Effect of Nonionic Surfactants on Dispersion and Polar Interactions in the Adsorption of Cellulases onto Lignin

Residual lignin in pretreated biomass impedes enzymatic hydrolysis. Nonionic surfactants are known to enhance the enzymatic hydrolysis of lignocellulosic biomass but their mechanisms of action are incompletely understood. This study investigates the effect of a nonionic surfactant, Tween 80, on the adsorption of cellulases onto model lignin substrates. Lignin substrates were prepared by spin coating of flat substrates with three different types of lignin: organosolv lignin, kraft lignin, and milled wood lignin. The functional group distributions in the lignins were quantitatively analyzed by ³¹P NMR spectroscopy. The surface energies and surface roughnesses of the substrates were determined by contact angle measurements and atomic force microscopy, respectively. Tween 80 and cellulase adsorption onto the lignin substrates was analyzed with a quartz crystal microbalance with dissipation monitoring. Tween 80 adsorbed rapidly and primarily (≥85%) via dispersion interactions onto the lignin substrates and effected solubilization of lignin molecules, most notably with organosolv lignin, having the largest dispersive surface energy component and smallest molar mass. Cellulase adsorption onto the lignin substrates was mostly irreversible and had both a rapid and a gradual adsorption stage. Rapid cellulase adsorption was primarily (≥64%) mediated by dispersion interactions. The subsequent gradual mass increase is postulated to involve swelling of the lignin substrates. Adsorbed Tween 80 rendered lignin surfaces more hydrophilic by increasing their polar surface energy component and reduced both the extent of rapid cellulase adsorption as well as the rate of the subsequent gradual mass increase. The effect of Tween 80 on the rate and extent of the gradual mass increase depended strongly on the chemical properties of the lignin

An Efficient, Regioselective Pathway to Cationic and Zwitterionic <i>N</i>‑Heterocyclic Cellulose Ionomers

Cationic derivatives of cellulose and other polysaccharides are attractive targets for biomedical applications due to their propensity for electrostatically binding with anionic biomolecules, such as nucleic acids and certain proteins. To date, however, relatively few practical synthetic methods have been described for their preparation. Herein, we report a useful and efficient strategy for cationic cellulose ester salt preparation by the reaction of 6-bromo-6-deoxycellulose acetate with pyridine or 1-methylimidazole. Dimethyl sulfoxide solvent favored this displacement reaction to produce cationic cellulose acetate derivatives, resulting in high degrees of substitution (DS) exclusively at the C-6 position. These cationic cellulose derivatives bearing substantial, permanent positive charge exhibit surprising thermal stability, dissolve readily in water, and bind strongly with a hydrophilic and anionic surface, supporting their potential for a variety of applications such as permeation enhancement, mucoadhesion, and gene or drug delivery. Expanding upon this chemistry, we reacted a 6-imidazolyl-6-deoxycellulose derivative with 1,3-propane sultone to demonstrate the potential for further elaboration to regioselectively substituted zwitterionic cellulose derivatives

Surface-Initiated Dehydrogenative Polymerization of Monolignols: A Quartz Crystal Microbalance with Dissipation Monitoring and Atomic Force Microscopy Study

This work highlights a real-time and label-free method to monitor the dehydrogenative polymerization of monolignols initiated by horseradish peroxidase (HRP) physically immobilized on surfaces using a quartz crystal microbalance with dissipation monitoring (QCM-D). The dehydrogenative polymer (DHP) films are expected to provide good model substrates for studying ligninolytic enzymes. The HRP was adsorbed onto gold or silica surfaces or onto and within porous desulfated nanocrystalline cellulose films from an aqueous solution. Surface-immobilized HRP retained its activity and selectivity for monolignols as coniferyl and <i>p</i>-coumaryl alcohol underwent dehydrogenative polymerization in the presence of hydrogen peroxide, whereas sinapyl alcohol polymerization required the addition of a nucleophile. The morphologies of the DHP layers on the surfaces were investigated via atomic force microscopy (AFM). Data from QCM-D and AFM showed that the surface-immobilized HRP-initiated dehydrogenative polymerization of monolignols was greatly affected by the support surface, monolignol concentration, hydrogen peroxide concentration, and temperature

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

Chitinase Activity on Amorphous Chitin Thin Films: A Quartz Crystal Microbalance with Dissipation Monitoring and Atomic Force Microscopy Study

