- The assembly, orientation, and structural features of nanoscale tubes composed of cyclic peptides, formed at the air− water interface, were detected by grazing incidence X-ray diffraction (GIXD). The peptide cyclo-[(l-Phe-dN-MeAla-) 4](1) exhibits two-dimensional crystallinity in which the plane of the peptide ring is parallel to the water interface. The peptide cyclo-[(l-Trp-d-Leu) 3-l-Ser-d-Leu](2) forms predominantly planar aggregates composed of several tubes, lying with their long axes parallel to the air− water interface. In contrast, the peptide cyclo-[(l-Trp-d-Leu) 4](3) exhibits a very low tendency to form ordered two-dimensional arrays of nanotubes. Films of peptides 2 and 3 as well as their mixtures with the phospholipid DPPA were transferred onto a solid support and visualized by scanning force microscopy (SFM).