Molecular packing of HSA, IgG, and fibrinogen adsorbed on silicon by AFM imaging

The adsorption of soft proteins onto different interfaces is mainly governed, although not only, by hydrophobic interactions. Nevertheless, when a hydrophilic surface is used as a substract, the electrostatic forces play an important role in the protein adsorption affecting on both surface-protein and protein-protein interactions. Nowadays, the tapping mode atomic force microscopy (TM-AFM) allows to obtain images from protein layers adsorbed on smooth surfaces, which provides much information about the spatial order of the immobilized molecules. In this paper, the molecular packing of three human proteins (albumin, IgG and fibrinogen) deposited on hydrophilic silicon plates was studied by TM-AFM, while adsorption kinetics were followed by ellipsometry. A theoretical analysis was also used to obtain some information concerning to the average area occupied by the adsorbed molecules. Adsorption isotherms were carried out at different pH values (from 4 to 9) and ionic strengths (from 2 to 300 mM) in order to change the electrostatic interactions during the adsorption process. The packing and degree of coverage of the proteins deposited at low ionic strength highly depends on the medium pH, although at the highest ionic strength tested, the screening effect of small ions surrounding the macromolecules decreases the intermolecular interaction. This leads to similar spatial order in the adsorbed state regardless the medium pH. The images obtained by AFM support the ellipsometry data and vice versa. (C) 1998 Elsevier Science S.A. All rights reserved.