The role of molecular structure, charge, and hydrophobicity in polyelectrolyte layer-by-layer assembly (LbL) of thin films has been studied using the model polypeptides poly-L-glutamatic acid (PLGA) and poly-L-lysine (PLL), quartz crystal microbalance (QCM), and circular dichroism spectroscopy (CD). The adsorption behavior of PLGA and PLL has been compared with the structure of these molecules in aqueous solution under the same conditions. The data show that the deposition of polypeptide per adsorption step scales with average secondary structure content, whether alpha helix or beta sheet. This is contrary to the expectation based on the view that hydrogen bonds are crucial to polypeptide film assembly, because secondary structure formation in a polypeptide reduces its intermolecular hydrogen-bonding potential. The data also show that polypeptide adsorption scales with ionic strength and chain length. Taken together, the results increase knowledge of polypeptide-based LbL thin film fabrication and will help to provide a firmer foundation for the use of natural or designed polypeptides in LbL.

Research on polypeptide multilayer films, coatings, and microcapsules is located at the intersection of several disciplines: synthetic polymer chemistry and physics, biomaterials science, and nanoscale engineering. The past few years have witnessed considerable growth in each of these areas. Unexplored territory has been found at the borders, and new possibilities for technology development are taking form from technological advances in polypeptide production, sequencing of the human genome, and the nature of peptides themselves. Most envisioned applications of polypeptide multilayers have a biomedical bent. Prospects seem no less positive, however, in fields ranging from food technology to environmental science. This review of the present state of polypeptide multilayer film research covers key points of polypeptides as materials, means of polymer production and film preparation, film characterization methods, focal points of current research in basic science, and the outlook for a few specific applications. In addition, it discusses how the study of polypeptide multilayer films could help to clarify the physical basis of assembly and stability of polyelectrolyte multilayers, and mention is made of similarities to protein folding studies.

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The method of thin film preparation known as layer-by-layer assembly is of growing interest for current and envisioned developments in bionanotechnology. Here, cysteine-containing 32mer peptides have been designed, synthesized, purified, and used to prepare polypeptide films. A range of methods-quartz crystal microbalance, Fourier transform infrared spectroscopy, circular dichroism spectroscopy, and high-performance liquid chromatography-have been used to probe the effect of ionic strength and polymer secondary structure in solution on peptide self-assembly, and on secondary structure formation and disulfide bond cross-linking in the multilayer film. The amount of designed peptide deposited per adsorption step of film fabrication increased with increasing ionic strength, as with conventional polyelectrolytes. Secondary structure content changed from random coil to beta sheet on incorporation of peptides into a film. "Peptide-inherent" cross-linking by disulfide bond formation increased film stability at acidic pH. Conditions for disulfide stabilization have been optimized. The results contribute to exploration of the physical basis of peptide self-assembly broaden the scope of applications of layer-by-layer assembly, particularly where biocompatibility and stability are key design concerns, and provide a basis for mass production of custom polypeptide thin films of high stability, even in harsh environments.

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