Amphiphilic laminin peptides at air/water interface--effect of single amino acid mutations on surface properties

The ultrastructure of fibronectin fibers pulled from a protein monolayer at the air-liquid interface and the mechanism of the sheet-to-fiber transition

Fibronectin is a globular protein that circulates in the blood and undergoes fibrillogenesis if stretched or under other partially denaturing conditions, even in the absence of cells. Stretch assays made by pulling fibers from droplets of solutions containing high concentrations of fibronectin have previously been introduced in mechanobiology, particularly to ask how bacteria and cells exploit the stretching of fibronectin fibers within extracellular matrix to mechano-regulate its chemical display. Our electron microscopy analysis of their ultrastructure now reveals that the manually pulled fibronectin fibers are composed of densely packed lamellar spirals, whose interlamellar distances are dictated by ion-tunable electrostatic interactions. Our findings suggest that fibrillogenesis proceeds via an irreversible sheet-to-fiber transition as the fibronectin sheet formed at the air-liquid interface of the droplet is pulled off by a sharp tip. This far from equilibrium process is driven by the externally applied force, interfacial surface tension, shear-induced fibronectin self-association, and capillary force-induced buffer drainage. The ultrastructural characterization is then contrasted with previous FRET studies that characterized the molecular strain within these manually pulled fibers. Particularly relevant for stretch-dependent binding studies is the finding that the interior fiber surfaces are accessible to nanoparticles smaller than 10 nm. In summary, our study discovers the underpinning mechanism by which highly hierarchically structured fibers can be generated with unique mechanical and mechano-chemical properties, a concept that might be extended to other bio- or biomimetic polymers.

Protein microcapsules: preparation and applications

Liposomes and polymerosomes generally represent the two most widely used carriers for encapsulating compounds, in particular drugs for delivery. While these are well established carriers, recent applications in biomedicine and food industry have necessitated the use of proteins as robust carriers that are stable under extreme acidic and basic conditions, have practically no toxicity and are able to withstand high shear force. This review highlights the different methods for using proteins as encapsulating materials and lists some biomedical applications of the microcapsules. The advantages and limitations in the capsules from the different preparation routes are enumerated.

Surface activity of amphiphilic helical beta-peptides from molecular dynamics simulation

The surface activity of beta-peptides is investigated using molecular simulations. The type and display of hydrophobic and hydrophilic groups on helical beta-peptides is varied systematically. Peptides with 2/3 hydrophobic groups are found to be surface active, and to adopt an orientation parallel to the air-water interface. For select beta-peptides, we also determine the potential of mean force required to bring a peptide to the air-water interface. Facially amphiphilic helices with 2/3 hydrophobic groups are found to exhibit the lowest free energy of adsorption. The adsorption process is driven by a favorable energetic term and opposed by negative entropic changes. The temperature dependence of adsorption is also investigated; facially amphiphilic helices are found to adopt orientations that are largely independent of temperature, while nonfacially amphiphilic helices sample a broader range of interfacial orientations at elevated temperatures. The thermodynamics of adsorption of beta-peptides is compared to that of 1-octanol, a well-known surfactant, and ovispirin, a naturally occurring antimicrobial peptide. It is found that the essential difference lies in the sign of the entropy of adsorption, which is negative for beta- and alpha-peptides and positive for traditional surfactants such as octanol.

Towards understanding structure-stability and surface properties of laminin peptide YIGSR and mutants

Properties of laminin peptide YIGSR and its mutated sequences YIGSD, YIGSS, YIGSN and YIGSQ have been investigated using molecular dynamics simulations (MDS) and Langmuir films at air/water interface. Simulation studies on laminin peptide YIGSR were performed in the isothermal-isobaric (N, P, T) ensemble, with run up to 5 ns in water as well as lipid environment at 298 K. From different initial configurations, shape transformations of the peptides on the timescale of nanoseconds were observed. The results showed YIGSR to be the most stable peptide with the order of minimized energy being YIGSR<YIGSQ<YIGSD<YIGSN<YIGSS. Subsequent experiments with newly synthesized amphiphilic derivatives of the mutated peptides were carried out for their monolayer formation and stability at air/water interface using surface pressure-molecular area (pi-A) and surface potential-molecular area (DeltaV-A) isotherms. The surface and interface activity of these compounds followed a similar trend as with the MDS studies. Results suggest that single amino acid mutation leads to large changes in minimized energy, surface activity and different rates to reach stable conformation. The native YIGSR is the most stable sequence with highest surface activity while YIGSS is least stable and has the lowest surface activity. This corroborated the results of the MDS.

Hydrodynamically coupled water in surface adsorbed amino acids as a tool to study hydrated peptides

The influence of different amino acid residues on properties of a protein surface is of great interest and importance. Hydrodynamically coupled water in the amino acids has the potential to be used as a tool to study surface properties of proteins. The contribution of this coupled water fraction in design of a hydropathy scale in surface adsorbed amino acid films on solid using quartz crystal microbalance is presented in this work. This scale compares well with the hydropathy scale of Guy reported in the literature and can be correlated with the solid/liquid interfacial tension and work of adhesion of the adsorbed amino acid films. Using Graphical Representation and Analysis of Surface Properties (GRASP) the free energy of transfer from Octanol to water for the amino acids has been estimated and shows approximately an inverse relationship with the coupled water fraction. This scale has been applied in a benchmark test for a native Laminin peptide YIGSR and its mutated sequences (with mutations carried out at 'Y and 'R' positions). The experimentally measured coupled water fractions seem to compare well with that obtained from the present scale assuming the total solvent fraction to be a linear function of the amino acids in the sequence. A survey of the protein data bank showed that sets of sequences based on this scale occur in membrane insertion domain or in trans-membrane proteins suggesting that the scale is suitable to study structure-function correlation in proteins.