Neutron Reflectometry and QCM-D Study of the Interaction of Cellulases with Films of Amorphous Cellulose

Nanomechanics of Lignin-Cellulase Interactions in Aqueous Solutions

Efficient enzymatic hydrolysis of cellulose in lignocellulose to glucose is one of the most critical steps for the production of biofuels. The nonproductive adsorption of lignin to expensive cellulase highly impedes the development of biorefinery. Understanding the lignin-cellulase interaction mechanism serves as a vital basis for reducing such nonproductive adsorption in their practical applications. Yet, limited report is available on the direct characterization of the lignin-cellulase interactions. Herein, for the first time, the nanomechanics of the biomacromolecules including lignin, cellulase, and cellulose were systematically investigated by using a surface force apparatus (SFA) at the nanoscale in aqueous solutions. Interestingly, a cation-π interaction was discovered and demonstrated between lignin and cellulase molecules through SFA measurements with the addition of different cations (Na⁺, K⁺, etc.). The complementary adsorption tests and theoretical calculations further confirmed the validity of the force measurement results. This finding further inspired the investigation of the interaction between lignin and other noncatalytic-hydrolysis protein (i.e., soy protein). Soy protein was demonstrated as an effective, biocompatible, and inexpensive lignin-blocker based on the molecular force measurements through the combined effects of electrostatic, cation-π, and hydrophobic interactions, which significantly improved the enzymatic hydrolysis efficiencies of cellulose in pretreated lignocellulosic substrates. Our results offer quantitative information on the fundamental understanding of the lignin-cellulase interaction mechanism. Such unraveled nanomechanics provides new insights into the development of advanced biotechnologies for addressing the nonproductive adsorption of lignin to cellulase, with great implications on improving the economics of lignocellulosic biorefinery.

Enhanced Thermostability and Enzymatic Activity of Cel6A Variants from Thermobifida fusca by Empirical Domain Engineering (Short Title: Domain Engineering of Cel6A)

Cellulases are a set of lignocellulolytic enzymes, capable of producing eco-friendly low-cost renewable bioethanol. However, low stability and hydrolytic activity limit their wide-scale applicability at the industrial scale. In this work, we report the domain engineering of endoglucanase (Cel6A) of Thermobifida fusca to improve their catalytic activity and thermal stability. Later, enzymatic activity and thermostability of the most efficient variant named as Cel6A.CBC was analyzed by molecular dynamics simulations. This variant demonstrated profound activity against soluble and insoluble cellulosic substrates like filter paper, alkali-treated bagasse, regenerated amorphous cellulose (RAC), and bacterial microcrystalline cellulose. The variant Cel6A.CBC showed the highest catalysis of carboxymethyl cellulose (CMC) and other related insoluble substrates at a pH of 6.0 and a temperature of 60 °C. Furthermore, a sound rationale was observed between experimental findings and molecular modeling of Cel6A.CBC which revealed thermostability of Cel6A.CBC at 26.85, 60.85, and 74.85 °C as well as structural flexibility at 126.85 °C. Therefore, a thermostable derivative of Cel6A engineered in the present work has enhanced biological performance and can be a useful construct for the mass production of bioethanol from plant biomass.

Chelator-mediated biomimetic degradation of cellulose and chitin

Non-enzymatic degradation of wood via a chelator-mediated Fenton (CMF) system is the primary method for initial attack in brown rot fungal decomposition of wood, the most common type of fungal degradation of terrestrial carbon biomass on the planet. In this study, the degradation of thin films of cellulose and chitin by a CMF system was investigated and compared to enzymatic hydrolysis. The kinetics of the rapid cellulose and chitin deconstruction and the morphologies of the degraded cellulose and chitin surfaces were studied by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM), respectively. The QCM-D results quantitatively indicated that ~90 wt% of the regenerated cellulose or chitin was capable of being deconstructed by CMF action alone. While enzymatic degradation was consistent with stripping of layers from the surface of the cellulose or chitin films, the CMF process exhibited a pronounced two stage process with a rapid initial depolymerization throughout the films. The initial degradation rates for both model surfaces by the CMF system were faster than enzyme action. This research suggests that the CMF process should be applicable for the deconstruction of a wide variety of polysaccharides over Fenton chemistry alone.

