pH-Dependent Selective Protein Adsorption into Mesoporous Silica

TL;DR

This study investigates how pH influences the selective adsorption of proteins into mesoporous silica, revealing charge-dependent binding behaviors and potential for protein separation based on isoelectric points.

Contribution

It demonstrates pH-dependent selective protein adsorption in mesoporous silica and explores competitive binding effects, offering insights for protein separation techniques.

Findings

Adsorption varies with pH and protein charge.

Electrostatic interactions dominate binding behavior.

Selective separation of proteins based on pI is possible.

Abstract

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.