Design of peptides for thin films, coatings and microcapsules for applications in biotechnology

Fluctuation-induced free energy of thin peptide films

We apply the Lifshitz theory of dispersion forces to find a contribution to the free energy of peptide films that is caused by the zero-point and thermal fluctuations of the electromagnetic field. For this purpose, using available information about the imaginary parts of the dielectric permittivity of peptides, an analytic representation for permittivity of a typical peptide along the imaginary frequency axis is devised. Numerical computations of the fluctuation-induced free energy are performed at room temperature for freestanding peptide films containing different fractions of water, and for similar films deposited on dielectric (SiO_{2}) and metallic (Au) substrates. It is shown that the free energy of a freestanding peptide film is negative and thus contributes to its stability. The magnitude of the free energy increases with increasing fraction of water and decreases with increasing thickness of a film. For peptide films deposited on a dielectric substrate, the free energy is nonmonotonous. It is negative for films thicker than 100nm, reaches the maximum value at some film thickness, but vanishes and changes its sign for films thinner than 100nm. The fluctuation-induced free energy of peptide films deposited on metallic substrate is found to be positive, which makes films less stable. In all three cases, simple analytic expressions for the free energy of sufficiently thick films are found. The obtained results may be useful to attain film stability in the next generation of organic microdevices with further reduced dimensions.

Surface orientation of polystyrene based polymers: steric effects from pendant groups on the phenyl ring

Near edge X-ray absorption fine structure (NEXAFS) coupled with molecular dynamics simulations were utilized to probe the orientation at the exposed surface of the polymer film for polystyrene type polymers with various pendant functional groups off the phenyl ring. For all the polymers, the surface was oriented so that the rings are nominally normal to the film surface and pointing outward from the surface. The magnitude of this orientation was small and dependent on the size of the pendant functional group. Bulky functional groups hindered the surface orientation, leading to nearly unoriented surfaces. Depth dependent NEXAFS measurements demonstrated that the surface orientation was localized near the interface. Molecular dynamics simulations showed that the phenyl rings were not oriented strongly around a particular "average tilt angle". In contrast, simulations demonstrate that the phenyl rings exhibit a broad distribution of tilt angles, and that changes in the tilt angle distribution with pendant functionality give rise to the observed NEXAFS response. The more oriented samples exhibit a higher probability of phenyl ring orientation at angles greater than 60 degrees relative to the plane of the films surface.

Straightforward and Effective Protein Encapsulation in Polypeptide-based Artificial Cells

A simple and straightforward approach to encapsulating an enzyme and preserving its function in polypeptide-based artificial cells is demonstrated. A model enzyme, glucose oxidase (GOx), was encapsulated by repeated stepwise adsorption of poly(L-lysine) and poly(L-glutamic acid) onto GOx-coated CaCO3 templates. These polypeptides are known from previous research to exhibit nanometer-scale organization in multilayer films. Templates were dissolved by ethylenediaminetetraacetic acid (EDTA) at neutral pH. Addition of polyethylene glycol (PEG) to the polypeptide assembly solutions greatly increased enzyme retention on the templates, resulting in high-capacity, high-activity loading of the enzyme into artificial cells. Assay of enzyme activity showed that over 80 mg-mL(-1) GOx was retained in artificial cells after polypeptide multilayer film formation and template dissolution in the presence of PEG, but only one-fifth as much was retained in the absence of PEG. Encapsulation is a means of improving the availability of therapeutic macromolecules in biomedicine. This work therefore represents a means of developing polypeptide-based artificial cells for use as therapeutic biomacromolecule delivery vehicles.