Cellulose Nano-Films as Bio-Interfaces

Cellulose, the most abundant polymer on earth, has enormous potential in developing bio-friendly, and sustainable technological products. In particular, cellulose films of nanoscale thickness (1-100 nm) are transparent, smooth (roughness <1 nm), and provide a large surface area interface for biomolecules immobilization and interactions. These attractive film properties create many possibilities for both fundamental studies and applications, especially in the biomedical field. The three liable-OH groups on the monomeric unit of the cellulose chain provide schemes to chemically modify the cellulose interface and engineer its properties. Here, the cellulose thin film serves as a substrate for biomolecules interactions and acts as a support for bio-diagnostics. This review focuses on the challenges and opportunities provided by engineering cellulose thin films for controlling biomolecules interactions. The first part reviews the methods for preparing cellulose thin films. These are by dispersing or dissolving pure cellulose or cellulose derivatives in a solvent to coat a substrate using the spin coating, Langmuir-Blodgett, or Langmuir-Schaefer method. It is shown how different cellulose sources, preparation, and coating methods and substrate surface pre-treatment affect the film thickness, roughness, morphology, crystallinity, swelling in water, and homogeneity. The second part analyses the bio-macromolecules interactions with the cellulose thin film interfaces. Biomolecules, such as antibodies and enzymes, are adsorbed at the cellulose-liquid interface, and analyzed dry and wet. This highlights the effect of film surface morphology, thickness, crystallinity, water intake capacity, and surface pre-treatment on biomolecule adsorption, conformation, coverage, longevity, and activity. Advance characterization of cellulose thin film interface morphology and adsorbed biomolecules interactions are next reviewed. X-ray and neutron scattering/reflectivity combined with atomic force microscopy (AFM), quartz crystal microbalance (QCM), microscopy, and ellipsometer allow visualizing, and quantifying the structural morphology of cellulose-biomolecule interphase and the respective biomolecules conformations, kinetics, and sorption mechanisms. This review provides a novel insight on the advantages and challenges of engineering cellulose thin films for biomedical applications. This is to foster the exploration at the molecular level of the interaction mechanisms between a cellulose interface and adsorbed biomolecules with respect to adsorbed molecules morphology, surface coverage, and quantity. This knowledge is to engineer a novel generation of efficient and functional biomedical devices.

Real-Time Adsorption of Exo- and Endoglucanases on Cellulose: Effect of pH, Temperature, and Inhibitors

Effective regulation of cellulase adsorption is key to improving the efficiencies of the two major bottlenecks of lignocellulose hydrolysis and cellulase recovery. In this work, we investigated the effect of inhibitors, pH, and temperature on the adsorption of exo- and endoglucanases (Cel7A and Cel7B, respectively) on cellulose using quartz crystal microgravimetry with dissipation. The addition of glucose and cellobiose can both inhibit the hydrolysis activity of Cel7A, whereas only cellobiose can inhibit that of Cel7B. Notably, the adsorption was favored by acidic conditions (pH ≤ 4.8) and low temperature, whereas alkaline conditions (pH 9 and 10) facilitated enzyme desorption, which is useful to guide the process of cellulase recovery. The adsorption and hydrolysis activity of Cel7A and Cel7B were both higher at 45 °C than at 25 °C. These findings pave the way to effective regulation of cellulase adsorption and thus improve lignocellulose conversion and cellulase recovery.

