Forces between carboxyl and amide groups measured by atomic force microscopy

The atomic force microscope (AFM) has been used to measure the interaction force between a hydrophilic (COOH-functionalised) AFM tip and an oligopeptide (Cys-Gly-Ala-Ala-Ala-Ala amide) self-assembled him as a function of their separation distance. These measurements produce force-distance curves characterised by two relative minima in the regions in which the tip is strongly attracted by the film surface. We have performed control experiments to prove the relationship of one of the two minima with the formation of hydrogen bonds between tip carboxyl groups and oligopeptide terminal amide groups. Under conditions less favourable to hydrogen-bond formation, curves with only one minimum are recorded. Other experiments performed by using the same functionalised tips and self-assembled alkanethiol films confirm that the less deep minimum is related to hydrogen bonding between the tip and the sample.