Thrust 2: Tuning the Strength of Hydrophobic Interactions using Immobilized Ions

C. Derek Ma, Chenxuan Wang, Claribel Acevedo-Vélez, Samuel H. Gellman, and Nicholas L. Abbott

Figure : (a) structure of the β-peptides that were designed to be globally amphiphilic (GA), the charging of -lysine (ammonium) residues proximal to a hydrophobic domain strengthens hydrophobic interactions between -peptides, and -arginine residues, in striking contrast to -lysine, largely eliminate hydrophobic interactions associated with neighboring hydrophobic domains of b-peptides. (b) Structure of mixed monolayers of organosulfur compounds on gold films that presented methyl groups along with either amine or guanidine groups, (c) schematic of the single force measurements, and (d) adhesion force histograms of mixed component methyl/amine/ammonium monolayers interacting with a hydrophobic AFM tip.

The dynamic structuring of water near non-polar molecules and surfaces mediates cohesive interactions (so-called hydrophobic interactions) that underlie a broad range of nano-scale phenomena, including wetting, protein folding and molecular self-assembly. In many systems, however, non-polar nanodomains are found in close proximity to polar and charged functional groups. Theoretical studies predict that immobilized hydrophilic groups dramatically alter the dynamic organization of water near non-polar nanodomains, but experimental evidence of such effects on hydrophobic interactions has been lacking. Thrust 2 of the Wisconsin NSEC has used chemical force spectroscopy to probe surfaces presenting methyl and amine/ammonium (Am) groups to reveal that protonation of amines can double the strength of hydrophobic interactions.

In striking contrast, guanidine/guanidinium (Gdm) groups, when immobilized with methyl groups, are found to eliminate measurable hydrophobic interactions. These pronounced and varied effects of immobilized charged groups on hydrophobic interactions were confirmed by independent force measurements using a biologically-inspired system comprised of conformationally well-defined oligomers of β-amino acids (β-peptides) that included β3-homolysine and β3-homoarginine residues. Overall, these results indicate that cations placed adjacent to non-polar nanodomains can substantially tune the strength of hydrophobic interactions.