Current characterization methods for cellulose nanomaterials

A new family of materials comprised of cellulose, cellulose nanomaterials (CNMs), having properties and functionalities distinct from molecular cellulose and wood pulp, is being developed for applications that were once thought impossible for cellulosic materials. Commercialization, paralleled by research in this field, is fueled by the unique combination of characteristics, such as high on-axis stiffness, sustainability, scalability, and mechanical reinforcement of a wide variety of materials, leading to their utility across a broad spectrum of high-performance material applications. However, with this exponential growth in interest/activity, the development of measurement protocols necessary for consistent, reliable and accurate materials characterization has been outpaced. These protocols, developed in the broader research community, are critical for the advancement in understanding, process optimization, and utilization of CNMs in materials development. This review establishes detailed best practices, methods and techniques for characterizing CNM particle morphology, surface chemistry, surface charge, purity, crystallinity, rheological properties, mechanical properties, and toxicity for two distinct forms of CNMs: cellulose nanocrystals and cellulose nanofibrils.

On the distinct binding modes of expansin and carbohydrate-binding module proteins on crystalline and nanofibrous cellulose: implications for cellulose degradation by designer cellulosomes

Transformation of cellulose into monosaccharides can be achieved by hydrolysis of the cellulose chains, carried out by a special group of enzymes known as cellulases.

Effect of Ionic Strength and Surface Charge Density on the Kinetics of Cellulose Nanocrystal Thin Film Swelling

This work explores cellulose nanocrystal (CNC) thin films (<50 nm) and particle-particle interactions by investigating film swelling in aqueous solutions with varying ionic strength (1-100 mM). CNC film hydration was monitored in situ via surface plasmon resonance, and the kinetics of liquid uptake were quantified. The contribution of electrostatic double-layer forces to film swelling was elucidated by using CNCs with different surface charges (anionic sulfate half ester groups, high and low surface charge density, and cationic trimethylammonium groups). Total water uptake in the thin films was found to be independent of ionic strength and surface chemistry, suggesting that in the aggregated state van der Waals forces dominate over double-layer forces to hold the films together. However, the rate of swelling varied significantly. The water uptake followed Fickian behavior, and the measured diffusion constants decreased with the ionic strength gradient between the film and the solution. This work highlights that nanoparticle interactions and dispersion are highly dependent on the state of particle aggregation and that the rate of water uptake in aggregates and thin films can be tailored based on surface chemistry and solution ionic strength.

Functionality of Immunoglobulin G and Immunoglobulin M Antibody Physisorbed on Cellulosic Films

The functionality and aging mechanism of antibodies physisorbed onto cellulosic films was investigated. Blood grouping antibodies immunoglobulin G (IgG) and immunoglobulin M (IgM) were adsorbed onto smooth cellulose acetate (CAF) and regenerated cellulose (RCF) films. Cellulose films and adsorbed IgG layers were characterized at the air and liquid interface by X-ray and neutron reflectivity (NR), respectively. Cellulose film 208 Å thick (in air) swell to 386 Å once equilibrated in water. IgG adsorbs from solution onto cellulose as a partial layer 62 Å thick. IgG and IgM antibodies were adsorbed onto cellulose and cellulose acetate films, air dried, and aged at room temperature for periods up to 20 days. Antibody functionality and surface hydrophobicity were measured everyday with the size of red blood cell (RBC) agglutinates (using RBC specific to IgG/IgM) and the water droplet contact angle, respectively. The functionality of the aged IgG/IgM decreases faster if physisorbed on cellulose than on cellulose acetate and correlates to surface hydrophobicity. IgG physisorbed on RCF or CAF age better and remain functional longer than physisorbed IgM. We found a correlation between antibody stability and hydrogen bond formation ability of the system, evaluated from antibody carbonyl concentration and cellulosic surface hydroxyl concentration. Antibody physisorbs on cellulose by weak dipole forces and hydrogen bonds. Strong hydrogen bonding contributes to the physisorption of antibody on cellulose into a non-functional configuration in which the molecule relaxes by rotation of hydophobic groups toward the air interface.

