Researchers at the Max Planck Institute for Polymer Research in Mainz in collaboration with the group of Prof. Schouten at the University of Groningen have, for the first time, investigated the electromechanical properties of polypeptides grown directly from a flat surface. The results were recently published in Science (Vol. 279, 2. Jan.).
One of the challenges of supramolecular chemistry is the manufacture of ultrathin layers with a perceptible and stable polar order. Such materials have potential in the construction of nano-scale piezo- and pyroelectric devices and for electrooptical applications. Following the approach by Whitesell and Chang (Science 1993, 261, 73) we have grown monomolecular films of 15 nanometer thickness of the helical polypeptide poly-gamma-benzyl-L-glutamate (PBLG) directly from a flat aluminum surface. Infrared spectra indicated that the a-helical polypeptide stands upright on the surface. Determination of the electric field-induced change in film thickness is a measure of the degree of polar order. A recently developed Nomarski optical interferometer (Winkelhahn, Winter, Neher, Applied Physics Letter 1994, 64, 1347), capable of detecting periodic thickness changes with sup-picometer resolution, provides such electromechanical data. The measured PBLG film polarization was found to be comparable to that of conventional ferroelectric materials. This result demonstrates that the polymerization starting from the surface forces the helical chains into a polar arrangement. Despite having piezoelectric coefficients less than those of most commercial materials, these piezoelectric-active films can be grown on a variety of electrodes, even on flexible substrates, without the need for subsequent poling.
An additional advantage of this electromechanical interferometer is the possibility to estimate the mechanical plate modulus of an ultrathin film. The coupling of the electrical and mechanical properties allows the direct probing of the mechanical moduli of polypeptides in the biological relevant a-helical conformation. Our experimental results verify, for the first time, the theoretical prediction for a single polypeptide in its helical form along the helical axis (Helfrich, Hentschke, Apel, Macromolecules 1994, 27, 472).
Journal
Science