Phase Transitions of an Interfacially Confined Amphiphilic Peptide

The work described here investigates the influences of temperature on the self-assembly of a rationally designed amphiphillic b-strand forming peptide, allowing us to explore the thermodynamics of pattern formation at the air/water interface. Using Langmuir-Blodgett (LB) trough, we observe the surface activity of peptide molecules via surface pressure-area isotherms. By taking images of structures in the compression and expansion isotherms using a Brewster Angle Microscopy, we are able to examine the effects of temperature on network formation. Additionally, we find the critical area or pressure for Beta 9H peptide where there is a transition of random structure to ordered fiber structure.

Periodically sequenced amphiphilic peptides can be designed to form nanostructure as a function of surface pressure at the air-water interface. These self-assembled architectures can then be used to create templates for biomaterials, sensing applications and molecular electronics. We explore several fundamental factors that affect the self-assembly of peptides in two-dimensions, including peptide concentration, temperature, pH, and electrolyte concentration. Our study shows that we can quantitatively describe the influence of temperature on the phase behavior of the confined peptide, and we can examine the parameters that govern the transition from a disordered to a fibrillar state.

This work involves peptides molecules that are rationally designed to form β-sheets that are confined to the air-water interface. The molecules are designed by selecting amino acids that have a high propensity of being found in a β-sheet and a periodicity to yield an amphiphilic architecture at a surface. The Beta 9H peptides shown in figure 1 contains 17 amino acids, including valine, glutamine, tryptophan, and histidine. Beta 9H has 9 residues (~3.5 nm) in between its two histidine residues.Theisoelectricpointofhistidine(pI=6.0)contributes to the difference in electrostatic repulsion of the overall peptide molecule in deionized water (pH ~5.5). Furthermore, the overall charge of the molecules can be tuned as a function of pH as shown in an earlier study. Here, we focus on the influences of temperature on these assemblies during both compression and expansion cycles in the Langmuir isotherms.

We characterize the phase behavior of these peptides using Langmuir Blodgett (LB) experiments and Brewster Angle Microscopy (BAM). The Langmuir trough measures the surface activity of the molecules by defining the surface pressure as a function of molecular area at a constant temperature (Langmuir isotherms). These isotherms show the phase behavior by compressing the confined molecules from a gaseous state to a two-dimensional liquid state, and, at high surface pressures, a solid state. Using Brewster Angle Microscopy, we are able to visualize phase behavior as a function of surface pressure. Moreover, by analyzing images of the peptide structure as a function of surface pressure, we observe the critical area or pressure where there is a transition of random structure to ordered fiber structures.