Peptide Surfactants Research Articles

Ranking the affinity of aromatic residues for carbon nanotubes by using designed surfactant peptides

A series of surfactant peptides were created to evaluate the affinity of aromatic AAs for single-walled carbon nanotubes in the absence of complications from peptide folding or self-association. Each surfactant peptide has a lipidlike architecture, with two Lys residues at the C-terminus as a hydrophilic head, five Val residues to form a hydrophobic tail, and the testing AA at the N-terminus. Raman and CD spectroscopic studies reveal that the surfactant peptides have a large unordered structural component which is independent of peptide concentration, suggesting that the peptides undergo minimal association under experimental conditions, thus removing this interference from interpretation of the peptide/carbon nanotube interactions. A lack of peptide self-association is also indicated by sedimentation equilibrium ultracentrifugation results. Optical spectroscopy of the peptide/carbon nanotube dispersions indicate that among the three aromatic AAs, tryptophan has the highest affinity for carbon nanotubes (both bundled and individual states) when incorporated into a surfactant peptide, while the Tyr-containing peptide is more selective for individual carbon nanotubes. Phe has the lowest overall affinity for carbon nanotubes. Raman spectra of dispersions made with SPF, SPY and SPW display similar types of nanotubes dispersed, although differences in the relative nanotube populations are observed by optical spectroscopy.

Peptide surfactants (Pepfactants) for switchable foams and emulsions

AbstractWe have developed a series of sustainable peptide surfactants (Pepfactants) capable of stabilizing foams and emulsions in a stimuli‐responsive manner, based on the reversible formation of a mechanically strong interfacial film. Under conditions where the interfacially adsorbed peptide forms a mechanically strong ‘film state’, foam or emulsion stabilization occurs as a direct result of the film strength. Under conditions where the interfacially adsorbed peptide forms a mobile ‘detergent state’, foam or emulsion stabilization is either reduced, or does not occur. Preformed foams or emulsions stabilized by Pepfactants undergo rapid phase coalescence when the film state is converted to the detergent state. Switching between film and detergent states is readily and reversibly achieved by a change in the bulk solution composition, such as a change in pH, or the addition or sequestering of metal ions. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.

Switchable Peptide Surfactants with Designed Metal Binding Capacity

By modifying a well-studied peptide sequence, we have designed two biosurfactants with the ability to reversibly and precisely control the stability of foams. Foam stabilization occurs when the peptide forms a cohesive interfacial film cross-linked by metal ions, while foam destabilization occurs when peptide-metal binding is disrupted. The parent sequence is an amphipathic peptide that adsorbs at fluid interfaces, but forms neither cohesive interfacial films nor stable foams at the concentrations tested. Two modified peptide sequences were designed in which internal sites were substituted with metal-binding histidine residues. The first derivative, AM1, contains two histidines and can undergo intermolecular cross-linking by metal at the air-water interface. AM1 forms cohesive interfacial films and stable foams in the presence of Zn(II), Co(II), or Ni(II), but not in the absence of metal ions. The second derivative, AFD4, has four histidine substitutions, and can undergo both intra- and intermolecular cross-linking by metal ions. AFD4 forms stronger interfacial films and more stable foams than AM1 in the presence of the same metal ions, and also undergoes helical structuring in solution in the presence of added metal ions. For both peptides, film formation and foam stabilization can be reversed by acidification of the bulk solution, or addition of a metal chelator.

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

We report the structure and Young's modulus of switchable films formed by peptide self-assembly at the air-water interface. Peptide surfactant AM1 forms an interfacial film that can be switched, reversibly, from a high- to low-elasticity state, with rapid loss of emulsion and foam stability. Using neutron reflectometry, we find that the AM1 film comprises a thin (approx. 15A) layer of ordered peptide in both states, confirming that it is possible to drastically alter the mechanical properties of an interfacial ensemble without significantly altering its concentration or macromolecular organization. We also report the first experimentally determined Young's modulus of a peptide film self-assembled at the air-water interface (E=80MPa for AM1, switching to E<20MPa). These findings suggest a fundamental link between E and the macroscopic stability of peptide-containing foam. Finally, we report studies of a designed peptide surfactant, Lac21E, which we find forms a stronger switchable film than AM1 (E=335MPa switching to E<4MPa). In contrast to AM1, Lac21E switching is caused by peptide dissociation from the interface (i.e. by self-disassembly). This research confirms that small changes in molecular design can lead to similar macroscopic behaviour via surprisingly different mechanisms.