Visualization and Quantification of IgG Antibody Adsorbed at the Cellulose-Liquid Interface

Quantification of adsorbed biomolecules (enzymes, proteins) at the cellulose interface is a major challenge in developing eco-friendly biodiagnostics. Here, a novel methodology is developed to visualize and quantify the adsorption of antibody from solution to the cellulose-liquid interface. The concept is to deuterate cellulose by replacing all nonexchangeable hydrogens from the glucose rings with deuterium in order to enhance the scattering contrast between the cellulose film surface and adsorbed antibody molecules. Deuterated cellulose (DC) was obtained from bacterial (Gluconacetobacter xylinus strain) cellulose, which was grown in heavy water (D₂O) media with a deuterated glycerol as a carbon source. For comparison, hydrogenated cellulose (HC) was obtained from cellulose acetate. Both HC and DC thin films were prepared on silicon substrate by spin coating. X-ray reflectivity (XR) shows the formation of homogeneous and smooth film. Neutron reflectivity (NR) at the liquid/film interface reveals swelling of the cellulose film by a factor of 2-3× its initial thickness. An Immunoglobulin G (IgG), used as a model antibody, was adsorbed at the liquid-solid interface of cellulose (HC) and deuterated cellulose (DC) films under equilibrium and surface saturation conditions. NR measurements of the IgG antibody layer adsorbed onto the DC film can clearly be visualized, in sharp contrast in comparison to the HC film. The average thickness of the IgG adsorbed layer onto cellulose films is 127 ± 5 Å and a partial monolayer is formed. Visualization and quantification of adsorbed IgG is shown by large difference in scattering length density (SLD) between DC (7.1 × 10^-6 Å^-2) and IgG (4.1 × 10^-6 Å^-2) in D₂O, which enhanced the scattering contrast in NR. Quartz crystal measurements (QCM-D) were used as a complementary method to NR to quantify the adsorbed IgG over the cellulose interface.

Cellulose: To depolymerize… or not to?

Oxidation of the primary OH groups in cellulose is a pivotal reaction both at lab and industrial scale, leading to the value-added products, i.e. oxidized cellulose which have tremendous applications in medicine, pharmacy and hi-tech industry. Moreover, the introduction of carboxyl moieties creates prerequisites for further cellulose functionalization through covalent attachment or electrostatic interactions, being an essential achievement designed to boost the area of cellulose-based nanomaterials fabrication. Various methods for the cellulose oxidation have been developed in the course of time, aiming the selective conversion of the OH groups. These methods use: nitrogen dioxide in chloroform, alkali metal nitrites and nitrates, strong acids alone or in combination with permanganates or sodium nitrite, ozone, and sodium periodate or lead (IV) tetraacetate. In the case of the last two reagents, cellulose dialdehydes derivatives are formed, which are further oxidized by sodium chlorite or hydrogen peroxide to form dicarboxyl groups. A major improvement in the cellulose oxidation was represented by the introduction of the stable nitroxyl radicals, such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). However, a major impediment for the researchers working in this area is related with the severe depolymerisation occurred during the TEMPO-mediated conversion of CH₂OH into COOH groups. On the other hand, the cellulose depolymerisation represent the key step, in the general effort of searching for alternative strategies to develop new renewable, carbon-neutral energy sources. In this connection, exploiting the biomass feed stocks to produce biofuel and other low molecular organic compounds, involves a high amount of research to improve the overall reaction conditions, limit the energy consumption, and to use benign reagents. This work is therefore focused on the parallelism between these two apparently antagonist processes involving cellulose, building a necessary bridge between them, thinking how the reported drawbacks of the TEMPO-mediated oxidation of cellulose are heading towards to the biomass valorisation, presenting why the apparently undesired side reactions could be turned into beneficial processes if they are correlated with the existing achievements of particular significance in the field of cellulose conversion into small organic compounds, aiming the general goal of pursuing for alternatives to replace the petroleum-based products in human life.

