(634a) Interplay Between Salt Concentration and Surface Functionality for Lysozyme Adsorption Capacity and Energetics On Mesostructured Cellular Foam Silica

Because many drugs and drug targets are proteins, protein separation plays a critical role in the pharmaceutical industry. Solid silica based materials are extensively used as adsorbents for protein separations.  Mesoporous cellular foam (MCF) silicas are valued for high adsorption capacity, controlled pore size, and fast intraparticle mass transport.  These properties are achieved using a unique three dimensional pore network consisting of narrow windows and larger cells.  Amine and hydrocarbon functional groups can be grafted to the MCF silica surface to modulate biomolecule adsorption.  An experimental system to explore lysozyme adsorption onto MCF silica (17nm windows and 34nm cells) was established at pH 5.2, where lysozyme and native MCF silica are oppositely charged.  Low ionic strength (0.01M acetate buffer) and high ionic strength (0.01M acetate buffer with 1M sodium sulfate) conditions were examined.  Batch adsorption isotherms established the equilibrium adsorption capacity and isotherm type.  Confocal laser scanning microscopy using fluorescently tagged lysozyme confirmed fast MCF pore saturation and complete lysozyme penetration.

The heat of lysozyme adsorption onto a packed MCF silica bed was be measured quantitatively in real time using flow microcalorimetry (FMC).  Lysozyme adsorption onto silica typically involves an initial exotherm attributed to electrostatic interactions between the lysozyme and silica surface, a secondary exotherm associated with protein re-orientation and secondary attachment, and an endotherm associated with protein desorption.  The enthalpies associated with these events are impacted both by functional groups on the silica surface and the salt concentration in the buffer.  In particular, ion pairing between lysozyme and the sodium cation occurs at high sodium sulfate concentration.  Sodium cation ion pairing increases the lysozyme net charge and reduces the Debye length.  The competing effects of MCF surface modification and salt concentration on lysozyme adsorption are elucidated by correlating fluorescent imaging, equilibrium capacity results, thermogram development, and thermodynamic analysis.