Energetics of biomolecule adsorption on mesostructured cellular foam silica

Heats of adsorption measured by flow microcalorimetry (FMC) were used to understand the energetics of biomolecule adsorption on mesostructured cellular foam (MCF) silica. Tryptophan (Trp), lysozyme (LYS), and bovine serum albumin (BSA) were used as probe molecules. The FMC results confirmed that attractive interactions (both electrostatic and van der Walls interactions) between the biomolecules and acid-washed MCF silica were the driving force for adsorption, even when the protein (BSA) and the surface were both negatively charged and repulsion interactions might be expected. Multiple exothermic events occurred, possibly because of multipoint interactions between biomolecules and MCF silica resulting from multiple binding sites on the biomolecule and a curved pore structure. Interestingly, the magnitude of the enthalpy of adsorption (ΔHTotal) increased with increasing biomolecule size at pH 5.2; the number of exothermic peaks corresponded to the number of binding regions on the biomolecule. In addition, standard Gibbs energies of adsorption (ΔG°) and entropy of adsorption (ΔS°) were calculated from batch adsorption isotherms and the measured enthalpy of adsorption. BSA adsorption energetics were significantly affected by changing the pH from 4 to 5.2. This effect was attributed to a dramatic conformational change of BSA as a function of pH. The energetics of adsorption provide invaluable insight into the mechanism biomolecule adsorption.

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► Thermograms provide insight into the mechanism of biomolecule adsorption.
► Complex multiple exotherms were observed.
► Adsorption thermodynamics trend with biomolecule size.
► Individual exotherms may correspond to specific binding regions of the biomolecule.
► Adsorption energetics can be correlated with conformational changes.