Proteinogenic amino acid residues that promote beta-sheet secondary structure are hydrophobic (e.g., Ile or Val) or only moderately polar (e.g., Thr). The design of peptides intended to display beta-sheet secondary structure in water typically requires one set of residues to ensure conformational stability and an orthogonal set, with charged side chains, to ensure aqueous solubility and discourage self-association. Here we describe new amino acids that manifest substantial beta-sheet propensity, by virtue of beta-branching, and also bear an ionizable group in the side chain.