Mechanistic aspects of protein/material interactions probed by atomic force microscopy

Physicochemical studies on the mechanisms of protein adsorption onto solid material surfaces have been extensively performed so far, mainly based on the analysis of factors such as the equilibrium adsorbed amount (adsorption isotherms), time-dependent change of adsorbed amount (adsorption kinetics), and conformational change of adsorbed protein. However, direct understanding of the strength of the molecular interaction between protein and the material surface has not been established yet. For this issue, the force measurement techniques of an atomic force microscope (AFM) using a protein-modified probe tip are recently becoming powerful tools to analyze the actual interaction forces between protein and material surfaces. In this mini review, we discuss the characteristics and interpretation of the AFM force-versus-distance curves (f-d curves) obtained with the protein-modified probe tip, and the relationship between the forces measured from the f-d curves and the driving forces in the natural process of protein adsorption. Relative degrees of each of the following contributions which determine the character of protein adsorption are clarified: (1) the intrinsic protein/material forces mediated by solvent, (2) the thermodynamic stability of protein/material adhesion interface, and (3) diffusion force of protein molecules. Within these driving forces, the latter two in particular are confirmed to play essential roles in determining the character of protein adsorption, based on the profiles of f-d curves.