Measurement of the interaction forces between proteins and iniferter-based graft-polymerized surfaces with an atomic force microscope in aqueous media

To investigate the characteristics of interaction forces between proteins and end-grafted polymer surfaces, force-versus-distance curves (f-d curves) were measured between protein-fixed probe tips (albumin (Alb) and lysozyme (Lyso)) and surfaces graft-polymerized with N,N-dimethylacrylamide (DMAAm) or acrylic acid (AAc) in an aqueous solution, using an atomic force microscope. DMAAm graft-polymerized surfaces with different chain lengths and AAc graft-polymerized surface were prepared by photopolymerization on a dithiocarbamate (iniferter)-immobilized surface. The effects of grafted chain length, grafting density, and electrostatic property of the grafted chain segments on the interaction forces in the processes of protein adsorption onto and desorption from the graft-polymerized surfaces were analyzed from the approaching and retracting traces of the observed f-d curves, respectively. (1) In the Alb/poly(DMAAm) system, steric repulsion was observed, in which the interaction range and the compressive force of the poly(DMAAm) layer linearly increased with increasing chain length of poly(DMAAm) except for very short chain lengths. Adhesion force was observed only for the poly(DMAAm) layer with short chains. (2) In the Alb/poly(AAc) system, repulsive force due to steric and electrostatic interactions, and "tooth-like" adhesion forces were observed. (3) In the Lyso/poly(AAc) system, electrostatic attraction and adhesion forces were observed. From observation 1, the grafting density, the elastic modulus of the poly(DMAAm) layer, and the conformation of the grafted chain ("mushroom" or "brush") were deduced and are discussed in relation to the characteristics of the interaction force with the proteins. From observations 2 and 3, it was found that a polyanionic surface can provide a significant adhesion force not only to positively charged proteins but also to negatively charged ones at physiological pH.