Crystalline Cyclic Peptide Nanotubes at Interfaces

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- d - N -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).