Hydrolysis of model cellulose films by cellulosomes: Extension of quartz crystal microbalance technique to multienzymatic complexes

Bacterial cellulosomes contain highly efficient complexed cellulases and have been studied extensively for the production of lignocellulosic biofuels and bioproducts. A surface measurement technique, quartz crystal microbalance with dissipation (QCM-D), was extended for the investigation of real-time binding and hydrolysis of model cellulose surfaces from free fungal cellulases to the cellulosomes of Clostridium thermocellum (Ruminiclostridium thermocellum). In differentiating the activities of cell-free and cell-bound cellulosomes, greater than 68% of the cellulosomes in the crude cell broth were found to exist unattached to the cell across multiple growth stages. The initial hydrolysis rate of crude cell broth measured by QCM was greater than that of cell-free cellulosomes, but the corresponding frequency drop (a direct measure of the mass of enzyme adsorbed to the film) of crude cell broth was less than that of the cell-free cellulosomes, consistent with the underestimation of the cell mass adsorbed using QCM. Inhibition of hydrolysis by cellobiose (0-10g/L), which is similar for crude cell broth and cell-free cellulosomes, demonstrates the sensitivity of the QCM to environmental perturbations of multienzymatic complexes. QCM measurements using multienzymatic complexes may be used to screen and optimize hydrolysis conditions and to develop mechanistic, surface-based models of enzymatic cellulose deconstruction.

Smooth deuterated cellulose films for the visualisation of adsorbed bio-macromolecules

Novel thin and smooth deuterated cellulose films were synthesised to visualize adsorbed bio-macromolecules using contrast variation neutron reflectivity (NR) measurements. Incorporation of varying degrees of deuteration into cellulose was achieved by growing Gluconacetobacter xylinus in deuterated glycerol as carbon source dissolved in growth media containing D2O. The derivative of deuterated cellulose was prepared by trimethylsilylation(TMS) in ionic liquid(1-butyl-3-methylimidazolium chloride). The TMS derivative was dissolved in toluene for thin film preparation by spin-coating. The resulting film was regenerated into deuterated cellulose by exposure to acidic vapour. A common enzyme, horseradish peroxidase (HRP), was adsorbed from solution onto the deuterated cellulose films and visualized by NR. The scattering length density contrast of the deuterated cellulose enabled accurate visualization and quantification of the adsorbed HRP, which would have been impossible to achieve with non-deuterated cellulose. The procedure described enables preparing deuterated cellulose films that allows differentiation of cellulose and non-deuterated bio-macromolecules using NR.

Tunable Semicrystalline Thin Film Cellulose Substrate for High-Resolution, In-Situ AFM Characterization of Enzymatic Cellulose Degradation

In the field of enzymatic cellulose degradation, fundamental interactions between different enzymes and polymorphic cellulose materials are of essential importance but still not understood in full detail. One technology with the potential of direct visualization of such bioprocesses is atomic force microscopy (AFM) due to its capability of real-time in situ investigations with spatial resolutions down to the molecular scale. To exploit the full capabilities of this technology and unravel fundamental enzyme-cellulose bioprocesses, appropriate cellulose substrates are decisive. In this study, we introduce a semicrystalline-thin-film-cellulose (SCFTC) substrate which fulfills the strong demands on such ideal cellulose substrates by means of (1) tunable polymorphism via variable contents of homogeneously sized cellulose nanocrystals embedded in an amorphous cellulose matrix; (2) nanoflat surface topology for high-resolution and high-speed AFM; and (3) fast, simple, and reproducible fabrication. The study starts with a detailed description of SCTFC preparation protocols including an in-depth material characterization. In the second part, we demonstrate the suitability of SCTFC substrates for enzymatic degradation studies by combined, individual, and sequential exposure to TrCel6A/TrCel7A cellulases (Trichoderma reesei) to visualize synergistic effects down to the nanoscale.

