Interactions between biomolecules and an iron-silica surface

• Synthesized high surface area iron-silica with cylindrical mesopores.
• Subtle differences between amino acid adsorption on iron-silica and on MCF silica.
• Enthalpic data differentiate nonspecific adsorption from bond formation.
• Lysozyme adsorbs in a different orientation on iron silica than on MCF.
• Lysozyme orientation is mediated by histidine.

An iron-silica adsorbent (1:1 ratio by mass) with a pore cell size of 11.4 nm and BET surface area of 610 m2 g−1 was synthesized and compared to a mesocellular foam (MCF) silica and iron oxide for amino acid (alanine, aspartic acid, lysine and histidine) and lysozyme adsorption. Amino acid batch adsorption loadings did not exceed estimated monolayer coverage and were generally quantitatively similar on iron-silica and MCF silica; the observed loadings followed the trend alanine > lysine/aspartic acid ≫ histidine. Multilayers were observed on the strongly positive iron oxide surface. The dynamic heat of biomolecule adsorption was directly measured with flow microcalorimetry (FMC); the enthalpies of adsorption of alanine, aspartic acid, lysine, and histidine on iron-silica were −0.046, −0.73, −5.3 and −14.3 kJ mol−1, respectively. Alanine adsorption was mediated by the surrounding water, aspartic acid adsorbed through weak electrostatic attractions, and lysine and histidine formed bonds during adsorption. Lysozyme monolayer loading was higher on iron-silica than on MCF silica, calorimetric studies indicate a different lysozyme adsorption mechanism on iron-silica and MCF silica.

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