Hamiltonian replica exchange simulations of glucose oxidase adsorption on charged surfaces

Acceleration of Enzyme-Catalyzed Reactions at Aqueous Interfaces through Enhanced Reaction Kinetics of Microdroplets

Enzyme-catalyzed reactions have the advantages of excellent selectivity, low cost, and mild reaction conditions, but the slow reaction kinetics limit their practical applications. Herein, a microdroplet generator that can continuously and rapidly generate water microdroplets with tunable size was designed and used for the study of an enzyme-catalyzed reaction in microdroplets. Using glucose oxidase as a model and resazurin as a fluorescence probe, the fluorescence intensity of the collected microdroplets sprayed into the gas phase was 35 times higher than that in the bulk system, demonstrating obvious reaction acceleration in the microdroplets. Mechanistic studies demonstrated that local concentration enrichment and enzyme reorientation at the gas-water interfaces play key roles in the acceleration of enzymatic reactions in microdroplets. Further, the potential application of the reaction system in glucose sensing was investigated. Finally, we also studied the reaction acceleration of enzymic catalysis at the oil-water interfaces. Online measurement of the fluorescence signal of microdroplets sprayed into the mineral oil revealed a reaction acceleration factor of 6.2. It was demonstrated that aqueous microdroplets provided a green, efficient, and convenient methodology for enzyme-catalyzed reactions.

Dynamic Co-Clustering and Self-Sorting in Interactive Protocell Populations

The design and implementation of collective actions in model protocell communities is an on-going challenge in synthetic protobiology. Herein, we covalently graft alginate or chitosan onto the outer surface of semipermeable enzyme-containing silica colloidosomes to produce hairy catalytic protocells with pH-switchable membrane surface charge. Binary populations of the enzymatically active protocells exhibit self-initiated stimulus-responsive changes in spatial organization such that the mixed community undergoes alternative modes of electrostatically induced self-sorting and reversible co-clustering. We demonstrate that co-clustering, but not self-sorting, mitigates signal attenuation in a binary community of enzyme-containing sender and receiver protocells due to increased proximity effects. The level of signal attenuation is correlated with a time-dependent pH-mediated switch in the spatial organization of the sender and receiver populations. Our results pave the way towards the development of programmable networks of adaptive life-like objects and could have implications for the development of interactive cytomimetic materials and agent-based robotics.

Screen-printed electrode designed with MXene/doped-polyindole and MWCNT/doped-polyindole for chronoamperometric enzymatic glucose sensor

The enzymatic glucose sensors as modified by MXene-dPIn and MWCNT-dPIn on a screen-printed carbon electrode (SPCE) were investigated. Herein, MXene was molybdenum carbide (Mo3C2) which has never been utilized and reported for glucose sensors. The biopolymer type to support the enzyme immobilization was examined and compared between chitosan (CHI) and κ-carrageenan (κC). MWCNT-dPIn obviously showed a larger electroactive surface area, lower charge transfer resistance and higher redox current than Mo3C2-dPIn, indicating that MWCNT-dPIn is superior to Mo3C2-dPIn. For the chitosan-based sensors, the sensitivity value of CHI-GOD/Mo3C2-dPIn is 3.53 μA mM-1 cm-2 in the linear range of 2.5-10 mM with the calculated LOD of 1.57 mM. The sensitivity value of CHI-GOD/MWCNT-dPIn is 18.85 μA mM-1 cm-2 in the linear range of 0.5-25 mM with the calculated LOD of 0.115 mM. For the κ-carrageenan based sensors, κC-GOD/MWCNT-dPIn exhibits the sensitivity of 15.80 μA mM-1 cm-2 and the widest linear range from 0.1 to 50 mM with the calculated LOD of 0.03 mM. The presently fabricated sensors exhibit excellent reproducibility, good selectivity, high stability, and disposal use. The fabricated glucose sensors are potential as practical glucose sensors as the detectable glucose ranges well cover the glucose levels found in blood, urine, and sweat for both healthy people and diabetic patients.

