Evolution of the Cellulose Microfibril through Gamma-Valerolactone-Assisted Co-Solvent and Enzymatic Hydrolysis

Abstract

Biomass recalcitrance during deconstruction remains a key bottleneck to affordable biomass processing technologies. A clear connection between the cell wall structure and biomass deconstruction is necessary to understand how lignocellulosic material is broken down to valuable monomeric components. Here, we monitor changes in the cellulose microfibril domains of poplar, sorghum, and switchgrass throughout gamma-valerolactone (GVL)–water co-solvent pretreatment and enzymatic hydrolysis using solid-state 13C cross-polarization magic angle spinning nuclear magnetic resonance spectroscopy (CP/MAS 13C-NMR) and wide-angle X-ray scattering (WAXS). Spectral fitting of NMR peaks corresponding to different cellulose microenvironments at the C4 carbon center suggests that a mildly acidic GVL–water co-solvent pretreatment of poplar leads to nearly full removal of xylan–cellulose linkages, which primes the cellulose for enzymatic attack. The spectral fitting also suggests that the pretreatment causes significant depletion of the inaccessible fibril surface domains with an increase in more thermally stable crystalline resonances (Iβ). WAXS confirmed a decrease in the lattice spacing between (200) crystalline planes with increasing co-solvent pretreatment severity. These results are interpreted as an opening of bound microfibril surfaces previously inaccessible to the co-solvent system, which leaves behind a more thermally stable, crystalline domain that is potentially prone to relaxation and recrystallization. Full conversion of residual GVL-pretreated biomass was achieved after the GVL co-solvent pretreatment at 140 °C using a commercial enzyme cocktail, CTec2, which contains different cellulases and other enzymes. Spectral fitting of enzymatically hydrolyzed samples by a single engineered cellulase, CelR, suggests that the residual cellulose recalcitrance is mainly due to the inability of CelR to digest the Iβ crystalline domain present in pretreated samples. This work helps to provide new information regarding the structure of the cell wall and recalcitrance throughout GVL–water mild acidolysis and CelR enzymatic biomass deconstruction by tracking the evolution of structural domains within the cellulose microfibril. This work further directs recommendations for improving the conversion and sugar yields in future studies. Our findings inform inquiry into larger questions of cellulose recalcitrance through GVL pretreatment and CelR enzymatic hydrolysis and give insight into subsequent required steps for full cellulose conversion with attention to the most recalcitrant cellulose structures.

Contribution of calcium ligands in substrate binding and product release in the Acetovibrio thermocellus glycoside hydrolase family 9 cellulase CelR

“…Our work has shown that AtCelR is effective as a single enzyme for hydrolysis of amorphous fraction of cellulose derived from gamma valerolactone-pretreated plant biomass ( 56 , 67 ), although the I β -like crystal form of cellulose produced by this pretreatment is resistant to hydrolysis by both AtCelR and commercial enzyme preparations ( 68 ). AtCelR cannot hydrolyze xylan, which limits its utility with less purified forms of plant biomass.…”

Recyclable Copper Indium Sulfide Quantum Dots for Photocatalytic Homocoupling of Arenediazonium Tetrafluoroborates

“…Besides, replacing the harmful organic solvents with nontoxic, less flammable, and safe reaction media is a key tool for green and sustainable chemical production. In this regard, γ-valerolactone (GVL) is easily synthesized from readily available and potentially renewable feedstocks and it has been used as an additive for food and as a green reaction medium in organic synthesis. − On the basis of the above considerations and with the intent to design highly performant catalysts for organic reactions, − herein, we presented our efforts to achieve the first photochemical synthesis of symmetrical biaryls by homocoupling of arenediazonium tetrafluoroborates using CuInS 2 QDs as the heterogeneous photocatalyst at room temperature in GVL (Scheme ).…”