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 di stinct film components with characteristic frictional, morphological, and adsorptive signatures. A high-friction continuous film 1-4 nm thic k strongly adheres to mica, while a low-friction component is more wea kly adsorbed as large islands on top of, or small domains within, the high-friction layer. The low-friction component exhibits a porous morp hology and fluid-like character and is selectively destroyed when the film is heated sufficiently. A high-force scanning procedure remarkabl y transforms the molecularly-rough high-friction film into the molecul arly-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 gel atin attached to the SFM tip. The anticipated network structure of gel atin is observed on the high-friction layer. The low-friction material is interpreted as moieties of intramolecularly-folded gelatin, with t hickness (1.5 +/- 0.2 nm) equal to the diameter of the collagen-fold t riple helix, containing substantial structural water. Analysis suggest s that differences in viscoelasticity account for the component-specif ic frictional dissipation.