Simultaneous observation of the spatial and temporal dynamics of single enzymatic catalysis using a solid-state nanopore

We developed a bipolar SiNx nanopore for the observation of single-molecule heterogeneous enzymatic dynamics. Single glucose oxidase was immobilized inside the nanopore and its electrocatalytic behaviour was real-time monitored via continuous recording of ionic flux amplification. The temporal heterogeneity in enzymatic properties and its spatial dynamic orientations were observed simultaneously, and these two properties were found to be closely correlated. We anticipate that this method offers new perspectives on the correlation of protein structure and function at the single-molecule level.

Interactions of Catalytic Enzymes with n-Type Polymers for High-Performance Metabolite Sensors

The tight regulation of the glucose concentration in the body is crucial for balanced physiological function. We developed an electrochemical transistor comprising an n-type conjugated polymer film in contact with a catalytic enzyme for sensitive and selective glucose detection in bodily fluids. Despite the promise of these sensors, the property of the polymer that led to such high performance has remained unknown, with charge transport being the only characteristic under focus. Here, we studied the impact of the polymer chemical structure on film surface properties and enzyme adsorption behavior using a combination of physiochemical characterization methods and correlated our findings with the resulting sensor performance. We developed five n-type polymers bearing the same backbone with side chains differing in polarity and charge. We found that the nature of the side chains modulated the film surface properties, dictating the extent of interactions between the enzyme and the polymer film. Quartz crystal microbalance with dissipation monitoring studies showed that hydrophobic surfaces retained more enzymes in a densely packed arrangement, while hydrophilic surfaces captured fewer enzymes in a flattened conformation. X-ray photoelectron spectroscopy analysis of the surfaces revealed strong interactions of the enzyme with the glycolated side chains of the polymers, which improved for linear side chains compared to those for branched ones. We probed the alterations in the enzyme structure upon adsorption using circular dichroism, which suggested protein denaturation on hydrophobic surfaces. Our study concludes that a negatively charged, smooth, and hydrophilic film surface provides the best environment for enzyme adsorption with desired mass and conformation, maximizing the sensor performance. This knowledge will guide synthetic work aiming to establish close interactions between proteins and electronic materials, which is crucial for developing high-performance enzymatic metabolite biosensors and biocatalytic charge-conversion devices.

Low-Denaturazing Glucose Oxidase Immobilization onto Graphite Electrodes by Incubation in Chitosan Solutions

In this work, glucose oxidase (GOx) has been immobilized onto graphite rod electrodes through an assisted-chitosan adsorption reaching an enzyme coverage of 4 nmol/cm2. The direct and irreversible single adsorption of the Flavine Adenine Dinucleotide (FAD) cofactor has been minimized by electrode incubation in a chitosan (CH) solution containing the enzyme GOx. Chitosan keeps the enzyme structure and conformation due to electrostatic interactions preventing FAD dissociation from the protein envelope. Using chitosan, both the redox cofactor FAD and the protein envelope remain in the active form as demonstrated by the electrochemistry studies and the enzymatic activity in the electrochemical oxidation of glucose up to a concentration of 20 mM. The application of the modified electrodes for energy harvesting delivered a power density of 119 µW/cm2 with a cell voltage of 0.3 V. Thus, chitosan presents a stabilizing effect for the enzyme conformation promoted by the confinement effect in the chitosan solution by electrostatic interactions. Additionally, it facilitated the electron transfer from the enzyme to the electrode due to the presence of embedded chitosan in the enzyme structure acting as an electrical wiring between the electrode and the enzyme (electron transfer rate constant 2.2 s−1). This method involves advantages compared with previously reported chitosan immobilization methods, not only due to good stability of the enzyme, but also to the simplicity of the procedure that can be carried out even for not qualified technicians which enable their easy implementation in industry.

