MOLECULAR PACKING OF HSA, IGG, AND FIBRINOGEN ADSORBED ON SILICON BY AFM IMAGING

The adsorption of soft proteins onto different interfaces is mainly go verned, although not only, by hydrophobic interactions. Nevertheless, when a hydrophilic surface is used as a substract, the electrostatic f orces play an important role in the protein adsorption affecting on bo th surface-protein and protein-protein interactions. Nowadays, the tap ping mode atomic force microscopy (TM-AFM) allows to obtain images fro m protein layers adsorbed on smooth surfaces, which provides much info rmation 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 theor etical analysis was also used to obtain some information concerning to the average area occupied by the adsorbed molecules. Adsorption isoth erms were carried out at different pH values (from 4 to 9) and ionic s trengths (from 2 to 300 mM) in order to change the electrostatic inter actions during the adsorption process. The packing and degree of cover age of the proteins deposited at low ionic strength highly depends on the medium pH, although at the highest ionic strength tested, the scre ening effect of small ions surrounding the macromolecules decreases th e intermolecular interaction. This leads to similar spatial order in t he adsorbed state regardless the medium pH. The images obtained by AFM support the ellipsometry data and vice versa. (C) 1998 Elsevier Scien ce S.A. All rights reserved.