Effect of oxidation on cellulose and water structure: a molecular dynamics simulation study

Sathish Kumar Mudedla,Kaisa Marjamaa,Suvi Arola,Markus B Linder,Elias Veijola,Anu Koivula,Maisa Vuorte,Maria Sammalkorpi

doi:10.1007/s10570-021-03751-8

Abstract

Enzymatic cleavage of glycocidic bonds is an important, green and biocompatible means to refine lignocellulosic biomass. Here, the effect of the resulting oxidation point defects on the structural and water interactions of crystalline cellulose {100} surface are explored using classical molecular dynamics simulations. We show that even single oxidations reduce the connections within cellulose crystal significantly, mostly via local interactions between the chains along the surface plane but also via the oxidation defects changing the structure of the crystal in direction perpendicular to the surface. Hydrogen bonding on the surface plane of cellulose is analyzed to identify onset of desorption of glucose chains, and the desorption probed. To assess the actual soluble product profile and their fractions resulting from lytic polysaccharide monooxygenase (LPMO) enzyme oxidation on real cellulose crystal samples, we employ High-Performance Anion-Exchange Chromatography with Pulsed Amperometric-Detection (HPAEC-PAD) technique. The findings demonstrate the LPMO oxidation results in soluble glucose fragments ranging from 2 to 8 glucose units in length. Additionally, significantly more oxidized oligosaccharides were released in LPMO treatment of AaltoCell than Avicel, the two studied microcrystalline cellulose species. This is likely to result from the large reactive surface area preserved in AaltoCell due to manufacturing process. Furthermore, as can be expected, the oxidation defects at the surfaces lead to the surfaces binding a larger amount of water both via direct influence by the defect but also the defect induced protrusions and fluctuations of the glucose chain. We quantify the enhancement of water interactions of cellulose crystals due to the oxidation defects, even when no desorption takes place. The molecular simulations indicate that the effect is most pronounced for the C1-acid oxidation (carboxylic acid formation) but present also for the other defects resulting from oxidation. The findings bear significance in understanding the effects of enzymatic oxidation on cellulose nanocrystals, the difference between cellulose species, and cleavage of soluble products from the cellulosic material.

Highlights

Cellulose is the most abundant biopolymer on Earth and a key component of e.g. plant biomass which provides the key staple of fermentable sugars for biofuel production and biotechnological products e.g. in polymer, cosmetics and chemical industries (Khalil et al 2012; Cao et al 2009; Lindman et al 2010; Zimmermann et al 2010; Kontturi et al 2006)
The structural and water interaction changes of crystalline cellulose {100} surface due to oxidation point defects and the glucose fragment desorption resulting from oxidation point defects at close separations were explored using classical molecular dynamics simulations
The cellooligosaccharides solubilization findings were corroborated with experimental characterization of lytic polysaccharide monooxygenase (LPMO) oxidation products on Avicel and AaltoCell substrates

Summary

Introduction

Cellulose is the most abundant biopolymer on Earth and a key component of e.g. plant biomass which provides the key staple of fermentable sugars for biofuel production and biotechnological products e.g. in polymer, cosmetics and chemical industries (Khalil et al 2012; Cao et al 2009; Lindman et al 2010; Zimmermann et al 2010; Kontturi et al 2006). Most of the applications and chemical products require conversion of pulp into soluble cellulosic derivatives or even monomeric sugars (Kontturi et al 2006): the most widespread method to obtain these is via cellulase conversion in which several enzymes cooperatively cleave and hydrolyze cellulose to glucose sugars (Paliwal et al 2012; Juergensen et al 2012; Bhat and Bhat 1997; Ward and Moo-Young 1989; Himmel et al 2007; Horn et al 2012).

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