Chapter 6
Macroscopic and Microscopic Interactions Between Albumin and Hydrophilic Surfaces 1,2
1,3
3
C. J. van Oss , W. Wu , and R. F. Giese 1
2
3
Downloaded by UNIV OF BATH on May 12, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0602.ch006
Departments of Microbiology, Chemical Engineering, and Geology, State University of New York at Buffalo, Buffalo, NY 14214-3078
On a macroscopic level, proteins such as serum albumin (HSA), dissolved in water, should in theory be sufficiently repelled by clean glass or silica particles not to adsorb to them at pH values significantly higher or lower than the PI of HSA of 4.85. Yet at pH < 8, HSA does adsorb to a moderate extent to such hydrophilic surfaces. An explanation is that on a microscopic level, discrete heterogeneous sites in the hydrophilic surface that are positively charged and electron acceptors (e.g., Ca ), locally attract negatively charged and electron donor moieties of the HSA surface. It can be shown via extended DLVO analysis (which includes Lewis acid-base interactions) that such moieties must be situated on HSA sites with a small radius of curvature. The adsorption of HSA to hydrophilic particles could be decreased by the admixture of Na EDTA. In the presence of this complexing agent, the HSA adsorption was 60% less with montmorillonite and 51% less with silica particles at pH 7.2. There was no free Ca in the particles or in the HSA solution. ++
2
++
The net macroscopic interaction between hydrophilic proteins and hydrophilic surfaces, such as glass or montmorillonite clay particles, immersed in aqueous media, at neutral pH, is strongly repulsive. Thus, under conditions where the macroscopicscale rules of Lifshtz-van der Waals (LW), Lewis acid-base (AB), and electrical double layer (EL) interactions are applicable, adsorption of hydrophilic proteins onto hydrophilic mineral surfaces should not occur. However, hydrophilic proteins, dissolved in water, do adsorb onto glass [albeit more sparsely than the adsorption of hydrophilic proteins onto hydrophobic surfaces (/)], as well as onto montmorillonite surfaces (2). The adsorption of proteins onto hydrophilic, high-energy surfaces, appears to occur through interactions between microscopic attractor sites on the high-energy surface, and the protein surface. For the microscopic attraction between such attractor sites to prevail, it must be relatively long-range in nature as well as strong enough to surmount the overall macroscopic repulsion. The most likely macroscopic attractors are plurivalent cations present in glass and montmorillonite surfaces, acting with
0097-6156/95/0602-0080$12.00/0 © 1995 American Chemical Society In Proteins at Interfaces II; Horbett, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
6. VAN OSS ET AL.
Interactions Between Albumin and Hydrophilic Surfaces 81
Downloaded by UNIV OF BATH on May 12, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0602.ch006
anionic sites on the protein. Concomitantly with the attraction caused by differences in electrostatic signs of charge, there also is a Lewis acid-base attraction between the accessible cationic sites which also function as Lewis acids, and the Lewis base sites that dominate the surface of dissolved protein. To determine the conditions that have to be fulfilled to enable the interaction energies between discrete microscopic attractor sites to overcome the overall macroscopic repulsion field, an analysis must be made of the interplay of all the free energy elements involved in these interactions, as a function of the distance between the interacting sites and surfaces. To that effect energy versus distance diagrams must be elaborated, taking into account L W and E l (3), as well as A B forces, as a function of distance, in the guise of extended D L V O diagrams (4, 5, 6) To test the putative influence on protein adsorption by plurivalent cationic sites imbedded in the surfaces of high-energy hydrophilic mineral materials, attempts were made to block the action of such cationic sites by means of complexing agents; see below. Theory On a macroscopic level, at all distances H between the outer surface of protein (1), and the adsorbing surface (2), immersed in water (w): AG™--AG% AG&AG*
CD
+
However as the L W , A B and E L free energies decay as a function of distance H according to different regimes, A G ^ O ? ) , A G ^ ( 0 and AG,^r(0 must each be determined separately before being combined into AG ^? (0. The expressions for the different types of A G ( 0 are, for a sphere with a radius R and a flat surface: T
1
l w 2
(
AG^y-lnRXAG^expi^-]
3
)
and 4
A Gfyfi) =Rety M 1 +exp( - K€)]
< >
2
where: the van der Waals, or Hamaker constant, (s
A ^ - n ^ G t : ;
>
and the apolar (LW) energy of interaction between two flat parallel bodies, A G , " ^ W ) at the minimum equilibrium distance,