Tuning curvature and stability of monoolein bilayers by designer lipid-like peptide surfactants.

This study reports the effect of loading four different charged designer lipid-like short anionic and cationic peptide surfactants on the fully hydrated monoolein (MO)-based Pn3m phase (Q224). The studied peptide surfactants comprise seven amino acid residues, namely A6D, DA6, A6K, and KA6. D (aspartic acid) bears two negative charges, K (lysine) bears one positive charge, and A (alanine) constitutes the hydrophobic tail. To elucidate the impact of these peptide surfactants, the ternary MO/peptide/water system has been investigated using small-angle X-ray scattering (SAXS), within a certain range of peptide concentrations (R≤0.2) and temperatures (25 to 70°C). We demonstrate that the bilayer curvature and the stability are modulated by: i) the peptide/lipid molar ratio, ii) the peptide molecular structure (the degree of hydrophobicity, the type of the hydrophilic amino acid, and the headgroup location), and iii) the temperature. The anionic peptide surfactants, A6D and DA6, exhibit the strongest surface activity. At low peptide concentrations (R = 0.01), the Pn3m structure is still preserved, but its lattice increases due to the strong electrostatic repulsion between the negatively charged peptide molecules, which are incorporated into the interface. This means that the anionic peptides have the effect of enlarging the water channels and thus they serve to enhance the accommodation of positively charged water-soluble active molecules in the Pn3m phase. At higher peptide concentration (R = 0.10), the lipid bilayers are destabilized and the structural transition from the Pn3m to the inverted hexagonal phase (H2) is induced. For the cationic peptides, our study illustrates how even minor modifications, such as changing the location of the headgroup (A6K vs. KA6), affects significantly the peptide's effectiveness. Only KA6 displays a propensity to promote the formation of H2, which suggests that KA6 molecules have a higher degree of incorporation in the interface than those of A6K.

Structure and conformation of the disulfide bond in dimeric lung surfactant peptides SP-B 1–25 and SP-B 8–25

Raman spectroscopy was used to determine the conformation of the disulfide linkage between cysteine residues in the homodimeric construct of the N-terminal alpha helical domain of surfactant protein B (dSP-B 1–25). The conformation of the disulfide bond between cysteine residues in position 8 of the homodimer of dSP-B 1–25 was compared with that of a truncated homodimer (dSP-B 8–25) of the peptide having a disulfide linkage at the same position in the alpha helix. Temperature-dependent Raman spectra of the S–S stretching region centered at ∼ 500 cm − 1 indicated a stable, although highly strained disulfide conformation with a χ(CS–SC) dihedral angle of ± 10° for the dSP-B 1–25 dimer. In contrast, the truncated dimer dSP-B 8–25 exhibited a series of disulfide conformations with the χ(CS–SC) dihedral angle taking on values of either ± 30° or 85± 20°. For conformations with χ(CS–SC) close to the ± 90° value, the Raman spectra of the 8–25 truncated dimers exhibited χ(SS–CC) dihedral angles of 90/180° and 20–30°. In the presence of a lipid mixture, both constructs showed a ν(S–S) band at ∼ 488 cm − 1 , corresponding to a χ(CS–SC) dihedral angle of ± 10°. Polarized infrared spectroscopy was also used to determine the orientation of the helix and β-sheet portion of both synthetic peptides. These calculations indicated that the helix was oriented primarily in the plane of the surface, at an angle of ∼ 60–70° to the surface normal, while the β structure had ∼ 40° tilt. This orientation direction did not change in the presence of a lipid mixture or with temperature. These observations suggest that: (i) the conformational flexibility of the disulfide linkage is dependent on the amino acid residues that flank the cysteine disulfide bond, and (ii) in both constructs, the presence of a lipid matrix locks the disulfide bond into a preferred conformation.

