The solution conformation of conantokin-T, a Gla-containing 21-residue peptide, (G1 EgammagammaY5QKMLgamma10NLRgammaA15EVKKN20A-amide), in the absence of divalent metal ions, was studied by use of two-dimensional proton NMR spectroscopy. The peptide is helical from the N-terminus to the C-terminus, as defined by upfield-shifted alpha-proton resonances and by characteristic NOE connectivities. Extensive interactions among the amino acid side-chains were identified from the NOESY spectra of this peptide in a buffered aqueous solution. Four hydrophobic residues Tyr5, Met8, Leu9, and Leu12 contact one another in a stable cluster, even in the presence of 6 M urea. The solution structure of conantokin-T is a well-defined alpha-helix, having RMSD values for the backbone and all heavy atoms of 0.40 A and 0.77 A, respectively. Potential repulsion between the negatively-charged side chains of Gla10 and Gla14 is minimized by a Gln6-Gla10 hydrogen bond and by an Arg13-Gla14 ion-pair interaction. The C-terminal amide and the Asn20 side-chain amide both interact with the backbone and minimize fraying at the C-terminal end of the alpha-helix. This study provides a basis to evaluate the changes in peptide conformation concomitant upon the binding of divalent metal ions. In addition, this investigation demonstrates that apo-conantokin-T has almost all of the favorable interactions that are known to contribute to helical stabilization in proteins and monomeric helices.