Sequence Dependent Behavior of Amphiphilic β-Peptides on Gold Surfaces

We describe the formation of self-assembled monolayers (SAMs) on gold by helical β-peptides and show that β-peptide sequence can determine whether or not the SAM displays internal order. β-Peptide sequence isomers were designed either to be globally amphiphilic ( GA ), i.e., to display segregated hydrophilic and lipophilic surface patches, or to display an even distribution of hydrophilic and lipophilic side chains ( iso -GA ). These isomers were compared in terms of two-dimensional self-assembly on a gold surface. β-Peptides derivatized with 3-mercapto-propionic acid, termed GA-SH and iso -GA-SH, display an N-terminal thiol for formation of gold-sulfur bonds. Comparison of ellipsometry measurements and the Amide I /Amide II ratios from infrared spectroscopy indicates that interactions between globally amphiphilic β-peptide molecules dictate the organization of the SAM formed from GA-SH . In contrast, the non-globally amphiphilic isomer iso -GA-SH formed a disordered monolayer. Comparison of the IR data for GA-SH and an analogue lacking a terminal thiol group, GA-NH 2, suggests that thiol-mediated chemisorption to the gold surface is not required for organization but facilitates the concentration of β-peptides at the surface. These studies suggest that sequence control among β-peptides is important for generating an ordered monolayer. Because the helices formed by appropriately designed β-peptides are more stable than helices formed by α-peptides, we suggest that β-peptide SAMs provide an avenue to the rational engineering of surfaces in which chemical groups are presented in precise and predictable arrangements.