Evaluating the Conformation of Adsorbed Proteins with Compositionally Sensitive SFM

Vishal Kanda, Truong C. Ta, Francis Nsiah and Mark T. McDermott*

Department of Chemistry, University of Alberta
Edmonton, Alberta T6G 2G2 Canada

The initial stages of biological response towards a biomaterial involve the spontaneous passivating of the foreign surface by protein films. The conformations adopted by proteins in a spontaneously adsorbed film on a biomaterial are important in governing further biological responses (e.g. cell binding and growth). Friction force imaging in contact mode scanning force microscopy (SFM) and the phase lag in tapping mode SFM can be used to generate images based upon spatial variations in surface chemical and physical properties. In the present work, friction and phase contrast imaging is shown to be sensitive to variations in the conformations of proteins adsorbed at the solid/liquid interface. Hence, these SPM imaging contrasts provides a means of directly observing substrate-induced differences in adsorbed proteins without the need for radioisotope or fluorophore labeling. Differences in the surface chemistries of self assembled monolayers (SAMs) are used to effect adsorption of proteins with varying degrees of conformational change from the native state. Efforts to understand the energy-dissipating mechanisms responsible for generation of phase contrast are presented. We use force curve (force vs. displacement) experiments to show the contributions of adhesion hysteresis and variations in mechanical properties of conformational disparate protein films to the phase contrast. Further evidence of the ability of different surface chemistries to induce varying degrees of conformational change is elucidated by surface plasmon resonance and infrared spectroscopic studies of antibody binding to adsorbed films.