Encapsulation of Myoglobin with a Mesoporous Silicate Results in New Capabilities

Evolution of myoglobin diffusion mechanisms: exploring pore and surface diffusion in a single silica particle

This study elucidates the mass transfer mechanism of myoglobin (Mb) within a single silica particle with a 50 nm pore size at various pH levels (6.0, 6.5, 6.8, and 7.0). Investigation of Mb distribution ratio (R) and distribution kinetics was conducted using absorption microspectroscopy. The highest R was observed at pH 6.8, near the isoelectric point of Mb, as the electrostatic repulsion between Mb molecules on the silica surface decreased. The time-course absorbance of Mb in the silica particle was rigorously analyzed based on a first-order reaction, yielding the intraparticle diffusion coefficient of Mb (Dp). Dp-(1 + R)-1 plots at different pH values were evaluated using the pore and surface diffusion model. Consequently, we found that at pH 6.0, Mb diffused in the silica particle exclusively through surface diffusion, whereas pore diffusion made a more substantial contribution at higher pH. Furthermore, we demonstrated that Mb diffusion was hindered by slow desorption, associated with the electrostatic charge of Mb. This comprehensive analysis provides insights into the diffusion mechanisms of Mb at acidic, neutral, and basic pH conditions.

Pore Size Dependence of Mass Transfer of Zinc Myoglobin in a Single Mesoporous Silica Particle

This study investigated the pore size dependence of the mass transfer of zinc myoglobin (ZnMb) in a single mesoporous silica particle through confocal fluorescence microspectroscopy. The ZnMb's fluorescence depth profile in the particle was analyzed by a spherical diffusion model, and the intraparticle diffusion coefficient was obtained. The intraparticle diffusion coefficient in the silica particle with various pore sizes (10, 15, 30, and 50 nm) was furthermore analyzed based on a pore and surface diffusion model. Although the mass transfer mechanism in all silica particles followed the pore and surface diffusion model, the adsorption and desorption of ZnMb affected the mass transfer depending on the pore size. The influence of the slow desorption of ZnMb became pronounced for large pore sizes (30 and 50 nm), which was revealed by simulation using a diffusion equation combined with the adsorption-desorption kinetics. The distribution of ZnMb was suppressed in small pore sizes (10 and 15 nm) owing to the adsorption of ZnMb onto the entrance of the pore. Thus, we revealed the mass transfer mechanism of ZnMb in the silica particle with different pore sizes.

Kinetic Analysis of the Mass Transfer of Zinc Myoglobin in a Single Mesoporous Silica Particle by Confocal Fluorescence Microspectroscopy

The adsorption/desorption mechanisms of biomolecules in porous materials have attracted significant attention because of their applications in many fields, including environmental, medical, and industrial sciences. Here, we employ confocal fluorescence microspectroscopy to reveal the diffusion behavior of zinc myoglobin (ZnMb, 4.4 nm × 4.4 nm × 2.5 nm) as a spherical protein in a single mesoporous silica particle (pore size of 15 nm). The measurement of the time course of the fluorescence depth profile of the particle reveals that intraparticle diffusion is the rate-limiting process of ZnMb in the particle. The diffusion coefficients of ZnMb in the particle for the distribution (D_dis) and release (D_re) processes are determined from the rate constants, e.g., D_dis = 1.65 × 10^-10 cm² s^-1 and D_re = 3.68 × 10^-10 cm² s^-1, for a 10 mM buffer solution. The obtained D values for various buffer concentrations are analyzed using the pore and surface diffusion model. Although surface diffusion is the main distribution process, the release process involves pore and surface diffusion, which have not been observed with small organic molecules; the mechanism of transfer of small molecules is pore diffusion alone. We demonstrate that the mass transfer kinetics of ZnMb in the silica particle can be explained well on the basis of pore and surface diffusion.

Structural Characterization of Proteins Adsorbed at Nanoporous Materials

A nanoporous material has been applied for the development of functional nanobiomaterials by utilizing its uniform pore structure and large adsorption capacity. The structure and stability of biomacromolecules, such as peptide, oligonucleotide, and protein, are primary factors to govern the performance of nanobiomaterials, so that their direct characterization methodologies are in progress. In this review, we focus on recent topics in the structural characterization of protein molecules adsorbed at a nanoporous material with uniform meso-sized pores. The thermal stabilities of the adsorbed proteins are also summarized to discuss whether the structure of the adsorbed protein molecules can be stabilized or not.