Complementary Powerful Techniques for Investigating the Interactions of Proteins with Porous TiO2 and Its Hybrid Materials: A Tutorial Review

Understanding the adsorption and interaction between porous materials and protein is of great importance in biomedical and interface sciences. Among the studied porous materials, TiO₂ and its hybrid materials, featuring distinct, well-defined pore sizes, structural stability and excellent biocompatibility, are widely used. In this review, the use of four powerful, synergetic and complementary techniques to study protein-TiO_2-based porous materials interactions at different scales is summarized, including high-performance liquid chromatography (HPLC), atomic force microscopy (AFM), surface-enhanced Raman scattering (SERS), and Molecular Dynamics (MD) simulations. We expect that this review could be helpful in optimizing the commonly used techniques to characterize the interfacial behavior of protein on porous TiO₂ materials in different applications.

Enzyme Immobilization on Gold Nanoparticles for Electrochemical Glucose Biosensors

More than 50 years have passed since Clark and Lyon developed the concept of glucose biosensors. Extensive research about biosensors has been carried out up to this day, and an exponential trend in this topic can be observed. The scope of this review is to present various enzyme immobilization methods on gold nanoparticles used for glucose sensing over the past five years. This work covers covalent bonding, adsorption, cross-linking, entrapment, and self-assembled monolayer methods. The experimental approach of each modification as well as further results are described. Designated values of sensitivity, the limit of detection, and linear range are used for the comparison of immobilization techniques.

Replica exchange dissipative particle dynamics method on threadlike micellar aqueous solutions

The self-assembly of surfactant molecules can spontaneously result in a variety of micelle morphologies, such as spherical micelles, threadlike micelles, and vesicles, and it is therefore crucial to predict and control the self-assembly to achieve a helpful process in the fields of materials chemistry and engineering. A dissipative particle dynamics (DPD) method used in a coarse-grained molecular simulation is applied to simulate various self-assembling soft matter systems because it can handle greater length and time scales than a typical molecular dynamics simulation (MD). It should be noted that the thorough sampling of a system is not assured at low temperatures because of large complex systems with coarse-grained representations. In this article, we demonstrate that the replica exchange method (REM) is very effective for even a DPD in which the energy barrier is comparatively lower than that of a MD. A replica exchange on DPD (REDPD) simulation for threadlike micellar aqueous solutions was conducted, and the values of the potential energy and the mean aggregation number were compared. As a result, the correct values and a self-assembled structure within a low-temperature range can only be obtained through the REDPD.

Charged polymeric additives affect the nucleation of lysozyme crystals

Charged polymers (PGA and PL) interact with lysozyme and then promote the heterogeneous nucleation of the crystals.

Bilirubin Oxidase Adsorption onto Charged Self-Assembled Monolayers: Insights from Multiscale Simulations

The efficient immobilization and orientation of bilirubin oxidase (BOx) on different solid substrates are essential for its application in biotechnology. The T1 copper site within BOx is responsible for the electron transfer. In order to obtain quick direct electron transfer (DET), it is important to keep the distance between the T1 copper site and electrode surface small and to maintain the natural structure of BOx at the same time. In this work, the combined parallel tempering Monte Carlo simulation with the all-atom molecular dynamics simulation approach was adopted to reveal the adsorption mechanism, orientation, and conformational changes of BOx from Myrothecium verrucaria (MvBOx) adsorbed on charged self-assembled monolayers (SAMs), including COOH-SAM and NH₂-SAM with different surface charge densities (±0.05 and ±0.19 C·m^-2). The results show that MvBOx adsorbs on negatively charged surfaces with a "back-on" orientation, whereas on positively charged surfaces, MvBOx binds with a "lying-on" orientation. The locations of the T1 copper site are closer to negatively charged surfaces. Furthermore, for negatively charged surfaces, the T1 copper site prefers to orient closer to the surface with lower surface charge density. Therefore, the negatively charged surface with low surface charge density is more suitable for the DET of MvBOx on electrodes. Besides, the structural changes primarily take place on the relatively flexible turns, coils, and α-helix. The native structure of MvBOx is well preserved when it adsorbs on both charged surfaces. This work sheds light on the controlling orientation and conformational information on MvBOx on charged surfaces at the atomistic level. This understanding would certainly promote our understanding of the mechanism of MvBOx immobilization and provide theoretical support for BOx-based bioelectrode design.