Changes in the secondary structure of adsorbed IgG and F(ab')(2) studied by FTIR spectroscopy

The purpose of the present study is to find a quantitative relationship between the adsorption behavior and the secondary structure of proteins. The adsorption of two monoclonal IgGs which differ in their isoelectric point and their corresponding F(ab')(2) fragments is followed in time. The proteins are adsorbed on hydrophilic silica and on hydrophobic methylated surfaces at different values of pH and ionic strength. The adsorption behavior is studied by measuring FTIR spectra of the adsorbed proteins. The adsorbed amount is related to the integrated area of the amide II region of the spectrum. The secondary structure of the adsorbed proteins is evaluated by analyzing the second derivatives of the amide I region. Quantitative information on the secondary structure is obtained by applying a fitting procedure which assumes the absorption bands for the different structural components to be Lorentzian shaped. The results show that for IgG the adsorbed amounts decrease with an increasing net charge density on the protein. This decrease is correlated to a reduction in the beta-sheet content which suggests that IgG molecules adsorb in a less compact conformation. The adsorption-induced reduction in the beta-sheet content is larger at hydrophobic methylated surfaces than at hydrophilic silica surfaces. The dependence of the amount of structural elements on the pH diminishes at higher ionic strength. F(ab')(2) fragments contain a higher fraction of beta-sheet content than whole IgG molecules, and these fractions are less strongly influenced by the adsorption conditions. Therefore, it can be concluded that the F(ab')(2) fragments have a higher structural stability toward adsorption than whole IgG molecules.