Designer Self‐Assembling Peptide Materials

Understanding of macromolecular materials at the molecular level is becoming increasingly important for a new generation of nanomaterials for nanobiotechnology and other disciplines, namely, the design, synthesis, and fabrication of nanodevices at the molecular scale from bottom up. Basic engineering principles for microfabrication can be learned through fully grasping the molecular self-assembly and programmed assembly phenomena. Self- and programmed-assembly phenomena are ubiquitous in nature. Two key elements in molecular macrobiological material productions are chemical complementarity and structural compatibility, both of which require weak and non-covalent interactions that bring building blocks together during self-assembly. Significant advances have been made during the 1990s at the interface of materials chemistry and biology. They include the design of helical ribbons, peptide nanofiber scaffolds for three-dimensional cell cultures and tissue engineering, peptide surfactants for solubilizing and stabilizing diverse types of membrane proteins and their complexes, and molecular ink peptides for arbitrary printing and coating surfaces as well as coiled-coil helical peptides for multi-length scale fractal structures. These designer self-assembling peptides have far reaching implications in a broad spectrum of applications in biology, medicine, nanobiotechnology, and nanobiomedical technology, some of which are beyond our current imaginations. [image: see text]

The global role of ppGpp synthesis in morphological differentiation and antibiotic production in Streptomyces coelicolor A3(2).

The induction of ppGpp synthesis in Streptomyces coelicolor influenced the expression of several genomic elements characteristic of streptomycete biology, including antibiotic gene clusters, conservons, and morphogenetic proteins.

Fluorescence, Circular Dichroism, Light Scattering, and Microscopic Characterization of Vesicles of Sodium Salts of Three N-Acyl Peptides

Aggregation behavior of three N-acyl peptide surfactants, sodium N-(4-n-dodecyloxybenzoyl)-L-alyl-L-valinate (SDBAV), L-valyl-L-alaninate (SDBVA), and L-valyl-L-valinate (SDBVV), were investigated. The amphiphiles have very low critical aggregation concentration (cac). Fluorescence anisotropy studies using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a fluorescent probe indicated formation of bilayer aggregates in dilute solution. Transmission electron micrographs showed the existence of large vesicles in dilute solution. Circular dichroism spectra suggested formation of helical aggregates. The vesicle formation was found to be more favored at neutral pH. Dynamic light scattering was used to measure hydrodynamic radius of the vesicles. The microviscosity of the vesicles formed by the amphiphiles was determined by use of fluorescence anisotropy and the lifetime of the DPH probe. The vesicles formed by the surfactants are stable at temperatures above body temperature and for a long period of time. Fluorescence probe studies, however, indicated transformation of vesicles to rod-like micelles at surfactant concentrations much higher than the cac value. Addition of sodium chloride also transformed the vesicles to rod-like micelles.

Designer short peptide surfactants stabilize G protein-coupled receptor bovine rhodopsin.

Membrane proteins play vital roles in every aspect of cellular activities. To study diverse membrane proteins, it is crucial to select the right surfactants to stabilize them for analysis. Despite much effort, little progress has been made in elucidating their structure and function, largely because of a lack of suitable surfactants. Here we report the stabilization of a G protein-coupled receptor bovine rhodopsin in solution, using a new class of designer short and simple peptide surfactants. These surfactants consist of seven amino acids with a hydrophilic head, aspartic acid or lysine, and a hydrophobic tail with six consecutive alanines. These peptide surfactants not only enhance the stability of bovine rhodopsin in the presence of lipids and the common surfactants n-dodecyl-beta-D-maltoside and octyl-D-glucoside, but they also significantly stabilize rhodopsin under thermal denaturation conditions, even after lipids are removed. These peptide surfactants are simple, versatile, effective, and affordable. They represent a designer molecular nanomaterial for use in studies of diverse elusive membrane proteins.

