Conantokins, which are neuroactive peptides found in the venom of marine snails, contain post-translationally modified &gamma-carboxyglutamate residues (Gla; γ). The Gla residues are integral for their structure, function, and subunit-selective antagonism of ion channels of the N-Methyl-D-Aspartate Receptors (NMDAR). The functional heterotetramer of the receptor is composed of GluN1 and GluN2 subunits which impart specific biochemical and physiological properties to the ion channels. The optimum spacing of Gla residues in conantokins stabilize the structure when complexed to divalent ions and mediate NMDAR-subunit selective properties. In this study, the structural and antagonistic properties of conantokins ConPr-1,-2, and -3 from Conus parius and ConRl-B from Conus rolani species were studied. The Conus parius family of conantokin peptides are GluN2B selective and adopt a low properties of α-helix conformation in the presence of Mg2+. These peptides also display the ability to diminish downstream signaling events. ConRl-B, ConPr-2, and ConPr-3, additionally contain the post-translationally modified 4-transhydroxyproline (HyP; O) residue. The Hyp10 residue in ConRl-B, not only changes the Gla positioning in the C-terminal part of ConRl-B, but also affects the structural properties of the peptide. A combination of biophysical techniques, e.g., CD, 1H NMR and ITC, was employed to study the structure and metal binding properties of Mg2+/ConRl-B. The structure determined by high-field 1H NMR, shows disruption in the α-helix due to HyP10 further interfering with Mg2+-binding, thus resulting in solvent accessible backbone amide protons. To assess the role of HyP and the molecular requisites for the unique structural and functional properties of ConRl-B, mutant peptides, ConG[10 ▼O] and ConRl-B[ΔKAO▼NQ] were synthesized. Although, the 3D structure and Mg2+ binding abilities of ConRl-B[ΔKAO▼NQ] were similar to Con-G, this peptide displays complete non-selectivity towards NMDAR. Local disruption of the &#945-helix backbone in Con-G on the introduction of HyP, differs from that of ConRl-B. Additionally, Con-G[10 ▼O] loses selectivity towards ion channels containing the GluN2B subunit. These results suggest that the natural occurrence of HyP in the ConRl-B backbone is important for high GluN2B selectivity. In conclusion, the presence of HyP residue changes the structural and functional properties of conantokins.