Assembly of Triple-Stranded β-Sheet Peptides at Interfaces

A 30-residue peptide, BS30, which incorporates two proline residues to induce reverse turns, was designed to form a triple-stranded β-sheet monolayer at the air−water interface. To discern the structural role of proline, a second peptide, BS30G, identical to BS30 but with glycine residues replacing proline, was prepared and examined in parallel fashion. Surface pressure−molecular area isotherms indicated a limiting area per molecule (ca. 460 Å^2) for BS30 that corresponds well to that estimated from the known dimensions of crystalline β-sheet monolayers (492 Å^2). Comparable measurements on BS30G yielded a smaller molecular area (380 Å^2). Grazing incidence X-ray diffraction measurements performed on the BS30 monolayer at nominal area per molecule of 500 Å^2, exhibited two Bragg peaks corresponding to 4.79 and 34.9 Å spacings, consistent with formation of triple-stranded β-sheet structures that assemble into two-dimensional crystallites at the air−water interface. Visualized by Brewster angle microscopy, BS30 monolayers displayed uniform, solidlike domains, whereas BS30G appeared to be disordered.