Reversible Photoisomerization of Self-Organized Cylindrical Peptide Assemblies at Air−Water and Solid Interfaces

A new photoswitchable peptide system composed of two flat, ring-shaped cyclic octapeptides with the sequence cyclo -[( l -Phe- d - Me N -Ala) 3 - l -Cys- d - Me N -Ala] tethered via an azobenzene moiety was investigated at the air−water interface, on mica, germanium, and quartz glass supports. Surprisingly, both the E - and Z -forms of the peptide system form very stable layers at the air−water interface. The surface pressure−area isotherms of each isomer are readily distinguished by plateau regions at 12.5 mN/m ( E -isomer) and 14.0 mN/m ( Z -isomer). Scanning force microscopy (SFM) was employed to scrutinize the structure of transferred Langmuir−Blodgett (LB) films on mica. The SFM and film balance measurements as well as the results from ATR-FT-IR spectroscopy indicate that the peptide cylinders are oriented predominantly perpendicular to the surface normal. SFM images demonstrate that at higher surface pressure a second peptide layer atop the first one is formed. The proposed model was further supported by temperature-dependent isotherms and high-resolution SFM images, revealing that the stacking process occurs in the plateau region. Remarkably, the azobenzene subunits could be reversibly isomerized at the air−water interface. The area per molecule increased upon isomerization from the E - to Z -isomer by 70 Å 2 at constant pressure. At constant area an increase in surface pressure of 1.3 mN/m was detected by switching from E to Z . As confirmed by UV/vis spectroscopy, peptide LB films transferred onto quartz glass also retain the ability to isomerize.