Conantokin-R (con-R) is a gamma-carboxyglutamate-containing 27-residue neuroactive peptide present in the venom of Conus radiatus, and acts as a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor. This peptide features a single disulfide bond, a type of structural element found in most classes of conotoxins, but not in other conantokins. The NMDA receptor antagonist activity of chemically synthesized con-R was determined through an assay involving inhibition of the spermine-enhanced binding of the NMDA receptor channel blocker, [(3)H]MK-801, to rat brain membranes, and yielded an IC(50) of 93 nM. This value represents a 2-5 times better potency than con-G or con-T, the other two characterized conantokins. Circular dichroism (CD) analysis of the metal-free form of con-R is indicative of a low alpha-helical content. There is an increase in alpha-helicity upon the addition of divalent cations, such as Ca(2+), Mg(2+), or Zn(2+). Isothermal titration calorimetry experiments showed one detectable Mg(2+) binding site with a K(d) of 6.5 microM, and two binding sites for Zn(2+), with K(d) values of 150 nM and 170 microM. Residue-specific information of the conformational state of con-R was obtained by two-dimensional (1)H-NMR. Analyses of the alpha-proton chemical shifts, NOE patterns, and hydrogen exchange rates of the peptide indicated an alpha-helical conformation for residues 1-19. Synthetic con-R-derived peptide variants, containing deletions of 7 and 10 amino acid residues from the carboxy-terminus of the wild-type peptide, displayed unaltered cation binding and NMDA receptor antagonist properties. The alpha-helical secondary structures of the two truncation peptides were more stable than full-length con-R, as evidenced by CD measurements and reduced backbone hydrogen exchange rates. These results provide experimental evidence that the structural elements common to the three conantokins thus far identified are the primary determinants for receptor function and cation binding/secondary structure stability.