Andrew S. Malcolm

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.

Tuneable Control of Interfacial Rheology and Emulsion Coalescence

Breaking point: Switchable peptide surfactants are used to demonstrate that the extent of cross-linking in an interfacial surfactant layer can control the rate of emulsion coalescence. Pictured is the rupture of an aqueous thin film where the peptide layer lacks sufficient strength to prevent hole formation, but nonetheless dramatically slows the rate of hole expansion.

Comparison of positional surfactant isomers for displacement of rubisco protein from the air―water interface

Protein-surfactant interaction, which is a function of the protein and surfactant characteristics, is a common phenomenon in a wide range of industrial applications. In this work, we used rubisco, the most abundant protein in nature, as a model protein and sodium dodecylbenzenesulfonate (SDOBS), one of the most widely used commercial surfactants, with two positional isomers (SDOBS-2 and SDOBS-6), as a model surfactant. We first examined the surface tension and the mechanical properties of interfacial mixed rubisco-SDOBS films adsorbed at the air-water interface. The concentration of rubisco in solution was fixed at 0.1 mg mL(-1) while the SDOBS concentration varied from 0 to 150 μM. Both the surface tension and the mechanical strength of the interfacial film decreased with increasing SDOBS concentration. Overall, the surface tension of a rubisco-SDOBS-6 mixture is lower than that of rubisco-SDOBS-2, while the mechanical strength of both systems is similar. Neutron reflection data suggest that rubisco protein is likely denatured at the interface. The populations of rubisco and SDOBS of the mixed systems at the interface were determined by combining non-deuterated and deuterated SDOBS to provide contrast variation. At a low surfactant concentration, SDOBS-6 has a stronger ability to displace rubisco from the air-water interface than SDOBS-2. However, when surfactant concentration reaches 50 μM, SDOBS-2 has a higher population than SDOBS-6, with more rubisco displaced from the interface. The results presented in this work suggest that the extent of protein displacement from the air-water interface, and hence the nature of the protein-surfactant interactions at the interface, are strongly affected by the position of surfactant isomerisation, which might allow the design of formulations for efficient removal of protein stains.

Mixed System of Eudragit S-100 with a Designed Amphipathic Peptide: Control of Interfacial Elasticity by Solution Composition

We report an interfacially active system based on an informational peptide surfactant mixed with an oppositely charged polyelectrolyte. The 21-residue cationic peptide, AM1, has previously been shown to respond reversibly to pH and metal ions at fluid interfaces, forming elastic films that can be rapidly switched to collapse foams or emulsions on demand. Here we report the reversible association of AM1 with the methacrylate-based anionic polymer Eudragit S-100. The strength of the association, in bulk aqueous solution, is modulated by added metal ions and by ionic strength. Addition of zinc ions to the peptide-polymer system promotes complex formation and phase separation, while addition of a chelating agent reverses the association. The addition of salt weakens peptide-polymer interactions in the presence or absence of zinc. At the air-water interface, Eudragit S-100 forms an elastic mixed film with AM1 in the absence of metal, under conditions where the peptide alone does not show interfacial elasticity. When zinc is present, the elasticity of the mixed film is increased, but the rate of interfacial adsorption slows due to formation of peptide-polymer complexes in bulk solution. An understanding of these interactions can be used to identify favorable foam-forming conditions in the mixed system.

Using nano-structured interfacial peptide films to create stimuli-responsive foams and emulsions

Proteins and peptides assemble nanostructured interfacial films when adsorbed at the fluid-fluid interface. We have measured the mechanical properties of these interfacial films with novel apparatus, the Cambridge interfacial tensiometer (CIT) and shown that the characteristic behaviour of the films can be modulated in response to changes in solution conditions. Changes in the mechanical properties of these interfacial films correspond to changes in the stability of both liquid droplets in emulsion systems and air bubbles in foam systems. We have developed a series of peptide surfactants (Pepfactants ) that are capable of stabilising foams and emulsions in a stimuli-responsive manner.