Heterogeneity and Specificity of Nanoscale Adhesion Forces Measured between Self-Assembled Monolayers and Lignocellulosic Substrates: A Chemical Force Microscopy Study

Abstract

Lack of fundamental understanding of cellulase interactions with different plant cell wall components during cellulose saccharification hinders progress toward achieving an economic production of biofuels from renewable plant biomass. Here, chemical force microscopy (CFM) was utilized to quantify the interactions between two surfaces that model either hydrophilic or hydrophobic functional groups of cellulases and a set of lignocellulosic substrates prepared through Kraft, sulfite, or organosolv pulping with defined chemical composition. The measured forces were then decoupled into specific and nonspecific components using the Poisson statistical approach. Heterogeneities in the distributions of forces as a function of the pretreatment method were mapped. Our results showed that hydrophobic domains and chemical moieties involved in hydrogen bonding and polar interactions were homogeneously distributed on all substrates but with distribution densities that varied with the type of the pretreatment method used to prepare substrates. In addition, we showed that increasing surface lignin coverage increased the heterogeneity of the substrates. When forces were decoupled, our results indicated that xylan reduced the strength of hydrogen bonding between the hydrophilic model surface and substrates. Permanent dipole-dipole interactions dominated the adhesion of the hydrophilic model surface to lignosulfonates, whereas hydrophobic interactions facilitated the adhesion of the hydrophobic model surface to Kraft lignin. We further showed that the structure of lignin determines the type of forces that dominate lignocellulosic interactions with other surfaces. Our findings suggest that nonproductive binding of cellulases to lignocellulosic biomass can be reduced by altering the hydrophobicity and/or chemical moieties involved in the polar interactions and by utilizing organosolv as a pretreatment method.

Effects of the Surface Morphology and Conformations of Lignocellulosic Biomass Biopolymers on Their Nanoscale Interactions with Hydrophobic Self-Assembled Monolayers

“…The fibers of lignocellulosic substrates were attached to silicon wafers for AFM experiments via a protocol described previously. 12 Briefly, piranha solution (4:1 H 2 SO 4 /H 2 O 2 )-cleaned silicon wafers were coated with 0.01% poly-L-lysine (PLL), and a 50 μL aliquot of a 1 mg/ mL water suspension of the fiber of interest was added to the PLLcoated silicon wafer for substrate attachment. The water in the suspension droplet was then allowed to evaporate overnight in a vacuumed desiccator.…”

“…The cantilever modification with methyl functional groups was performed as described previously. 12 Briefly, gold-coated AFM cantilevers were immersed in a 1 mM ethanolic solution of 1-octadecanethiol for 2 h. The cantilevers were then washed with 190 proof ethanol and dried in a stream of nitrogen before force measurements.…”

“…AFM Force Measurements and Analysis of Nanoscale Adhesion Forces Measured between the Hydrophobic Cantilevers and Lignocellulosic Biomass. AFM force measurements were previously 12 conducted using a PicoForce scanning probe microscope with a Nanoscope IIIa controller (Bruker Inc., Santa Barbara, CA) in force−volume (FV) mode. Magnitudes of adhesion forces acting between a methyl-functionalized cantilever and a reference substrate were determined from force−distance retraction curves using the Nanoscope Analysis 1.5 software (Bruker, Camarillo, CA).…”

Microscopic Techniques for the Analysis of Micro and Nanostructures of Biopolymers and Their Derivatives

“…The measured forces are then mapped and compared to the surface structures detected by the topological imaging for further analysis. These measurements are proven to be helpful in identification purposes, to determine compatibility between two materials, and to predict interactions between materials [219]. CFM has proven its use in the analysis of biomaterial and biopolymer fields, and has been used in the characterization of cellulose [220], chitosan [82], and DNA [221].…”

Variation in leaf anatomy, vascular traits and nanomechanical cell-wall properties among European beech (Fagus sylvatica L.) provenances

“…It was reported that biodegradation rapidly decreases xylem toughness measured by dynamic tests (Clausen 2010). Adhesive forces and interactions between neighbouring cells are essential for both cell development and tissue morphogenesis (Lommel et al 2013;Arslan et al 2015). In addition, changes in adhesion may be a good indicator of alterations in cell wall biopolymer distribution and orientation as well as chemical alterations in cell walls (Ďurkovič et al 2013).…”