Foaming properties of a peptide designed to form stimuli-responsive interfacial films.

We have designed an amphipathic peptide, AM1, that can self-assemble at the air-water interface to form an interfacial ensemble capable of switching between a mechanically strong cohesive film state and a mobile detergent state in response to changes in the solution conditions. The mechanical properties of the AM1 ensemble in the cohesive film state are qualitatively equivalent to the protein β-LG, while in the mobile detergent state they are equivalent to the low molecular weight surfactant, SDS. In this work the foaming properties of AM1 are compared to those of β-LG and SDS at the same weight concentration and it is found that AM1 adsorbs rapidly to the interface, initially forming a dense foam like that formed by SDS and superior to β-LG. In addition, under solution conditions where interfacially adsorbed AM1 forms a cohesive film state the foam stability is high, comparable to β-LG. However when the interfacially adsorbed AM1 forms a foam under detergent-state conditions, the foam stability is poor. We have achieved control of foam stability through the design of a peptide that exhibits stimuli-responsive changes in the extent of intermolecular interactions between peptide molecules adsorbed at the air-water interface. These results illustrate the exciting potential of peptide surfactants to form a new class of stimuli-responsive foaming agents.

Molecular Dynamics Study of the Lung Surfactant Peptide SP-B1–25 with DPPC Monolayers: Insights into Interactions and Peptide Position and Orientation

We have performed molecular dynamics simulations of the interactions of the peptide SP-B1–25, which is a truncated version of the full pulmonary surfactant protein SP-B, with dipalmitoylphosphatidylcholine monolayers, which are the major lipid components of lung surfactant. Simulations of durations of 10–20ns show that persistent hydrogen bonds form between the donor atoms of the protein and the acceptors of the lipid headgroup and that these bonds determine the position, orientation, and secondary structure of the peptide in the membrane environment. From an ensemble of initial conditions, the most probable equilibrium orientation of the α-helix of the peptide is predicted to be parallel to the interface, matching recent experimental results on model lipid mixtures. Simulations of a few mutated analogs of SP-B1–25 also suggest that the charged amino acids are important in determining the position of the peptide in the interface. The first eight amino acids of the peptide, also known as the insertion sequence, are found to be essential in reducing the fluctuations and anchoring the peptide in the lipid/water interface.

Extracellular matrix-like surfactant polymers containing arginine-glycine-aspartic acid (RGD) peptides.

We report on a novel series of biomimetic polymers exhibiting interfacial properties similar to the extracellular matrix. A series of well-defined surfactant polymers were synthesized by simultaneously incorporating arginine-glycine-aspartic acid (RGD) peptide, dextran oligosaccharide, and hexyl ligands with controlled feed ratios onto a poly(vinyl amine) (PVAm) backbone. The peptide sequence was H-GSSSGRGDSPA-NH(2) (Pep) having a hydrophilic extender at the amino terminus and capped carboxy terminus. The peptide-to-dextran (Pep:Dex) ratios were varied to create surfactants having 0, 25, 50, 75, and 100 mol-% peptide relative to dextran. The surfactants were characterized by IR, NMR and atomic force microscopy (AFM) for composition and surface active properties. AFM confirmed full surface coverage of PVAm(Pep)(100%) on graphite, and supported the mechanism of interdigitation of hexyl ligands between surfactant molecules within a specified range of hexyl chain densities. the attachment and growth of human pulmonary artery endothelial cells on the PVAm(Pep)(100%) surface was identical to the fibronectin positive control. Cell adhesion decreased dramatically with decreasing peptide density on the surfactant polymers. Molecular model of a peptide surfactant polymer, consisting of poly(vinyl amine) backbone with peptide, dextran oligosaccharide and hexyl branches coupled to the polymer chain.