Chitinases are widely distributed in nature and have wide-ranging pharmaceutical and biotechnological applications. This work highlights a real-time and label-free method to assay Chitinase activity via a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). The chitin substrate was prepared by spincoating a trimethylsilyl chitin solution onto a silica substrate, followed by regeneration to amorphous chitin (RChi). The QCM-D and AFM results clearly showed that the hydrolysis rate of RChi films increased as Chitinase (from <i>Streptomyces griseus</i>) concentrations increased, and the optimal temperature and pH for Chitinase activity were around 37 °C and 6–8, respectively. The Chitinase showed greater activity on chitin substrates, having a high degree of acetylation, than on chitosan substrates, having a low degree of acetylation

Role of (1,3)(1,4)-β-Glucan in Cell Walls: Interaction with Cellulose

(1,3)­(1,4)-β-d-Glucan (mixed-linkage glucan or MLG), a characteristic hemicellulose in primary cell walls of grasses, was investigated to determine both its role in cell walls and its interaction with cellulose and other cell wall polysaccharides in vitro. Binding isotherms showed that MLG adsorption onto microcrystalline cellulose is slow, irreversible, and temperature-dependent. Measurements using quartz crystal microbalance with dissipation monitoring showed that MLG adsorbed irreversibly onto amorphous regenerated cellulose, forming a thick hydrogel. Oligosaccharide profiling using <i>endo</i>-(1,3)­(1,4)-β-glucanase indicated that there was no difference in the frequency and distribution of (1,3) and (1,4) links in bound and unbound MLG. The binding of MLG to cellulose was reduced if the cellulose samples were first treated with certain cell wall polysaccharides, such as xyloglucan and glucuronoarabinoxylan. The tethering function of MLG in cell walls was tested by applying <i>endo</i>-(1,3)­(1,4)-β-glucanase to wall samples in a constant force extensometer. Cell wall extension was not induced, which indicates that enzyme-accessible MLG does not tether cellulose fibrils into a load-bearing network

KTN-BMP-2 interaction.

<p>Association (a) and dissociation (d) electrostatic interaction profiles of BMP-2 and the KTN monolayer in PBS and in water. BMP-2 analytes were successively flowed, at incrementally increasing concentrations: A) in PBS (pH 7.4) at 0.2, 0.4, 0.9, 1.7, 3.5, and 6.9 μM, B) in PBS (pH 4.5) at 0.03, 0.05, 0.1, 0.2, 0.4, 0.9, and 1.7 μM, and C) in water (pH 7) at 0.03, 0.05, 0.1, 0.2, and 0.4 μM. KTN-BMP-2 electrostatic attraction was strongest in water (KD = 1.1 × 10–7 M). In the presence of PBS salts at physiological pH, binding association was slightly weakened (KD = 3.2 × 10–5 M). Acidification of the PBS eliminated any binding between BMP-2 and KTN.</p

KTN bulk release.

<p>KTN gel bulk degradation in vitro at A-B) constant pH = 7.4, and C-D) constant [KCl] = 154 mM for over a period of 28 days at 37°C. Faster degradation occurred at longer time points, lower [KCl], and higher pH levels.</p

Comparison between reduced (KTN) and oxidized (KOS) keratin biomaterials.

<p>KTN can form disulfide linkages to produce a stable scaffold; but KOS cannot, due to the sulfonic acid modification of thiol groups. Consequently, the electrostatic properties are also altered, resulting in more negatively-charged KOS scaffolds, prone to more rapid hydrolytic degradation than KTN scaffolds. 10 mM NaOH solvent was used for wetting and soaking.</p

XPS analysis.

<p>A) Wide-scan XPS spectra of gold surfaces treated with 10 mM NaOH solvent, KOS, and KTN. Overnight incubation of KTN led to no detectable Au signals. The inset graph shows that, compared to the solvent group, KOS has very similar concentration levels of carbon, nitrogen, oxygen, sulfur, and gold, while KTN has elevated amounts of protein elements (C, N, and O) but decreased Au. B) Near-scan analysis displays the formation of an amide (O = C-N) peak at 288 eV, corresponding to KTN protein deposition on gold. Unbound and gold-bound KTN thiols were also detected at 163.6 and 162.5 eV, respectively. Partial adsorption of KTN on gold shifted the Au4f peaks to slightly lower energies.</p