Context dependence of the assembly, structure, and stability of polypeptide multilayer nanofilms

Polyelectrolyte multilayer nanofilms and nanocomposites have shown considerable promise for the rational development of multifunctional materials with wide-ranging properties. Polypeptides are a distinctive and largely unexplored class of polyelectrolytes in this context. Methods now exist for the synthesis of peptides with control at the level of the amino acid sequence, and for the preparation of these polymers in massive quantities. Here, we analyze the roles of six designed 32mer peptides in the fabrication, structure, and stability of multilayer nanofilms prepared by layer-by-layer self-assembly. The data show that amino acid sequence and the specific combination of anionic and cationic peptides together have a marked impact on nanofilm growth behavior, secondary structure content, and density in experimental studies. The same factors determine physical properties of the corresponding interpolypeptide complexes in molecular dynamics simulations.

Controlled loading and release of a model drug from polypeptide multilayer nanofilms

A major concern of medicine today is the sustained release of therapeutic compounds. Delivery vehicles for such compounds must be biocompatible. Ideally, loading a drug into the delivery vehicle will be a simple process, and vehicle properties will allow control over the drug release profile under desired conditions. Here, polypeptide multilayer nanofilms have been prepared by electrostatic layer-by-layer self-assembly to study the post-fabrication loading and release of a model therapeutic, methylene blue (MB). Drug loading and release have been characterized by optical spectroscopy for different peptide designs at different pH values, and film surface morphology has been characterized by atomic force microscopy (AFM). Differences in peptide structure have been found to influence MB loading and release under otherwise fixed conditions. Release is also influenced by pH, salt concentration, and number of "capping" layers. Although more research will be needed to exhaust the potential of polypeptide multilayer films, present results would suggest that the technology holds considerable promise for applications in medicine.

Antimicrobial polypeptide multilayer nanocoatings

A multilayer coating (or film) of nanometer-thick layers can be made by sequential adsorption of oppositely charged polyelectrolytes on a solid support. The method is known as layer-by-layer assembly (LBL). No special apparatus is required for LBL and nanofilms can be prepared under mild, physiological conditions. A multilayer nanofilm in which at least one of the constituent species is a polypeptide is a polypeptide multilayer nanofilm. The present work was aimed at assessing whether polypeptide multilayer nanofilms with specific antimicrobial properties could be prepared by incorporation of a known antimicrobial agent in the film structure, in this case the edible protein hen egg white lysozyme (HEWL). The chicken enzyme is widely employed as a human food preservative. An advantage of LBL in this context is that the nanofilm is fabricated directly on the surface of interest, eliminating the need to incorporate the antimicrobial in other packaging materials. Here, nanofilms were made of poly(L-glutamic acid) (PLGA), which is highly negatively charged in the mildly acidic pH range, and HEWL, which has a high net positive charge at acidic pH. We show that PLGA/HEWL nanofilms inhibit growth of the model microbe Microccocus luteus in the surrounding liquid medium. The amount of HEWL released from PLGA/HEWL films depends on the number of HEWL layers and therefore on the total quantity of HEWL in the films. This initial study provides a sketch of the scope for further development of LBL in the area of antimicrobial polypeptide multilayer films. Potential applications of such films include strategies for food preservation and coatings for implant devices.

Polypeptide multilayer nanofilm artificial red blood cells

Reliable encapsulation of hemoglobin (Hb) within polypeptide multilayer nanofilms has been achieved by a template-based approach, and protein functionality has been demonstrated postencapsulation. The method is general in scope and could be useful for many other encapsulants. Met-Hb was adsorbed onto 5 microm-diameter CaCO3 microparticles, and the Hb-coated particles were encapsulated within a multilayer nanofilm of poly(L-glutamic acid) (PLGA) and poly(L-lysine) (PLL) by layer-by-layer assembly. The CaCO3 templates were then dissolved within the PLGA/PLL nanofilms by addition of ethylenediaminetetraacetic acid. Encapsulation of Hb was proved by fluorescence microscopy, the pH-dependence of retention of Hb was determined by visible wavelength absorbance, and conversion of the encapsulated met-Hb to deoxy-Hb and oxy-Hb was demonstrated by spectroscopic analysis of the Soret absorption peak under various conditions. It thus has been shown that control of Hb oxygenation within polypeptide multilayer nanofilm artificial cells is possible, and that Hb thus encapsulated can bind, release, and subsequently rebind molecular oxygen. This work therefore represents an advance in the development of polypeptide multilayer film artificial red blood cells.