Differential Scanning Calorimetry Study on the Adsorption of Myoglobin at Mesoporous Silicas: Effects of Solution pH and Pore Size

In the present study, pore adsorption behavior of globular myoglobin (Mb) at mesoporous silicas was examined utilizing the low-temperature differential scanning calorimetry (DSC) method. The DSC method relies on a decrease in heat of fusion for the pore water upon adsorption of Mb. The amount and structure of Mb adsorbed into the mesoporous silica were examined by DSC and optical absorption spectroscopy. The results indicated that the pore adsorption behavior of Mb strongly depended on the solution pH and pore size of mesoporous silica. For the adsorption of Mb (diameter = 3.5 nm) into mesoporous silica with narrow pores (pore diameter = 3.3 nm) at a pH ranging from 7.0 to 3.7, the penetration of both folded and denatured Mb molecules was confirmed. The folded Mb could penetrate into large mesoporous silica pores (pore diameter = 5.3 and 7.9 nm), whereas the penetration of the denatured Mb molecules was completely inhibited. The distribution of folded Mb at mesoporous silica depended on the pore size; almost all folded Mb molecules located inside mesoporous silica pores of diameters 3.3 and 5.3 nm, whereas the Mb molecules distributed at bot internal and external pore surfaces of mesoporous silica with 7.9 nm in pore diameter. These pore adsorption behaviors suggest that aggregation or stacking of the Mb molecules at the pore entrance regions of the large pores affected the pore adsorption behavior.

Characterization of Myoglobin Adsorption into Mesoporous Silica Pores by Differential Scanning Calorimetry

Adsorption of protein molecules into the pores of a porous material is an important process for chromatographic separation of proteins and synthesis of nanoscale biocatalyst systems; however, there are barriers to developing a method for analyzing the process quantitatively. The purpose of this study is to examine the applicability of differential scanning calorimetry (DSC) for quantitative analysis of protein adsorption into silica mesopores. For this purpose myoglobin, a globular protein (diameter: 35.2 Å) was selected, and its adsorption onto mesoporous silica powders with uniform pore diameters (pore diameters: 39 and 64 Å) was measured by adsorption assay and DSC experiments. Our results confirmed that the adsorption of myoglobin into the silica mesopores induced significant changes in the positions and areas of freezing/melting peaks of the pore water. The decrease in heat of fusion of the pore water after myoglobin adsorption could be utilized to quantify the amount of myoglobin inside the silica mesopores. The advantages of DSC include its applicability to small wet mesoporous silica samples.

Successful Mesoporous Silica Encapsulation of Optimally Functional EcDOS (E. coli Direct Oxygen Sensor), a Heme-based O2-Sensing Phosphodiesterase

The heme-based O₂ sensor from Escherichia coli, EcDOS, exerts phosphodiesterase activity towards cyclic-di-GMP (c-di-GMP), an important second messenger that regulates biofilm formation, virulence, and other important functions necessary for bacterial survival. EcDOS is a two-domain protein composed of an N-terminal heme-bound O₂-sensing domain and a C-terminal functional domain. O₂ binding to the heme Fe(II) complex in the O₂-sensing domain substantially enhances the catalytic activity of the functional domain, a property with potentially promising medical applications. Mesoporous silica is a useful material with finite-state machine-like features suitable for mediating numerous enzymatic functions. Here, we successfully encapsulated EcDOS into mesoporous silica, and demonstrated that encapsulated EcDOS was substantially activated by CO, an alternative signaling molecule used in place of O₂, exhibiting the same activity as the native enzyme in aqueous solution. Encapsulated EcDOS was sufficiently stable to exert its enzymatic function over several experimental cycles under aerobic conditions at room temperature. Thus, the present study demonstrates the successful encapsulation of the heme-based O₂ sensor EcDOS into mesoporous silica and shows that the native gas-stimulated function of EcDOS is well conserved. As such, this represents the first application of mesoporous silica to an oxygen-sensing-or any gas-sensing-enzyme.