Tailor-made functional surfaces based on cellulose-derived materials

As one of the most abundant natural materials in nature, cellulose has revealed enormous potential for the construction of functional materials thanks to its sustainability, non-toxicity, biocompatibility, and biodegradability. Among many fascinating applications, functional surfaces based on cellulose-derived materials have attracted increasing interest recently, as platforms for diagnostics, sensoring, robust catalysis, water treatment, ultrafiltration, and anti-microbial surfaces. This mini-review attempts to cover the general methodology for the fabrication of functional cellulose surface and a few popular applications including bioactive and non-adhesive (i.e., anti-fouling and anti-microbial) surfaces.

In situ stability of substrate-associated cellulases studied by DSC

This work shows that differential scanning calorimetry (DSC) can be used to monitor the stability of substrate-adsorbed cellulases during long-term hydrolysis of insoluble cellulose. Thermal transitions of adsorbed enzyme were measured regularly in subsets of a progressing hydrolysis, and the size of the transition peak was used as a gauge of the population of native enzyme. Analogous measurements were made for enzymes in pure buffer. Investigations of two cellobiohydrolases, Cel6A and Cel7A, from Trichoderma reesei, which is an anamorph of the fungus Hypocrea jerorina, showed that these enzymes were essentially stable at 25 °C. Thus, over a 53 h experiment, Cel6A lost less than 15% of the native population and Cel7A showed no detectable loss for either the free or substrate-adsorbed state. At higher temperatures we found significant losses in the native populations, and at the highest tested temperature (49 °C) about 80% Cel6A and 35% of Cel7A was lost after 53 h of hydrolysis. The data consistently showed that Cel7A was more long-term stable than Cel6A and that substrate-associated enzyme was less long-term stable than enzyme in pure buffer stored under otherwise equal conditions. There was no correlation between the intrinsic stability, specified by the transition temperature in the DSC, and the long-term stability derived from the peak area. The results are discussed with respect to the role of enzyme denaturation for the ubiquitous slowdown observed in the enzymatic hydrolysis of cellulose.

Addition of a carbohydrate-binding module enhances cellulase penetration into cellulose substrates

INTRODUCTION

Cellulases are of great interest for application in biomass degradation, yet the molecular details of the mode of action of glycoside hydrolases during degradation of insoluble cellulose remain elusive. To further improve these enzymes for application at industrial conditions, it is critical to gain a better understanding of not only the details of the degradation process, but also the function of accessory modules.

METHOD

We fused a carbohydrate-binding module (CBM) from family 2a to two thermophilic endoglucanases. We then applied neutron reflectometry to determine the mechanism of the resulting enhancements.

RESULTS

Catalytic activity of the chimeric enzymes was enhanced up to three fold on insoluble cellulose substrates as compared to wild type. Importantly, we demonstrate that the wild type enzymes affect primarily the surface properties of an amorphous cellulose film, while the chimeras containing a CBM alter the bulk properties of the amorphous film.

CONCLUSION

Our findings suggest that the CBM improves the efficiency of these cellulases by enabling digestion within the bulk of the film.

Effects of sulfate groups on the adsorption and activity of cellulases on cellulose substrates

Pretreatment of lignocellulosic biomass with sulfuric acid may leave sulfate groups on its surface that may hinder its biochemical conversion. This study investigates the effects of sulfate groups on cellulase adsorption onto cellulose substrates and the enzymatic hydrolysis of these substrates. Substrates with different sulfate group densities were prepared from H2SO4- and HCl-hydrolyzed and partially and fully desulfated cellulose nanocrystals. Adsorption onto and hydrolysis of the substrates was analyzed by quartz crystal microbalance with dissipation monitoring (QCM-D). The surface roughness of the substrates, measured by atomic force microscopy, increased with decreasing sulfate group density, but their surface accessibilities, measured by QCM-D H2O/D2O exchange experiments, were similar. The adsorption of cellulose binding domains onto sulfated substrates decreased with increasing sulfate group density, but the adsorption of cellulases increased. The rate of hydrolysis of sulfated substrates decreased with increasing sulfate group density. The results indicated an inhibitory effect of sulfate groups on the enzymatic hydrolysis of cellulose, possibly due to nonproductive binding of the cellulases onto the substrates through electrostatic interactions instead of their cellulose binding domains.

