Vibrational spectroscopy in conjunction with X-ray and gravimetric methods has been used to study the structural stability of a hydrated polypeptide prepared in a genetically engineered strain of Escherichia coli. The sample adopts a beta-sheet conformation in the solid state with a well-defined crystalline stem length and fold surfaces believed to be decorated with carboxylic acid groups. Ionization and subsequent hydration of these acid groups are found to have a major effect on the crystal packing and chain conformation. We have also established that the structural changes accompanying hydration of this model polypeptide occur in a stepwise fashion. First, because of their high accessibility, the carboxylic acid or carboxylate groups on the lamellar surfaces can readily interact with water molecules. In the second step, water penetrates into the regions between the hydrogen-bonded sheets; however, the resulting expansion in the intersheet distance can occur without altering the chain conformation. Lastly, when the water content is high, the hydrogen-bonded sheets are disrupted, leading to a change in chain conformation from beta-strands to helical or disordered chains.