The interfacial structure and Young's modulus of peptide films having switchable mechanical properties

Droplet shape control using microfluidics and designer biosurfactants

Many uses of emulsion droplets require precise control over droplet size and shape. Here we report a 'shape-memorable' micro-droplet formulation stabilized by a polyethylene glycol (PEG)-modified protein -surfactant, the droplets are stable against coalescence for months and can maintain non-spherical shapes for hours, depending on the surface coverage of PEGylated protein. Monodisperse droplets with aspect ratios ranging from 1.0 to 3.4 were controllably synthesized with a flow-focusing microfluidic device. Mechanical properties of the interfacial protein network were explored to elucidate the mechanism behind the droplet shape conservation phenomenon. Characterization of the protein film revealed that the presence of a PEG layer at interfaces alters the mechanical responses of the protein film, resulting in interfacial networks with improved strength. Taking advantage of the prolonged stabilization of non-spherical droplets, we demonstrate functionalization of the droplet interface with accessible biotins. The stabilization of micro-droplet shape with surface-active proteins that also serve as an anchor for integrating functional moieties, provides a tailorable interface for diverse biomimetic applications.

Biofunctionality of Enzymatically Derived Peptides from Codfish (Gadus morhua) Frame: Bulk In Vitro Properties, Quantitative Proteomics, and Bioinformatic Prediction

Protein hydrolysates show great promise as bioactive food and feed ingredients and for valorization of side-streams from e.g., the fish processing industry. We present a novel approach for hydrolysate characterization that utilizes proteomics data for calculation of weighted mean peptide properties (length, molecular weight, and charge) and peptide-level abundance estimation. Using a novel bioinformatic approach for subsequent prediction of biofunctional properties of identified peptides, we are able to provide an unprecedented, in-depth characterization. The study further characterizes bulk emulsifying, foaming, and in vitro antioxidative properties of enzymatic hydrolysates derived from cod frame by application of Alcalase and Neutrase, individually and sequentially, as well as the influence of heat pre-treatment. All hydrolysates displayed comparable or higher emulsifying activity and stability than sodium caseinate. Heat-treatment significantly increased stability but showed a negative effect on the activity and degree of hydrolysis. Lower degrees of hydrolysis resulted in significantly higher chelating activity, while the opposite was observed for radical scavenging activity. Combining peptide abundance with bioinformatic prediction, we identified several peptides that are likely linked to the observed differences in bulk emulsifying properties. The study highlights the prospects of applying proteomics and bioinformatics for hydrolysate characterization and in food protein science.

Comparing Surfactant Structures at "Soft" and "Hard" Hydrophobic Materials: Not All Interfaces Are Equivalent

The interfacial structures of a range of amphiphilic molecules are studied with both "soft" and "hard" hydrophobic substrates. Neutron reflection and quartz crystal microbalance with dissipation measurements highlight the differences between the adsorbed structures adopted by sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (C₁₆TAB), and the "AM1" surface active peptide. At the soft siloxane/water interface, small molecular surfactants form loosely packed layers, with the hydrophobic tails penetrating into the oily layer, and an area per surfactant molecule that is significantly less than previously reported for the air/water interface. Neutron reflection measurements, supported by quartz crystal microbalance studies, indicate that for C₁₆TAB, approximately 30 ± 8% of the alkyl tail penetrates into the poly(dimethylsiloxane) (PDMS) layer, whereas 20 ± 5% of the alkyl tail of SDS is located in the PDMS. For the engineered peptide surfactant AM1 (21 residues), it was found that one face of the α helix penetrated into the PDMS film. In contrast, penetration of the surfactant tails was not observed against hard solidlike hydrophobic surfaces made from octadecyltrichlorosilane (OTS) for any of the molecular species studied. At the OTS/water interface, C₁₆TAB and SDS were seen to adsorb as larger aggregates and not as monolayers. Amphiphilic adsorption (amount, structural conformation) at the PDMS/water interface is shown to be different from that at both the air/water interface and the hard OTS/water interface, illustrating that interfacial structures cannot be predicted by the surfactant packing parameter alone. The bound PDMS layer is shown to be a useful proxy for the oil/water interface in surface and stabilization studies, with hydrophobic components of the molecules able to penetrate into the oily PDMS.