Enzymes immobilized in mesoporous silica: a physical-chemical perspective

Mesoporous materials as support for immobilized enzymes have been explored extensively during the last two decades, primarily not only for biocatalysis applications, but also for biosensing, biofuels and enzyme-controlled drug delivery. The activity of the immobilized enzymes inside the pores is often different compared to that of the free enzymes, and an important challenge is to understand how the immobilization affects the enzymes in order to design immobilization conditions that lead to optimal enzyme activity. This review summarizes methods that can be used to understand how material properties can be linked to changes in enzyme activity. Real-time monitoring of the immobilization process and techniques that demonstrate that the enzymes are located inside the pores is discussed by contrasting them to the common practice of indirectly measuring the depletion of the protein concentration or enzyme activity in the surrounding bulk phase. We propose that pore filling (pore volume fraction occupied by proteins) is the best standard for comparing the amount of immobilized enzymes at the molecular level, and present equations to calculate pore filling from the more commonly reported immobilized mass. Methods to detect changes in enzyme structure upon immobilization and to study the microenvironment inside the pores are discussed in detail. Combining the knowledge generated from these methodologies should aid in rationally designing biocatalyst based on enzymes immobilized in mesoporous materials.

Electrochemical enzymatic biosensor with long-term stability using hybrid mesoporous membrane

An acetylcholinesterase-immobilized sensor unit was successfully prepared by encapsulating the enzyme within hybrid mesoporous silica membranes (F127-MST).

Spin-label ESR with nanochannels to improve the study of backbone dynamics and structural conformations of polypeptides

Nanochannels of mesoporous silica materials were previously found useful for reducing the tumbling motion of encapsulated biomolecules while leaving the biomolecular structure undisturbed. Here we show that experiments of cw-ESR distance measurement in nano-confinement can benefit immediately from the above mentioned features of sufficiently slow molecular tumbling, enabling more accurate determination of interspin distances throughout the temperature range, from 200 to 300 K. A 26-residue prion protein peptide, which can fold into either a helical or hairpin structure, as well as its variants, are studied by using ESR. By comparing the spectra obtained in vitrified bulk solutions vs. mesopores, the spectra from the latter display typical slow-motional lineshapes, thereby enabling dipolar anisotropy to be unambiguously revealed throughout the temperature range, whereas the spectra from the former are dominated by the disordering of the side chain and the rotational tumbling of the peptide. The spectral changes regarding the two secondary structures in nano-confinement are found to show a strong correlation with the dynamic properties of the backbones. The effect of viscosity agent perturbation on the motion of an R1 nitroxide side chain, a commonly employed probe, could be substantial in a bulk solution condition, though it is absolutely absent in nanochannels. Under nano-confinement, the probe is proven sufficiently sensitive to the backbone motions. Overall, the distance distributions determined from the mesopore studies not only describe the conformational structures (by average distances), but also the backbone dynamics (by distribution widths) of the spin-labeled peptides.

Effects of surface curvature and surface chemistry on the structure and activity of proteins adsorbed in nanopores

The interactions of proteins with the surface of cylindrical nanopores are systematically investigated to elucidate how surface curvature and surface chemistry affect the conformation and activity of confined proteins in an aqueous, buffered environment. Two globular proteins, lysozyme and myoglobin, with different catalytic functions, were used as model proteins to analyze structural changes in proteins after adsorption on ordered mesoporous silica SBA-15 and propyl-functionalized SBA-15 (C(3)SBA-15) with carefully controlled pore size. Liquid phase ATR-FTIR spectroscopy was used to study the amide I and II bands of the adsorbed proteins. The amide I bands showed that the secondary structures of free and adsorbed protein molecules differ, and that the secondary structure of the adsorbed protein is influenced by the local geometry as well as by the surface chemistry of the nanopores. The conformation of the adsorbed proteins inside the nanopores of SBA-15 and C(3)SBA-15 is strongly correlated with the local geometry and the surface properties of the nanoporous materials, which results in different catalytic activities. Adsorption by electrostatic interaction of proteins in nanopores of an optimal size provides a favorably confining and protecting environment, which may lead to considerably enhanced structural stability and catalytic activity.

