Reversible active switching of the mechanical properties of a peptide film at a fluid–fluid interface

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Entry by Richie Tay for AP 225 Fall 2012

General

Authors: Annette Dexter, Andrew Malcolm and Anton Middelberg

Publication: Dexter, A. et al. Reversible active switching of the mechanical properties of a peptide film at a fluid–fluid interface. Nature Materials 5, 502-506 (2006)

Keywords: emulsion, surfactant

Introduction

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

Figure 1. Switching of the mechanical properties of assembled AM1 at the air–water interface. (a) AM1 without metal ions (dotted line); after addition of ZnSO₄ (dashed line); and after subsequent addition of EDTA (solid line). (b) AM1 in the presence of ZnSO₄ at pH 7.4 (solid line); after acidification to pH 3.8 (dotted line); and after returning the bulk solution to pH 7.4 (dashed line). Figure from Ref. [1]

Figure 2. Reversible stabilization of toluene-in-water emulsion by AM1. Toluene phase was stained red and aqueous phase stained blue. (a) Both vials start off at pH 7.4; no additions were made to the left-hand vial, whereas an aliquot of H₂SO₄ was added to the right-hand vial with stirring. (b) 10 sec, (c) 20 sec, (d) 10 min after the addition of H₂SO₄. Figure from Ref. [1]

Figure 3. Reversible stabilization of air-in-water foam by AM1. (a) Foam was stable on standing for 10 min at pH 7.4. (b) It collapsed completely within 1 min after adding H₂SO₄. (c) A new foam was prepared from the acidified solution, but (d) it collapsed completely in 1 min. (e) The solution was neutralized with NaOH and a new foam was prepared. (f) This was stable on standing for 10 min. Figure from Ref. [1]

References

[1] Dexter, A. et al. Reversible active switching of the mechanical properties of a peptide film at a fluid–fluid interface. Nature Materials 5, 502-506 (2006)