Design of Modular Peptide Surfactants and Their Surface Activity

Designed peptide surfactants offer a number of advanced properties over conventional petrochemical surfactants, including biocompatibility, sustainability, and tailorability of the chemical and physical properties through peptide design. Their biocompatibility and degradability make them attractive for various applications, particularly for food and pharmaceutical applications. In this work, two new peptide surfactants derived from an amphiphilic peptide surfactant (AM1) were designed (AM-S and C₈-AM) to better understand links between structure, interfacial activity, and emulsification. Based on AM1, which has an interfacial α-helical structure, AM-S and C₈-AM were designed to have two modules, that is, the α-helical AM1 module and an additional hydrophobic moiety to provide for better anchoring at the oil-water interface. Both AM-S and C₈-AM at low bulk concentration of 20 μM were able to adsorb rapidly at the oil-water interface and reduced interfacial tension to equilibrium values of 17.0 and 8.4 mN/m within 400 s, respectively. Their relatively quick adsorption kinetics allowed the formation of nanoemulsions with smaller droplet sizes and narrower size distribution. AM-S and C₈-AM at 800 μM bulk concentration could make nanoemulsions of average diameters 180 and 147 nm, respectively, by simple sonication. With respect to the long-term stability, a minimum peptide concentration of 400 μM for AM-S and a lower concentration of 100 μM for C₈-AM were demonstrated to effectively stabilize nanoemulsions over 3 weeks. Compared to AM1, the AM-S nanoemulsion retained its stimuli-responsive function triggered by metal ions, whereas the C₈-AM nanoemulsions did not respond to the stimuli as efficiently as AM-S because of the strong anchoring ability of the hydrophobic C8 module. The two-module design of AM-S and C₈-AM represents a new strategy in tuning the surface activity of peptide surfactants, offering useful information and guidance of future designs.

Insights into the Role of Biomineralizing Peptide Surfactants on Making Nanoemulsion-Templated Silica Nanocapsules

We recently developed a novel approach for making oil-core silica-shell nanocapsules using designed bifunctional peptides (also called biomineralizing peptide surfactants) having both surface activity and biomineralization activity. Using the bifunctional peptides, oil-in-water nanoemulsion templates can be readily prepared, followed by the silicification directed exclusively onto the oil droplet surfaces and thus the formation of the silica shell. To explore their roles in the synthesis of silica nanocapsules, two bifunctional peptides, AM1 and SurSi, were systematically studied and compared. Peptide AM1, which was designed as a stimuli-responsive surfactant, demonstrated quick adsorption kinetics with a rapid decrease in the oil-water interfacial tension, thus resulting in the formation of nanoemulsions with a droplet size as small as 38 nm. Additionally, the nanoemulsions showed good stability over 4 weeks because of the formation of a histidine-Zn(2+) interfacial network. In comparison, the SurSi peptide that was designed by modularizing an AM1-like surface-active module with a highly cationic biosilicification-active module was unable to effectively reduce the oil-water interfacial tension because of its high molecular charge at neutral pH. The slow adsorption resulted in the formation of less stable nanoemulsions with a larger size (60 nm) than that of AM1. Besides, both AM1 and SurSi were found to be able to induce biomimetic silica formation. SurSi produced well-dispersed and uniform silica nanospheres in the bulk solution, whereas AM1 generated only irregular silica aggregates. Consequently, well-defined silica nanocapsules were synthesized using SurSi nanoemulsion templates, whereas silica aggregates instead of nanocapsules predominated when templating AM1 nanoemulsions. This finding indicated that the capability of peptide surfactants to form isolated silica nanospheres might play a role in the successful fabrication of silica nanocapsules. This fundamental study provides insights into the design of bifunctional peptides for making silica nanocapsules.

Unusually Large Young's Moduli of Amino Acid Molecular Crystals

Young's moduli of selected amino acid molecular crystals were studied both experimentally and computationally using nanoindentation and dispersion-corrected density functional theory. The Young modulus is found to be strongly facet-dependent, with some facets exhibiting exceptionally high values (as large as 44 GPa). The magnitude of Young's modulus is strongly correlated with the relative orientation between the underlying hydrogen-bonding network and the measured facet. Furthermore, we show computationally that the Young modulus can be as large as 70-90 GPa if facets perpendicular to the primary direction of the hydrogen-bonding network can be stabilized. This value is remarkably high for a molecular solid and suggests the design of hydrogen-bond networks as a route for rational design of ultra-stiff molecular solids.

