Sequence-Directed Assembly of β-peptide Oligomers at Phospholipid Membranes (2011)

AFM height images of DPPC/DOPC (1:1) supported lipid bilayers before (A and D) and after exposure to β-peptides (100 µM in Tris-HCl buffer, pH 7.4) GA (B and C) and iso-GA (E and F). DPPC gel-phase domains (lighter features) extend ~ 1nm above the DOPC surrounding fluid matrix. Inset in Figure 2B shows the height analysis for the defects observed after exposure to GA. Images were obtained in 10 mM Tris/ 100mM NaCl buffer, pH 7.4.

Past studies within Thrust 2 have shown that β-peptide nanorods display antifungal activity that is strongly dependent upon sequence and thus nanopatterning of chemical groups projected from the β-peptide helix. The exact mechanism of action of antifungal β-peptides is not fully understood but results from these past studies hint that it involves disruption of the cell membrane. Motivated by these previous observations, recent investigations based on in situ imaging using atomic force microscopy (AFM) by NSEC graduate student Claribel Acevedo have provided important insight into the impact of chemical nanopatterning on the interactions of β-peptide nanorods with model lipid membranes. In this study, β-peptides were designed to be either globally amphiphilic (GA), i.e., display a global segregation of side chains bearing hydrophobic and cationic functional groups, or non-globally amphiphilic (iso-GA), i.e., display a more uniform distribution of hydrophobic and cationic functional groups in three-dimensions. The interactions of these sequence isomers with a model membrane comprised of supported lipid bilayers of dioleoylglycero-phosphocholine (DOPC) and dipalmitoylglycero-phosphocholine (DPPC) were studied. The lipid mixture was selected because it is known to phase-separate at room temperature (Figure 1A and Figure 1D), which allowed observation of the effect of β-peptides on membrane domain structure. The results in Figure 1B/C show that GA interacts with the membrane to induce disruption of the lipid bilayer structure. Specifically, the formation of transient defects (~ 1nm in depth) at the DOPC fluid phase and the DPPC/DOPC domain boundaries was observed. These defects disappeared over time and the DOPC gel domains became miscible in the surrounding DOPC fluid phase (phase-separation is no longer observed in Figure 2C). In contrast, a change in the lipid bilayer structure was not observed with iso-GA. These results thus show striking structural evidence of sequence-dependent interactions between β-peptides and model phospholipid membranes. Studies with model membranes that more closely mimic the properties of cell membranes (i.e., eukaryotic and bacterial cells) are ongoing.