Enhancement in thermal stability and resistance to denaturants of lipase encapsulated in mesoporous silica with alkyltrimethylammonium (CTAB)

We assembled a highly durable conjugate with both a high-density accumulation and a regular array of lipase, by encapsulating it in mesoporous silica (FSM) with alkyltrimethylammonium (CTAB) chains on the surface. The activity for hydrolyzing esters of the lipase immobilized in mesoporous silica was linearly related to the concentration of lipase, whereas that of non-immobilized lipase showed saturation due to self-aggregation at a high concentration. The lipase conjugate also had increased resistance to heating when stayed in the silica coupling with CTAB. In addition, encapsulating the enzyme with FSM coupled CTAB caused the lipase to remain stable even in the presence of urea and trypsin, suggesting that the encapsulation prevented dissociation and denaturing. This conjugate had much higher activity and much higher stability for hydrolyzing esters when compared to the native lipase. These results show that FSM provides suitable support for the immobilization and dispersion of proteins in mesopores with disintegration of the aggregates.

Proteins in mesoporous silicates

Mesoporous silicates (MPS) have an ordered pore structure with dimensions comparable to many biological molecules. They have been extensively explored as supports for proteins and enzymes in biocatalytic applications. Since their initial discovery, novel syntheses methods have led to precise control over pore size and structure, particle size, chemical composition, and stability, thus allowing the adsorption of a wide variety of biological macromolecules, such as heme proteins, lipases, antibody fragments, and proteases, into their structures. This Review discusses the application of ordered, large-pore, functionalized mesoporous silicates to immobilize proteins for biocatalysis.

The ensemble of hetero-proteins in inorganic nanochannels

The assembly and proper alignment of two heterofluorescent proteins (sGFP and DsRed) in the mesoporous channels of ethanol-treated FSM6.2 (a folded-sheet mesoporous material with a pore diameter of 6.2 nm) was confirmed using a fluorescence resonance energy transfer (FRET) technique. The sGFP-DsRed-FSM6.2 conjugate showed a large decrease in the emission of donor (sGFP) fluorescence, indicating that the conjugate functions as an energy transfer system through the combination of the two heteroproteins, due to the successful encapsulation of the sGFP-DsRed pairs in the mesopores. Fluorescence spectral analysis demonstrated that the proteins were highly dispersed and homogeneously encapsulated in the mesopores of FSM6.2, even at high concentration, although they spontaneously aggregated and showed a red shift in solution at the concentration corresponding to that in the conjugate. Furthermore, an increase in the amount of sGFP and DsRed adsorbed to the pores of FSM6.2 led to a decrease in the distance between these proteins, resulting in enhancement of FRET efficiency.

Encapsulation of single enzyme in nanogel with enhanced biocatalytic activity and stability

Single protein encapsulated into nanogels with uniformed size and controllable shell thickness were prepared by surface acryloylation of a protein molecule followed by aqueous in situ polymerization. Compared to its free counterpart, the encapsulated protein exhibits similar biocatalytic behavior and significantly improved stability at high temperature and in the presence of organic solvent.

Encapsulation of Hemoglobin in Mesoporous Silica (FSM)—Enhanced Thermal Stability and Resistance to Denaturants

Hemoblogin (Hb), which is a typical oligomeric protein, was introduced into the pores of mesoporous silica (FSM: folded-sheet mesoporous material) that had a diameter of 7.5 nm. Soret CD spectra of Hb-FSM-7.5 conjugates showed a peak that was identical to that of free Hb. This suggests that Hb retained its highly ordered structure in the mesoporous silica. In addition, the UV-visible absorption spectrum showed that Hb had an increased resistance to heat denaturation in the silica. Even after heat treatment at 85 degrees C, Hb-FSM-7.5 retained its ligand-binding activity. The stability of Hb-FSM-7.5 was examined further by measuring its peroxidase-like activity. Encapsulation of Hb resulted in the retention of activity in the presence of high NaCl or Gdn-HCl levels. This suggests that encapsulation prevented dissociation and denaturing. Thus, it seems that the mesopores created a favorable environment for the oligomeric protein to perform its function, even under harsh conditions.

Effective immobilization of subunit protein in mesoporous silica modified with ethanol

Ethoxylated FSM-type mesoporous silica (folded-sheet mesoporous material) with a pore diameter of 6.2 nm (FSM6.2) remarkably enhances rigidly of the structure in aqueous solutions. The esterified material could be used successfully as an adsorbent to accommodate subunit protein, methemoglobin (Fe(3+)). Furthermore, methemoglobin (Fe(3+)) in the pores of ethoxy-FSM is maintained a peroxidase activity similar to the native, indicating methemoglobin retains its fore subunit structure in the pores of FSM6.2.