Determining the structure of interfacial peptide films: comparing neutron reflectometry and molecular dynamics simulations

The peptides AM1 and Lac21E self-organize into switchable films at an air-water interface. In an earlier study, it was proposed that both AM1 and Lac21E formed monolayers of α-helical peptides based on consistency with neutron reflectivity data. In this article, molecular dynamics simulations of assemblies of helical and nonhelical AM1 and Lac21E at an air-water interface suggest some tendency for the peptides to spontaneously adopt an α-helical conformation. However, irrespective of the structure of the peptides, the simulations reproduced not only the structural properties of the films (thickness and distribution of the hydrophobic and hydrophilic amino acids) but also the experimental neutron reflectivity measurements at different contrast variations. This suggests that neutron reflectometry alone cannot be used to determine the structure of the peptides in this case. However, together with molecular dynamics simulations, it is possible to obtain a detailed understanding of peptide films at an atomic level.

Insights into the role of protein molecule size and structure on interfacial properties using designed sequences

Mixtures of a large, structured protein with a smaller, unstructured component are inherently complex and hard to characterize at interfaces, leading to difficulties in understanding their interfacial behaviours and, therefore, formulation optimization. Here, we investigated interfacial properties of such a mixed system. Simplicity was achieved using designed sequences in which chemical differences had been eliminated to isolate the effect of molecular size and structure, namely a short unstructured peptide (DAMP1) and its longer structured protein concatamer (DAMP4). Interfacial tension measurements suggested that the size and bulk structuring of the larger molecule led to much slower adsorption kinetics. Neutron reflectometry at equilibrium revealed that both molecules adsorbed as a monolayer to the air-water interface (indicating unfolding of DAMP4 to give a chain of four connected DAMP1 molecules), with a concentration ratio equal to that in the bulk. This suggests the overall free energy of adsorption is equal despite differences in size and bulk structure. At small interfacial extensional strains, only molecule packing influenced the stress response. At larger strains, the effect of size became apparent, with DAMP4 registering a higher stress response and interfacial elasticity. When both components were present at the interface, most stress-dissipating movement was achieved by DAMP1. This work thus provides insights into the role of proteins' molecular size and structure on their interfacial properties, and the designed sequences introduced here can serve as effective tools for interfacial studies of proteins and polymers.

Stimuli-responsive peptide nanostructures at the fluid-fluid interface

The self-organization of peptide-based nanostructures at a confined fluid-fluid interface, for example, the air-water or oil-water interface, is important in the context of stabilizing macroscopic soft-matter foams and emulsions. The unique ability to design interfacial nanostructures by controlling the subtle cooperativity that drives peptide self-assembly, and the ability to switch molecular cooperativity by facile triggers such as pH, opens new vistas for controlling macroscopic soft matter in industries as diverse as healthcare and industrial processing. Here we describe research aimed at developing new understanding into soft-matter formation and control, through variation of peptide sequence and bulk conditions. Macroscopic foaming and microfluidic emulsification studies prove particularly useful in visualizing and hence understanding the synergistic link between molecular design, mesoscopic interfacial properties, and bulk soft-matter stability.

Co-delivery of antigen and a lipophilic anti-inflammatory drug to cells via a tailorable nanocarrier emulsion

Nanotechnology promises new drug carriers that can be tailored to specific applications. Here we report a new approach to drug delivery based on tailorable nanocarrier emulsions (TNEs), motivated by a need to co-deliver a protein antigen and a lipophilic drug for specific inhibition of nuclear factor kappa B (NF-κB) in antigen presenting cells (APCs). Co-delivery for NF-κB inhibition holds promise as a strategy for the treatment of rheumatoid arthritis. We used a highly surface-active peptide (SAP) to prepare a nanosized emulsion having defined surface properties predictable from the SAP sequence. Incorporating the lipophilic drug into the oil phase at the time of emulsion formation enabled its facile packaging. The SAP is depleted from bulk during emulsification, allowing simple subsequent addition of the drug-loaded oil-in-water emulsion to a solution of protein antigen. Decoration of emulsion surface with antigen was achieved via electrostatic deposition. In vitro data showed that the TNE prepared this way was internalized and well-tolerated by model APCs, and that good suppression of NF-κB expression was achieved. This work reports a new type of nanotechnology-based carrier, a TNE, which can potentially be tailored for co-delivery of multiple therapeutic components, and can be made using simple methods using only biocompatible materials.