Plant carbohydrate binding module enhances activity of hybrid microbial cellulase enzyme

A synthetic, highly active cellulase enzyme suitable for in planta production may be a valuable tool for biotechnological approaches to develop transgenic biofuel crops with improved digestibility. Here, we demonstrate that the addition of a plant derived carbohydrate binding module (CBM) to a synthetic glycosyl hydrolase improved the activity of the hydrolase in releasing sugar from plant biomass. A CEL-HYB1-CBM enzyme was generated by fusing a hybrid microbial cellulase, CEL-HYB1, with the CBM of the tomato (Solanum lycopersicum) SlCel9C1 cellulase. CEL-HYB1 and CEL-HYB1-CBM enzymes were produced in vitro using Pichia pastoris and the activity of these enzymes was tested using carboxymethylcellulose, MUC, and native crystalline cellulose assays. The presence of the CBM substantially improved the endoglucanase activity of CEL-HYB1, especially against the native crystalline cellulose encountered in Sorghum bicolor plant cell walls. These results indicate that addition of an endogenous plant derived CBM to cellulase enzymes may enhance hydrolytic activity.

Interactions of a cationic cellulose derivative with an ultrathin cellulose support

The adsorption behavior of cellulose-4-[N-methylammonium]butyrate chloride (CMABC) on two hydrophilic substrates is studied, namely nanometric cellulose model thin films and silicon dioxide substrates. The adsorption is quantified in dependence of electrolyte concentration and pH value using a quartz crystal microbalance with dissipation (QCM-D). In case of CMABC, at high ionic strengths (25-100 mM NaCl) high adsorption is observed at pH 7 (Δf(3): -15 to -17 Hz) while at lower ionic strengths (1-10 mM) less CMABC (Δf(3): -2 to -12 Hz) is deposited on the cellulose surfaces as indicated by the frequency changes using QCM-D. A change in pH value from 7 to 8 reveals an increase in adsorption. Atomic force microscopy shows that the coating of cellulose thin films with CMABC changes the morphology from a fibrillar to a particle like structure on the surface. The surface wettability with water increases with an increasing amount of CMABC on the surface compared to neat cellulose model films. At lower pH values (3 and 5), CMABC does not adsorb onto the cellulose model thin films. XPS is used to validate the results and to determine the nitrogen content of the surfaces. In addition, adsorption of CMABC onto another hydrophilic and negatively charged substrate, silicon dioxide coated quartz crystals, cannot be detected at different pH values and electrolyte concentrations as proven by QCM-D.

Competitive sorption kinetics of inhibited endo- and exoglucanases on a model cellulose substrate

For the first time, the competitive adsorption of inhibited cellobiohydrolase I (Cel7A, an exoglucanase) and endoglucanase I (Cel7B) from T. longibrachiatum is studied on cellulose. Using quartz crystal microgravimetry (QCM), sorption histories are measured for individual types of cellulases and their mixtures adsorbing to and desorbing from a model cellulose surface. We find that Cel7A has a higher adsorptive affinity for cellulose than does Cel7B. The adsorption of both cellulases becomes irreversible on time scales of 30-60 min, which are much shorter than those typically used for industrial cellulose hydrolysis. A multicomponent Langmuir kinetic model including first-order irreversible binding is proposed. Although adsorption and desorption rate constants differ between the two enzymes, the rate at which each surface enzyme irreversibly binds is identical. Because of the higher affinity of Cel7A for the cellulose surface, when Cel7A and Cel7B compete for surface sites, a significantly higher bulk concentration of Cel7B is required to achieve comparable surface enzyme concentrations. Because cellulose deconstruction benefits significantly from the cooperative activity of endoglucanases and cellobiohydrolases on the cellulose surface, accounting for competitive adsorption is crucial to developing effective cellulase mixtures.

