Surface Force Measurements of Electrostatic and Hydrogen-Bonding Interactions between Bilayers of Glycine Amphiphiles

Abstract

Using the surface-force apparatus (SFA), we have made sensitive force and adhesion measurements between Langmuir−Blodgett (LB) bilayers of a novel glycine-containing amphiphile at various pHs. The adhesion between the glycine headgroups is remarkably strong (F/R = −80 ± 5 mN/m) and leads to the extraction of amphiphiles from the bilayers on surface separation. By SFA measurements and Fourier transform infrared spectroscopy on LB layers of glycine amphiphile variants, we determined that hydrogen bonding between amide and carbonyl groups on opposing surfaces is responsible for the strong adhesion. The strong adhesion is accompanied by a 7.5 Å interpenetration of the glycine headgroups. At higher pH, charging of the carboxylic acid termini gives rise to electrostatic repulsion between the surfaces and reduces the strength of adhesion. Above pH 8.0, the surface charging blocks interpenetration and the adhesion is extinguished. Analysis of the forces required for amphiphile extraction shows that these amide hydrogen bonds are much stronger than those in aqueous environments. This observation has important implications in the study of protein stability and the design of self-assembled biomaterials.