Probing Biopolymers with Scanning Force Methods: Adsorption, Structure, Properties, and Transformation of Gelatin on Mica

Scanning force microscopy of thin gelatin films on mica reveals two distinct film components with characteristic frictional, morphological, and adsorptive signatures. A high-friction continuous film 1–4 nm thick strongly adheres to mica, while a low-friction component is more weakly adsorbed as large islands on top of, or small domains within, the high-friction layer. The low-friction component exhibits a porous morphology and fluid-like character and is selectively destroyed when the film is heated sufficiently. A high-force scanning procedure remarkably transforms the molecularly-rough high-friction film into the molecularly-smooth low-friction component if a sufficient amount of water is present in or on the film. The nanostructure of both the high-and low-friction components is imaged using a nanometer-scale asperity of gelatin attached to the SFM tip. The anticipated network structure of gelatin is observed on the high-friction layer. The low-friction material is interpreted as moieties of intramolecularly-folded gelatin, with thickness (1.5 ± 0.2 nm) equal to the diameter of the collagen-fold triple helix, containing substantial structural water. Analysis suggests that differences in viscoelasticity account for the component-specific frictional dissipation.