Dynamic adsorption and structure of interfacial bilayers adsorbed from lipopeptide surfactants at the hydrophilic silicon/water interface: effect of the headgroup length

Lipopeptides are an important group of biosurfactants expressed by microorganisms. Because they are well-known for being biocompatible, biodegradable, and highly surface active, they are attractive for a wide range of applications. Natural lipopeptide surfactants are however impure; it is hence difficult to use them for exploring the structure-function relation. In this work, a series of cationic lipopeptide surfactants, C(14)K(n) (n = 1-4), where C denotes the myristic acyl chain and K denotes lysine (Lys), have been synthesized, and their interfacial behavior has been characterized by studying their adsorption at the silicon/water interface (bearing a thin native oxide layer) using spectroscopic ellipsometry and neutron reflection (NR). The dynamic adsorption was marked by an initial fast step within the first 2-3 min followed by a slow molecular relaxation process over the subsequent 20-30 min. The initial rate of time-dependent adsorption and the equilibrated adsorbed amount showed a steady decrease with increasing n, indicating the impact of the molecular size, structure, and charge. NR revealed the formation of sandwiched bilayers from C(14)K(n), similar to conventional surfactants such as nonionic C(12)E(6) and cationic C(16)TAB. However, the electrostatic attraction between K and the silica surface caused confinement of the K groups, forcing the head segments into a predominantly flat-on conformation. This characteristic structural feature was confirmed by the almost constant thickness of the headgroup regions ranging from 8 to 11 Å as determined from NR combined with partial deuterium labeling to the acyl tail. An increase in area per molecular pair with n resulted directly from increasing the footprint. As a result, the hydrophobic back-to-back tail mixing and acyl chain tilting rose with n. The extent of chain-head intermixing became so intensified that the C(14)K(4) bilayer could be approximated to a uniform layer distribution.

Interfacial and emulsifying properties of designed β-strand peptides

The structural and surfactant properties of a series of amphipathic β-strand peptides have been studied as a function of pH. Each nine-residue peptide has a framework of hydrophobic proline and phenylalanine amino acid residues, alternating with acidic or basic amino acids to give a sequence closely related to known β-sheet formers. Surface activity, interfacial mechanical properties, electronic circular dichroism (ECD), droplet sizing and zeta potential measurements were used to gain an overview of the peptide behavior as the molecular charge varied from ±4 to 0 with pH. ECD data suggest that the peptides form polyproline-type helices in bulk aqueous solution when highly charged, but may fold to β-hairpins rather than β-sheets when uncharged. In the uncharged state, the peptides adsorb readily at a macroscopic fluid interface to form mechanically strong interfacial films, but tend to give large droplet sizes on emulsification, apparently due to flocculation at a low droplet zeta potential. In contrast, highly charged peptide states gave a low interfacial coverage, but retained good emulsifying activity as judged by droplet size. Best emulsification was generally seen for intermediate charged states of the peptides, possibly representing a compromise between droplet zeta potential and interfacial binding affinity. The emulsifying properties of β-strand peptides have not been previously reported. Understanding the interfacial properties of such peptides is important to their potential development as biosurfactants.

Interfacial assembly of proteins and peptides: recent examples studied by neutron reflection

Through reviewing a number of recent neutron reflection studies of interfacial adsorption of peptides and proteins, this paper aims to demonstrate the significance of this technique in studying interfacial biomolecular processes by illustrating the typical structural details that can be derived. The review will start with the introduction of relevant theoretical background, followed by an outline of representative biomolecular systems that have recently been studied to indicate the technical strengths of neutron reflection.