New procedure to record the rupture of bonds between macromolecules and the surface of the quartz crystal microbalance (QCM)

It is shown that an increase in the amplitude of QCM shear oscillations during frequency scanning around the resonance frequency is accompanied (at a definite voltage) by distortions in the amplitude-frequency dependence for QCM. We demonstrated that these distortions are connected to the rupture of macromolecules from the QCM surface. It is shown that the identification of the rupture of particles and macromolecules from the QCM surface can be carried out by relying on the analysis of these distortions of the amplitude-frequency dependence. The distortions were distinguished as a signal. The number of broken bonds can be estimated from the value of this distortion signal, and the threshold voltage applied to the system can be used to estimate the rupture force to high accuracy. Using the proposed method, we estimated the strength of a physical bond, which was 3 pN. This procedure can be useful for studying biological objects and represents an advanced step in the development of the REVS (rupture event scanning) technique.

Interactions of endoglucanases with amorphous cellulose films resolved by neutron reflectometry and quartz crystal microbalance with dissipation monitoring

A study of the interaction of four endoglucanases with amorphous cellulose films by neutron reflectometry (NR) and quartz crystal microbalance with dissipation monitoring (QCM-D) is reported. The endoglucanases include a mesophilic fungal endoglucanase (Cel45A from H. insolens), a processive endoglucanase from a marine bacterium (Cel5H from S. degradans ), and two from thermophilic bacteria (Cel9A from A. acidocaldarius and Cel5A from T. maritima ). The use of amorphous cellulose is motivated by the promise of ionic liquid pretreatment as a second generation technology that disrupts the native crystalline structure of cellulose. The endoglucanases displayed highly diverse behavior. Cel45A and Cel5H, which possess carbohydrate-binding modules (CBMs), penetrated and digested within the bulk of the films to a far greater extent than Cel9A and Cel5A, which lack CBMs. While both Cel45A and Cel5H were active within the bulk of the films, striking differences were observed. With Cel45A, substantial film expansion and interfacial broadening were observed, whereas for Cel5H the film thickness decreased with little interfacial broadening. These results are consistent with Cel45A digesting within the interior of cellulose chains as a classic endoglucanase, and Cel5H digesting predominantly at chain ends consistent with its designation as a processive endoglucanase.

Enzymatic digestion of partially and fully regenerated cellulose model films from trimethylsilyl cellulose

Partially and fully regenerated cellulose model films from trimethylsilyl cellulose (TMSC) were prepared by a time dependent regeneration approach. These thin films were characterized with contact angle measurements and attenuated total reflectance infrared spectroscopy (ATR-IR). In order to get further insights into the completeness of the regeneration we studied the interaction of cellulase enzymes from Trichoderma viride with the cellulose films using a quartz crystal microbalance with dissipation (QCM-D). To support the results from the QCM-D experiments capillary zone electrophoresis (CZE) and atomic force microscopy (AFM) were applied. The changes in mass and energy dissipation due to the interaction of the enzymes with the substrates were correlated with the surface wettability and elemental composition of the regenerated films. The highest interaction activity between the films and the enzyme, as well as the highest cellulose degradation, was observed on fully regenerated cellulose films, but some degradation also occurred on pure TMSC films. The enzymatic degradation rate correlated well with the rate of regeneration. It was demonstrated that CZE can be used to support QCM-D data via the detection of enzyme hydrolysis products in the eluates of the QCM-D cells. Glucose release peaked at the same time as the maximum mass loss was detected via QCM-D. It was shown that a combination of QCM-D and CZE together with enzymatic digestion is a reliable method to determine the conversion rate of TMSC to cellulose. In addition QCM-D and AFM revealed that cellulase is irreversibly bound to hydrophobic TMSC surfaces, while pure cellulose is digested almost completely in the course of hydrolysis.