- Grazing incidence X-ray diffraction measurements were performed on monolayer films of three amphiphilic and acidic β-sheet peptides having the general sequence Pro-Y-(Z-Y)5-Pro, with Y = Asp or Glu and Z = Phe or Leu denoted, PFD-5, PLE-5, PLD-5, and the 1:1 molar ratio mixture of PLD-5 and PLE-5. The crystalline domains of these peptides exhibited compressibility and elasticity of the crystalline unit cell indicated by changes in diffraction patterns on compression. Higher compressibility values appeared to be associated with more favorable cross-strand interactions between peptides with the larger side chains, whereas PLD-5, decorated by the smaller side chain amino acids, exhibited the lowest crystalline compressibility. Diffraction patterns provided evidence for a new subunit cell generated by the pleated β-strand motif in an apparently favorable mode of cross-strand intermolecular packing in β-sheets. The study contributes to the understanding of β-sheet flexibility at interfaces with relevance to natural proteins and designed biomaterials composed of β-sheet peptides.