Amphiphilic peptides comprising alternating hydrophilic and hydrophobic amino acid residues were designed to form super-secondary structures composed of self-assembled beta -strands as monolayers at the air-water interface. Insights provided by in situ grazing-incidence X-ray diffraction (GIXD), surface pressure vs area isotherms, and Fourier transform infrared spectroscopy allow structural characterization of the assembled nanostructures and rational correlation with the peptide sequence. Peptides seven to seventeen amino acids in length were found to form crystalline arrays with coherence lengths in the range of 100 to 1000 Angstrom. Two-dimensional registry of the self-assembled peptides was induced by placement of proline residues at the peptide termini. The films were found to intercalate ordered arrays of ions between juxtaposed beta -sheet ribbons to generate peptide-ion composite phases.