A 12 amino acid peptide, model BB, was designed to adopt a beta-hairpin tertiary structure in water that might be stabilized by a variety of local, nonlocal, polar, and nonpolar interactions. The conformational properties of model BB with and without an intramolecular disulfide bond (BB-O and BB-R, respectively) were characterized by NMR and CD spectroscopy. The set of observed short- and medium-range nOes were consistent with the formation of stable beta-hairpin-like structures by both BB-R and BB-O. BB-O adopts two distinct conformations that differ from each other in the designed reverse turn segment. A reasonably well-defined set of structures was obtained by using restraints from the NMR data in distance geometry calculations. None of the beta-hairpin-like structures contain a beta-sheet hydrogen bonding network. The distinctive feature of intrastrand and cross-strand pairing of threonine residues observed in all of the calculated structures suggests that hydrophobic interactions between the gamma-methyl groups of threonine residues may be the structure-determining interaction in model BB. The implications of these results for the formation of beta-sheets during protein folding, the aggregation of peptides as beta-sheets, and the de novo design of independently folding beta-hairpin-like peptides are considered.