Integration of Photosynthetic Protein Molecular Complexes in Solid-State Electronic Devices

Plants and photosynthetic bacteria contain protein−molecular complexes that harvest photons with nearly optimum quantum yield and an expected power conversion efficiency exceeding 20%. In this work, we demonstrate the integration of electrically active photosynthetic protein−molecular complexes in solid-state devices, realizing photodetectors and photovoltaic cells with internal quantum efficiencies of approximately 12%. Electronic integration of devices is achieved by self-assembling an oriented monolayer of photosynthetic complexes, stabilizing them with surfactant peptides, and then coating them with a protective organic semiconductor.

Design of molecular biological materials using peptide motifs

Two complementary fabrication technologies are employed in the production of materials and tools. In the ‘top-down’ approach, materials and tools are manufactured by stripping down an entity into its parts, e.g. carve a boat from a tree trunk. This contrasts sharply with the ‘bottom-up’ approach, in which materials and tools are assembled bit by bit to produce supra-structures and architectures, e.g. build a ship using wood strips. The bottom-up approach is likely to become an integral part of materials manufacture in the coming decades. This approach requires a deep understanding of individual molecular building blocks, their structures, assembling properties and dynamic behaviors. Two key elements in molecular materials manufacture are chemical complementarity and structural compatibility, both of which confer the weak and noncovalent interactions that bind building blocks together during self-assembly. Following Nature's leads, significant advances have been made at the interface of materials chemistry and biology, including the design of helical ribbons, peptide nanofiber scaffolds for three-dimensional cell cultures and tissue engineering, peptide surfactants, peptide detergents for solubilizing, stabilizing and crystallizing diverse types of membrane proteins and their complexes.

Positively Charged Surfactant-like Peptides Self-assemble into Nanostructures

A new type of surfactant peptide designed to mimic the properties of cationic lipid systems is described. These cationic surfactant peptides are approximately 2 nm in length with a cationic, hydrophilic head consisting of one to two residues of lysine or histidine followed by a hydrophobic tail of six alanine, valine, or leucine residues. In water, these surfactant peptides form ordered structures with dynamic behaviors. At pH below the pI values of the peptides, dynamic light scattering showed two distinct structural populations with average diameters of 50 nm (>95%) and 100−200 nm (<5%). Transmission electron microscopy visualization of quick-frozen samples revealed these populations to likely represent nanotubes and nanovesicles, respectively, showing great interplay between them. Above the pI, these structures are absent, having further self-assembled into large membranous sheets. These cationic surfactant peptides are distinct from other anionic surfactant peptides and have different applications, po...

Molecular self-assembly of surfactant-like peptides to form nanotubes and nanovesicles.

Several surfactant-like peptides undergo self-assembly to form nanotubes and nanovesicles having an average diameter of 30-50 nm with a helical twist. The peptide monomer contains 7-8 residues and has a hydrophilic head composed of aspartic acid and a tail of hydrophobic amino acids such as alanine, valine, or leucine. The length of each peptide is approximately equal to 2 nm, similar to that of biological phospholipids. Dynamic light-scattering studies showed structures with very discrete sizes. The distribution becomes broader over time, indicating a very dynamic process of assembly and disassembly. Visualization with transmission electron microscopy of quick-freeze/deep-etch sample preparation revealed a network of open-ended nanotubes and some vesicles, with the latter being able to "fuse" and "bud" out of the former. The structures showed some tail sequence preference. Many three-way junctions that may act as links between the nanotubes have been observed also. Studies of peptide surfactant molecules have significant implications in the design of nonlipid biological surfactants and the understanding of the complexity and dynamics of the self-assembly processes.

