On the Compatibility Criteria for Protein Encapsulation inside Mesoporous Materials

Abstract

The properties of the enzyme pepsin, relevant to its incorporation inside the channels of mesoporous silica materials in the preparation of bioinorganic hybrids, are highlighted by molecular dynamics simulations of aqueous solutions of the protein under conditions optimal for encapsulation in SBA-15. The protein size, shape, flexibility and surface properties are calculated with the aim of deriving general accessibility/compatibility criteria favouring encapsulation inside mesoporous systems.

Disentangling protein–silica interactions

“…The enzyme has optimal activity in the 1-4 pH range, minimal activity above pH 5 and is denatured at pH 6-7 [24,25]. Molecular dynamics simulations on substrate-free pepsin in a pH = 3.6 aqueous solution indicated stability of the enzyme in the medium adopted for the preparation of the pepsin-SBA-15 composite: the active site structure is preserved, relevant residues maintain relative positioning consistent with crystallographic data, and only small fluctuations characterize the pepsin surface charge distribution [26]. Moreover, at the hybrid's fabrication conditions, the average pepsin dimensions amount to approximately 4.5 × 5.0 × 6.6 nm [5,26].…”

“…Interestingly, owing to the weak surface-protein interaction, the protein shows a significant rotational mobility: indeed, different residues are found in closest contact with the surface along the trajectory and significant variations of the protein orientation are detected. It is to point out that pepsin presents a very complex charge distribution on its surface, with a predominance of negatively charged regions, but also with positively charged patches [26]. The non-uniform charge distribution on pepsin, together with its water-cations-mediated interaction with silica, may therefore explain the absence of a preferred orientation of the enzyme with respect to the surface.…”

“…The starting configurations of the enzyme were based on the 4PEP crystallographic structure of porcine pepsin at 1.8 Å resolution available from the protein data bank [21]. The total electrostatic charge of pepsin, −6|e|, was determined previously by calculating the pKa of the protein side chains at pH = 3.6 [26], where the protein loading in SBA-15 is maximal. In particular, His53, Lys319, Arg307 and Arg315 were modelled as positively charged residues, whereas 10 Asp residues (11,87,118,138,171,195,215,242,290,314) were unprotonated (negatively charged).…”

Supramolecular Organization in Confined Nanospaces

“…For example, molecular dynamics simulations showed that, in the presence of Na + and K + cations, a negatively charged peptide (pepsin) solvated by a water droplet can be attracted by a negatively charged silica surface . The peptide‐silica interaction was rationalized thanks to a detailed study of the electrostatic potential of the peptide surface, which exhibited both negative and positive regions . Such model allowed to explain why encapsulation of pepsin in SBA‐15 occurs at pH‐conditions where both protein and mesoporous silica are negatively charged …”

Micro, Meso and Macro Porous Materials on Medicine

“…Immobilization onto inert inorganic materials may cope with stability and compatibility problems, thus favoring advance in technological applications. The success of such a strategy depends on proper selection of the hybrid materials that can support or host the bioactive components while maintaining its activity and selectivity [22] [23]. Some examples of this application are:…”