| dc.description.abstract | Ellipsometry was used to investigate the influence of ionic strength (I) and pH on the adsorption of bovine serum
albumin (BSA) or β-lactoglobulin (BLG) onto preabsorbed layers of two polycations: poly(diallyldimethylammonium
chloride) (PDADMAC) or poly(4-vinylpyridine bromide) quaternized with linear aliphatic chains of two (QPVP-C2) or
five (QPVP-C5) carbons. Comparisons among results for the three polycations reveal hydrophobic interactions, while
comparisons between BSA and BLG-proteins of very similar isoelectric points (pI)-indicate the importance of protein
charge anisotropy. At pH close to pI, the ionic strength dependence of the adsorbed amount of protein (Γ) displayed
maxima in the range 10<I<25mMcorresponding to Debye lengths close to the protein radii. Visualization of protein
charge by Delphi suggested that these ionic strength conditions corresponded to suppression of long-range repulsion
between polycations and protein positive domains, without diminution of short-range attraction between polycation
segments and locally negative protein domains, in a manner similar to the behavior of PE-protein complexes in
solution.1-4 This description was consistent with the disappearance of the maxima at pH either above or below pI. In the
former case, Γ values decrease exponentially with I1/2, due to screening of attractions, while in the latter case adsorption
of both proteins decreased at low I due to strong repulsion. Close to or below pI both proteins adsorbed more strongly
onto QPVP-C5 than onto QPVP-C2 or PDADMAC due to hydrophobic interactions with the longer alkyl group.
Above pI, the adsorption was more pronounced with PDADMAC because these chains may assume more loosely
bound layers due to lower linear charge density. | |