Cooperative tuneable interactions between a designed peptide biosurfactant and positional isomers of SDOBS at the air-water interface

Rationally designed peptide biosurfactant AM1 was mixed with sodium dodecyl benzene sulfonate (SDOBS) to self-assemble a mixed surfactant-biosurfactant layer at the air-water interface. Under optimal conditions in the presence of Zn2+, the interfacial elasticity of the mixed layer was approximately 5-fold higher than for biosurfactant alone. Two head positional isomers, SDOBS-2 and SDOBS-6, were compared for their ability to enhance interfacial film strength. SDOBS-6 forms a stronger layer with AM1 than does SDOBS-2. The highest interfacial elasticity of the AM1/SDOBS-6 layer was 640 mN m(-1) whereas the maximum value for the AM1/SDOBS-2 layer was 440 mN m(-1). Neutron reflection was used to investigate the structure of AM1/SDOBS films at varied bulk SDOBS concentrations. Both deuterated and nondeuterated SDOBS-2 and SDOBS-6 were used to provide contrast variation. It was shown that there is cooperative interaction between AM1 and SDOBS at low SDOBS concentration in the presence of 100 microM Zn2+, promoting AM1 adsorption atthe interface to form a two-layered structure of AM1 resulting in a mechanically strong interfacial film. In the presence of EDTA, only a single AM1 layer was formed at the same SDOBS concentration, and the film did not show lateral force transmission capability. Further increasing the SDOBS concentration to a molar excess of > 10x decreased the peptide population at the interface and resulted in a mechanically weak layer. Compared to SDOBS-6, SDOBS-2 depletes AM1 at a lower bulk concentration. These results demonstrate that the film strength of a self-assembled surfactant-biosurfactant mixed layer can be fine tuned by changing the isomer type and concentration of surfactant and by adding or removing metal ions.

Importance of LPS structure on protein interactions with Pseudomonas aeruginosa

Atomic force microscopy (AFM) was used to quantify the adhesion forces between Pseudomonas aeruginosa PAO1 and AK1401, and a representative model protein, bovine serum albumin (BSA). The two bacteria strains differ in terms of the structure of their lipopolysaccharide (LPS) layers. While PAO1 is the wild-type expressing a complete LPS and two types of saccharide units in the O-antigen (A(+) B(+)), the mutant AK1401 expresses only a single unit of the A-band saccharide (A(+) B(-)). The mean adhesion force (F(adh)) between BSA and AK1401 was 1.12 nN, compared to 0.40 nN for F(adh) between BSA and PAO1. In order to better understand the fundamental forces that would control bacterial-protein interactions at equilibrium conditions, we calculated interfacial free energies using the van Oss-Chaudhury-Good (VCG) thermodynamic modeling approach. The hydrogen bond strength was also calculated using a Poisson statistical analysis. AK1401 has a higher ability to participate in hydrogen bonding with BSA than does PAO1, which may be because the short A-band and absence of B-band polymer allowed the core oligosaccharides and lipid A regions to be more exposed and to participate in hydrogen and chemical bonding. Interactions between PAO1 and BSA were weak due to the dominance of neutral and hydrophilic sugars of the A-band polymer. These results show that bacterial interactions with protein-coated surfaces will depend on the types of bonds that can form between bacterial surface macromolecules and the protein. We suggest that strategies to prevent bacterial colonization of biomaterials can focus on inhibiting these bonds.

Surface movement in water of splendipherin, the aquatic male sex pheromone of the tree frog Litoria splendida

The aquatic sex pheromone splendipherin (GLVSSIGKALGGLLADVVKSKGQPA-OH) of the male green tree frog Litoria splendida moves across the surface of water to reach the female. Surface pressure and X-ray reflectometry measurements confirm that splendipherin is a surface-active molecule, and are consistent with it having an ordered structure, whereby the hydrophilic portion of the peptide interacts with the underlying water and the hydrophobic region is adjacent to the vapour phase. The movement of splendipherin over the surface of water is caused by a surface pressure gradient. In order to better define the structure of splendipherin at the water/air interface we used 2D NMR studies of the pheromone with the solvent system trifluoroethanol/water (1 : 1 v/v). In this solvent system, splendipherin adopts a bent alpha helix from residues V3 to K21. The bending of the helix occurs in the centre of the peptide in the vicinity of G11 and G12. The region of splendipherin from V3 to G11 has well-defined amphipathicity, whereas the amphipathicity from G12 to A25 is reduced by K19 and P24 intruding into the hydrophobic and hydrophilic regions respectively. A helical structure is consistent with X-ray reflectometry data.