Assessment of protein incorporation into SBA-15 particles and their structural changes

The ordered mesoporous silica SBA-15 is a promising platform for protein immobilization and delivery, since adsorption into its porous matrix may improve protein structural stability. Here, three proteins-myoglobin (Mb), concanavalin A (ConA), and soybean trypsin inhibitor (STI)-were loaded into conventional SBA-15 particles with an average mesopore diameter of 9.3 nm to investigate structural changes in their predominant content after adsorption. Powdered biocomposites were prepared by loading each protein at two silica-to-protein weight ratios (10:1 and 5:1). Protein structures were evaluated by synchrotron radiation circular dichroism and fluorescence spectroscopy following resuspension in buffer solutions (pH 7.0 and 2.2). The structural changes were qualitatively analyzed and found to be more influenced by the silica-to-protein ratio for Mb and ConA, while buffer pH had a lesser effect on the proteins. A 10:1 ratio better preserved the α-helical spectral features of Mb, whereas a 5:1 ratio favored the retention of the β-sheet signature of ConA; in both cases, the aromatic residue microenvironment remained largely preserved. In contrast, STI, rich in irregular structures, exhibited smaller spectral changes relative to its native structure in both ratios, while the aromatic residue was less exposed to the solvent. Overall, the results reinforce the potential of SBA-15 as a protein carrier by retaining the main features of the protein's secondary structure under specific conditions. The simultaneous comparison of three structurally distinct proteins reveals variable adsorption behaviors and highlights the silica-to-protein ratio as a protein-dependent parameter, reinforcing the importance of optimizing adsorption conditions to develop more stable silica-protein systems.