 1H NMR spectroscopy. NOE-derived distances, refined by an iterative relaxation matrix approach, as well as dihedral and chirality restraints were used in high-temperature biphasic simulated annealing calculations. Fourteen minimum energy structures out of 50 subjected to the SA simulations were chosen for evaluation; these 14 structures have a final RMS deviation of 0.76 +/- 0.31 and 1.35 +/- 0.34 A for the backbone and heavy atoms, respectively. The overall structure is unusually well-defined due to a large helical component around the two disulfide bridges. The principal backbone folding motif may be common to a subclass of alpha-conotoxins. There are two distinct surfaces on the molecule almost at right angles to one another. One entirely consists of the hydrophobic residues Gly1, Cys2, Cys3, Leu15, and Cys16. The second comprises the hydrophilic residues Glu11, His12, Ser13, and Asn14. These surfaces on the ligand could be essential for the subtype-specific recognition of the receptor.