Reversible active switching of the mechanical properties of a peptide film at a fluid–fluid interface
Entry by Richie Tay for AP 225 Fall 2012
Authors: Annette Dexter, Andrew Malcolm and Anton Middelberg
The ability to control the properties of fluid–fluid interfaces is useful in industrial processes that rely on foams and emulsions, such as oil recovery, waste-water treatment, food processing and pharmaceutical formulation. Surfactants stabilize foams and emulsions by lowering the interfacial tension and generating electrostatic and/or steric barriers to coalescence. They fall into two broad classes: the low-molecular-weight detergents (e.g. polar lipids) we are familiar with, which have high lateral mobility in the interface; and polymers (including proteins), which have limited lateral mobility but form a cohesive interfacial film that prevents the rupture of thin films between bubbles or droplets.
Here the authors designed a peptide surfactant capable of switching from the less-stabilizing "detergent state" to the more-stabilizing "film state" using external triggers. The 21-residue peptide, AM1 (Ac-MKQLADSLHQLARQVSRLEHA-CONH2), forms an <math>\alpha</math>-helix at air- or oil-water interfaces. Histidine residues in the bulk aqueous phase orient towards neighboring peptide molecules at the interface, allowing the helices to be cross-linked in the presence of zinc ions to form a cohesive "film". This cross-linking can be reversed in the presence of EDTA (a Zn2+ chelator) or at low pH (when the His residues are uncharged). The authors demonstrate the ability of this stimuli-responsive surfactant to reversibly stabilize emulsions and foams.
Results and Discussion
The authors first looked at the mechanical properties of an air-water interface containing 5 μM of AM1 in different architectures. Interfacial stress-strain curves were obtained using an interfacial tensiometer. In the absence of Zn2+, the interfacial elasticity modulus (initial slope of the interfacial stress vs. strain curve in the low-strain elastic region) was less than 30 mN/m and the maximum interfacial stress was only 0.5 mN/m, corresponding to a "loose-knit" detergent state. With the addition of 100 μM Zn2+, the elasticity modulus increased to 121 mN/m, and both the maximum interfacial tensile stress (6.9 mN/m) and minimum interfacial compressive stress (−8.7 mN/m) were also greatly enhanced (Fig. 1a), showing that a film state was formed that could transmit significant force in the plane of the interface. The surface tension of the air-water interface was the same with or without Zn2+; this, combined with neutron-reflectivity measurements showing an approximate monolayer of peptide in both cases, showed that the difference between the film and detergent states arose from changes in interaction between peptide molecules at the interface (and not from multilayering or desorption of peptide). Importantly, this switch in architecture was reversible, as the addition of EDTA abolished the film state and reversed the increase in force transmission (Fig. 1a, dashed line vs. solid line). The film state could also be disrupted by acidification of the bulk solution to pH 3.6 (Fig. 1b, dotted line), and restored with subsequent neutralization of the bulk solution (Fig. 1b, dashed line).
They moved on to test the effects of AM1 architecture on the stability of oil-in-water emulsions. They prepared a 20% v/v toluene-in-water emulsion containing AM1 and Zn2+, which was stable over 20 hr on standing (Fig. 2a). The measured emulsion activity index (EAI) of AM1 was 360 m2 of interface stabilized per gram of peptide, which compares well with SDS at the same concentration (673 m2/g). This stability was abolished on acidification of the emulsion (Fig. 2b-d) or addition of EDTA; coalescence occurred within seconds and was complete after several minutes. Similarly, if the AM1 solution started off acidic or with added EDTA, the dispersion formed on homogenization with toluene coalesced within seconds, and a stable emulsion was not formed.
A similar stability reversal was seen with AM1 in air-water foams. A foam stabilized by AM1 at pH 7.4 rapidly collapsed on acidification (Fig. 3b), and the acidified solution failed to form a stable foam (Fig. 3d). Neutralization of the aqueous solution restored the foam stabilization ability of AM1 (Fig. 3f). The same effects on foam stability were also observed when EDTA was used to chelate added metal ions. Thus, reversible film formation by AM1 correlates well with foam stability as well as emulsion stability.
In summary, the authors have designed an amphipathic peptide with switchable interfacial architecture that responds reversibly to bulk solution composition. Although chemical surfactants exist that can switch interfacial tension in response to pH, UV irradiation, metal binding and other triggers, those interfacial changes are insufficient to produce rapidly switchable emulsions and foams. This peptide surfactant thus presents a facile means to both stabilize emulsions and rapidly demulsify them to recover the oil and water phases. Further, the possibility of low-cost bioproduction of peptides and the inherent biodegradability of peptides makes this class of surfactants a uniquely sustainable technology. Beyond the novelty of a switchable surfactant system, I find this paper interesting because it demonstrates the use of surfactants in controlling interfacial properties other than surface tension, in this case, the interfacial "stiffness".