Pore-Size Dependent Protein Adsorption and Protection from Proteolytic Hydrolysis in Tailored Mesoporous Silica Particles

Protein adsorption and interactions with mesoporous silica are of interest for a broad range of applications including drug delivery, chemical synthesis, biosensors, and bioseparations. A major challenge in designing mesoporous silica supports for tailored protein interaction is the differentiation of protein interactions at the surface of the particle from interactions within the pore, important features when considering mesoporous silica as a protective support for active proteins. In this investigation, the location of Enhanced Green Fluorescent Proteins (EGFPs) adsorbed on tailored mesoporous silica particles is examined as a function of pore diameter using proteolytic hydrolysis to distinguish between accessible and inaccessible proteins. Pore size control is achieved by tuning the hydrothermal aging temperature (60-110 °C) during synthesis, where the synthesis results in 5-15 μm diameter spherical particles appropriate for imaging by confocal scanning laser microscopy (CSLM). In low pH environments, EGFP unfolds within pores and on the surface of particles, rendering it susceptible to proteolytic hydrolysis by the protease Pepsin A. Upon return to neutral pH, un-hydrolyzed EGFP regains its fluorescence and can be visualized within the mesoporous particles. The pore-size dependent loading and protection of EGFP (2.4 nm diameter×4.2 nm) from proteolytic attack by Pepsin A (7.3 nm×3.6 nm×5.4 nm) is demonstrated by the retention of fluorescence in 7.3 nm pores. Larger-pored materials (>9 nm) provide diminishing protection for EGFP, and the protection is greatly reduced with increasing pore size and pore size distribution breadth. Proteolytic hydrolysis is used to delineate the activity of pore-loaded versus surface-bound proteins and to establish that there is an optimal pore diameter for loading EGFP while protecting it from attack by a larger proteolytic enzyme.