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

COMING SOON!

Entry by Richie Tay for AP 225 Fall 2012

## General

Authors: Annette Dexter, Andrew Malcolm and Anton Middelberg

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 $\alpha$-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]

The authors first looked at the mechanical properties of an air-water interface containing different AM1 architectures.