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

Greg Haugstad and Wayne L. Gladfelter * Department of Chemistry, University of Minnesota

Elizabeth B. Weberg, Rolf T. Weberg and Timothy D. Weatherill Medical Products Division, E. I. du Pont de Nemours and Co.

Abstract

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 (1.50.2 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.