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

Abstract

We report the structure and Young's modulus of switchable films formed by peptide selfassembly 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. 15 Å) 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 airwater interface (EZ80 MPa for AM1, switching to E!20 MPa). 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 (EZ335 MPa switching to E!4 MPa). In contrast to AM1, Lac21E switching is caused by peptide dissociation from the interface (i.e. by selfdisassembly). This research confirms that small changes in molecular design can lead to similar macroscopic behaviour via surprisingly different mechanisms.

Interfacial assembly of proteins and peptides: recent examples studied by neutron reflection

“…Since the late 1990s, its use in the study of different biointerfacial phenomena has also been reported. The biomolecular systems that have so far been studied include proteins (Lu et al ,d, 1999a(Lu et al ,b,c, 2003bSu et al 1998aSu et al ,b,c, 1999Su et al , 2000Fragneto et al 2000b;Holt et al 2000;Nylander et al 2001;Armstrong et al, 2004;Cooper et al 2005;Xu et al 2006bXu et al , 2007Xu et al , 2008Pan et al 2008), peptides (Holt et al 2000;Lu et al 2003aLu et al , 2004Lad et al 2007;Middelberg et al 2008;Zhao et al 2009), biosurfactants (Follows et al 2007) and lipid bilayers (Fragneto et al 2000a. A number of neutron reflection studies have also focused on interfacial interactions from binary mixtures of protein/surfactant (Lu et al 1998a,b;Green et al 2000aGreen et al ,b, 2001, protein or peptide/lipid (Schmidt et al 1992;Lu et al 2001;Miano et al 2007;Valincius et al 2008), protein/polymer Hollmann et al 2007Hollmann et al , 2008; Ganzevles et al 2008), DNA/polymer (Zhao et al 2008a), DNA/surfactant (Zhang et al 2008) and DNA/ lipid (Wu et al 2006).…”

Insights into the role of protein molecule size and structure on interfacial properties using designed sequences

“…While the presence of the additional aspartate residue (with its negative charge) has the effect of shifting the theoretical pI slightly from 7.3 for DAMP1 to 6.7 for DAMP4, our earlier study of the foaming and film-forming properties of DAMP1 and DAMP4 [16] indicated that their behaviours were not significantly influenced by the presence of these additional residues. DAMP1 and DAMP4 are based on the surface-active peptide AM1 [17], which was designed to adsorb as a single a-helix at the air-water or oil-water interface (monolayer structure was confirmed by neutron reflectometry [18]). It is expected that DAMP1 will behave similarly.…”

Bioproduction and self-assembly of designer peptides

“…This peptide adsorbed to the air-water interface and lowered interfacial tension as a function of molecular charge. 194 At low pH, where glutamate residues are partially neutralised, Lac21E readily adsorbed to the interface, lowering interfacial tension and forming an interfacial film. Conversely, at high pH, where glutamate residues are deprotonated, peptide desorbed from the interface and film formation was lost.…”