Stimulated release of small molecules from polyelectrolyte multilayer nanocoatings

Free thiol-containing polyelectrolytes serve simultaneously as a material for self-assembly of a multilayer nanocoating and as a carrier of small molecules for release from the coating in response to an environmental cue.

Physics of polypeptide multilayer films

Polypeptide multilayer films are promising for the development of coatings for implant devices, biosensors, and artificial cells. This paper discusses aspects of the physics of these films. Three sub-topics in the physics of peptide adsorption in multilayer film assembly covered here are peptide structure at the film/solid support interface, adsorbed layer thickness, and dynamics of peptide adsorption. A synopsis of work in these areas is preceded by an introduction to the subject and a review of some aspects of polymer theory.

A molecular dynamics study of the physical basis of stability of polypeptide multilayer nanofilms

Electrostatic layer-by-layer assembly (LBL) is a versatile method of fabricating ultrathin multilayer films, coatings, and microcapsules from materials in solution, notably, oppositely charged polyelectrolytes in water. Polypeptides, a special type of polyelectrolyte, have recently shown promise for a range of applications in biotechnology and medicine, for example, artificial cells, drug delivery systems, cell/tissue scaffolds, artificial viruses, and implantable device coatings. Poly(L-lysine) (PLL) and poly(L-glutamic acid) (PLGA) at neutral pH are model oppositely charged polypeptides. Experimental studies have shown that PLL/PLGA multilayer films contain a substantial amount of beta-sheets. Here, we present findings of a molecular dynamics (MD) study of the physical basis of interaction between PLL and PLGA in multilayer film models. Simulations have been carried out to study structural and dynamical properties of PLL/PLGA aggregates in beta-sheet conformation. The results suggest that hydrophobic interactions, in addition to electrostatics interactions, play a significant role in PLL/PLGA multilayers. The preferred orientation of peptides in the beta-sheet structures is antiparallel within sheets and parallel between sheets. Intersheet hydrogen-bond formation is more likely the result of peptide association than the cause. The approach provides a general means to understand better how various types of noncovalent interactions contribute to the structure and stability of polypeptide multilayer films.

Polypeptide multilayer films

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.

High-capacity functional protein encapsulation in nanoengineered polypeptide microcapsules

Addition of polyethylene glycol to aqueous assembly solutions of oppositely charged polypeptides enables high-capacity "loading" of functional protein in biocompatible microcapsules by template-supported layer-by-layer nanoassembly.

Perturbation of nanoscale structure of polypeptide multilayer thin films

Multilayer thin films formed by sequential deposition of oppositely charged polypeptides on a charged surface are known from previous studies to comprise a mixture of types of secondary structure. Here, study of the perturbation of polypeptide film structure by deposition of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) on the film surface has revealed differences in behavior attributable to physical properties of the peptides. The methods of analysis were circular dichroism spectroscopy (CD), ultraviolet spectroscopy (UVS), and quartz crystal microbalance (QCM). Films made of poly(L-lysine) (PLL) and poly(L-glutamic acid) (PLGA) with an average charge per monomer of about 1 were substantially more susceptible to perturbation of structure than films made of designed polypeptides with an average charge per monomer of about 0.5, despite preparation under identical conditions. PLL-PLGA films showed loss or gain of material and change in secondary structure content on perturbation, whether made of high molecular mass (ca. 90 kDa) or low molecular mass (ca. 14 kDa) polymers. By contrast, films made of very low molecular mass (ca. 3.5 kDa) designed polypeptides showed little change in secondary structure content. The data suggest that the penetrability of PSS or PAH into a film and therefore film density can depend substantially on the polypeptides of which it is made and the character of intermolecular interactions.