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-MeN-Ala)(3)-L-Cys-D-N-Me-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 Angstrom(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.