ROLE OF SURFACTANT PROTEIN A (SP-A)/LIPID INTERACTIONS FOR SP-A FUNCTIONS IN THE LUNG

Surfactant protein A (SP-A), an oligomeric glycoprotein, is a member of a group of proteins named collections that contain collagen-like and Ca2+ -dependent carbohydrate recognition domains. SP-A interacts with a broad range of amphipathic lipids (glycerophospholipids, sphingophospholipids, glycosphingolipids, lipid A, and lipoglycans) that are present in surfactant or microbial membranes. This review summarizes SP-A/lipid interaction studies regarding the lipid system used (i.e., phospholipid vesicles, phospholipid monolayers, and lipids immobilized on silica or adsorbed on a solid support). The effect of calcium, ionic strength, and pH on the binding of SP-A to lipids and the subsequent lipid aggregation process is discussed. Current evidence suggests that hydrophobic-binding forces are involved in the peripherical association of SP-A to membranes. It is also proposed that fluid and liquid-ordered phase coexistence in surfactant membranes might favor partition of SP-A into those membranes. The binding of SP-A to surfactant membranes containing hydrophobic surfactant peptides makes possible the formation of a membrane reservoir in the alveolar fluid that is protected by SP-A against inactivation and improves the rate of surfactant film formation. In addition, the interaction of SP-A with membranes might enhance the affinity of SP-A for terminal carbohydrates of glycolipids or glycoproteins on the surface of invading microorganisms.

Secondary structure in lung surfactant SP-B peptides: IR and CD studies of bulk and monolayer phases

Pulmonary surfactant protein SP-B is known to facilitate adsorption and spreading of surfactant components across the air/water interface. This property appears essential for in vivo function in the alveolar subphase and at the air/alveolar surface. Three peptides with amino acid sequences based on SP-B containing predicted α-helical regions (SP-B1-20, SP-B9-36A, SP-B40-60A) have been synthesized to probe structure-function relationships and protein-lipid interaction in bulk phase and monolayer environments. IR and CD studies are reported along with traditional surface pressure-molecular area (π-A) isotherms and IR reflection-absorption spectroscopy (IRRAS) investigations conducted at the air/water interface. In bulk phase, helix-promoting environments (methanol and aqueous dispersions of lipid vesicles), SP-B1-20 and SP-B9-36A contained significant amounts of α-helical structure, whereas varying degrees of α-helix, random coil, and β-sheet were observed in aqueous solutions and monolayers. The most striking behavior was observed for SP-B9-36A, which displayed reversible surface pressure-induced β-sheet formation. Bulk phase lipid melting curves and monolayer experiments with peptide-lipid mixtures showed subtle differences in the degree of bulk phase interaction and substantial differences in peptide surface activity. The uniqueness of IRRAS is emphasized as the importance of evaluating secondary structure in both bulk phase and monolayer environments for lung surfactant peptide mimics is demonstrated.

Role of Surfactant Protein a (SP-A)/Lipid Interactions for SP-A Functions in the Lung

Surfactant protein A (SP-A), an oligomeric glycoprotein, is a member of a group of proteins named collectins that contain collagen-like and Ca(2+)-dependent carbohydrate recognition domains. SP-A interacts with a broad range of amphipathic lipids (glycerophospholipids, sphingophospholipids, glycosphingolipids, lipid A, and lipoglycans) that are present in surfactant or microbial membranes. This review summarizes SP-A/lipid interaction studies regarding the lipid system used (i.e., phospholipid vesicles, phospholipid monolayers, and lipids immobilized on silica or adsorbed on a solid support). The effect of calcium, ionic strength, and pH on the binding of SP-A to lipids and the subsequent lipid aggregation process is discussed. Current evidence suggests that hydrophobic-binding forces are involved in the peripherical association of SP-A to membranes. It is also proposed that fluid and liquid-ordered phase coexistence in surfactant membranes might favor partition of SP-A into those membranes. The binding of SP-A to surfactant membranes containing hydrophobic surfactant peptides makes possible the formation of a membrane reservoir in the alveolar fluid that is protected by SP-A against inactivation and improves the rate of surfactant film formation. In addition, the interaction of SP-A with membranes might enhance the affinity of SP-A for terminal carbohydrates of glycolipids or glycoproteins on the surface of invading microorganisms.