  The adsorption of lysozyme, cytochrome c and myoglobin, similar-sized
globular proteins of approximately 1.5 nm radius, into the mesoporous silica
material Santa Barbara Amorphous-15 (SBA-15) with 3.3 nm mean pore radius has
been studied photometrically for aqueous solutions containing a single protein
type and for binary protein mixtures. Distinct variations in the absolute and
relative adsorption behavior are observed as a function of the solution's
pH-value, and thus pore wall and protein charge. The proteins exhibit the
strongest binding below their isoelectric points pI, which indicates the
dominance of electrostatic interactions between charged amino acid residues and
the -OH groups of the silica surface in the mesopore adsorption process.
Moreover, we find for competitive adsorption in the restricted, tubular pore
geometry that the protein type which shows the favoured binding to the pore
wall can entirely suppress the adsorption of the species with lower binding
affinity, even though the latter would adsorb quite well from a single
component mixture devoid of the strongly binding protein. We suggest that this
different physicochemical behavior along with the large specific surface and
thus adsorption capability of mesoporous glasses can be exploited for
separation of binary mixtures of proteins with distinct pI by adjusting the
aqueous solution's pH.

Soft Condensed Matter;Mesoscale and Nanoscale Physics;Materials Science;Biological Physics;Chemical Physics