Local Site Selectivity and Conformational Structures in the Glycosidic Bond Scission of Cellobiose
ABSTRACT Car-Parrinello molecular dynamics combined with metadynamics simulations were used to study the acid-catalyzed hydrolysis of cellobiose (CB) in aqueous solution. The hydrolysis was studied in two steps. Step 1 involves the proton transfer from solvent to CB and dissociation of the glycosidic bond to β-glucose and oxacarbenium ion species. Step 2 involves the formation of α-glucose from oxacarbenium and regeneration of the acid proton species. Step 1 is endothermic, while Step 2 is exothermic. The overall activation free energy of CB hydrolysis is 32.5 kcal mol(-1), and the overall reaction free energy is -5.9 kcal mol(-l), consistent with available experimental data. We observe that a stepwise mechanism generally described in the literature for Step 1 is not significantly favored relative to a concerted β-1,4' linkage dissociation process.
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ABSTRACT: This paper reports a systematic investigation on the primary decomposition mechanism and kinetics of cellobiose under hydrothermal conditions at 200–275 °C and a wide initial concentration range of 10–10,000 mg L–1. Isomerization of cellobiose to cellobiulose (glucosyl-fructose) and glucosyl-mannose dominates the primary reactions of cellobiose decomposition, contributing to 71–93% of cellobiose decomposition depending on reaction conditions. In contrast, cellobiose hydrolysis to glucose makes only limited contributions (6–27% depending on reaction conditions) to the primary decomposition of cellobiose. This indicates that hydroxyl ions have a more significant effect to catalyze the isomerization reactions to produce cellobiulose and glucosyl-mannose. The catalytic effect of hydronium ions is weak probably because of the high affinity of hydronium ions for water molecules, which reduces the availability of hydronium ions for catalyzing the hydrolysis reaction. At increased temperatures, the affinity of hydronium ions for water molecules decreases because of the weakened hydrogen bonds in water, leading to an increase in the selectivity of the acid-catalyzed hydrolysis reaction. A higher initial cellobiose concentration also promotes hydrolysis reaction due to the formation of acidic products at the early stage of cellobiose decomposition. As a result of the reduced molar ratio of ion product to cellobiose, the activation energies of both isomerization and hydrolysis reactions increase with an increase in initial concentration, leading to an increase in the apparent activation energy of cellobiose hydrothermal conversion.Industrial & Engineering Chemistry Research 08/2014; 53(38):14607–14616. DOI:10.1021/ie5027309 · 2.24 Impact Factor
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ABSTRACT: Lignocellulosic biomass is an alternate and renewable source of carbon. However, due to high oxygen content and diverse functionality, its conversion to fuels and chemicals is technologically challenging. Since physico-chemical characteristics of biomass and its derived components are very different from petroleum, fundamental understanding of their interactions with catalysts and solvents and of their behavior during thermochemical processing needs to be developed. In the present paper, we provide a perspective on how multiscale molecular modeling can assist in developing the science of biomass processing. The scope of this paper is limited to liquid phase catalytic and pyrolytic conversion of biomass. Car–Parrinello molecular dynamics (CPMD), a multiscale method that combines quantum mechanics and classical molecular dynamics and is an excellent choice to simulate biomass interactions in the condensed phase, is discussed. An overview of metadynamics, a method to accelerate CPMD dynamics, is also given. Revealing the chemistry of biomass pyrolysis, identifying liquid phase catalytic reaction mechanisms and developing a fundamental understanding of the role of solvents in biomass processing are the three main areas highlighted in this paper. Molecular modeling based investigations in these areas are reviewed and key findings are summarized. Limitations of the current approaches are discussed and the relevance of multiscale methods like CPMD and metadynamics is discussed. Potential studies that could implement multiscale molecular modeling methods to solve some of the challenging problems in developing biomass conversion technology are elaborated and an outlook is provided.Chemical Engineering Science 02/2015; 121:217-235. DOI:10.1016/j.ces.2014.08.019 · 2.61 Impact Factor
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ABSTRACT: A novel surfactant 3-(dodecylimino)butan-2-one-oxime (DMBO) was synthesized. The metallomicelle La(DMBO)2 was prepared and used as mimic β-glucosidase to catalyze the hydrolysis of cellobiose in weakly alkaline aqueous solution at relative low temperature (80-110ºC). This study indicated that the functional metallomicelle displayed effective catalytic activity for hydrolysis of cellobiose to monosaccharide (glucose, fructose and 1,6-anhydroglucose) and glucosyl-erythrose. The conversion of cellobiose and selectivity of monosaccharide could reach 38.5% and 71.1% for reaction 10h at pH9.0 and 95ºC, respectively. The possible reaction pathways of cellobiose hydrolysis were proposed and the catalysis reaction rate constant kcat and Michaelis constant Km for the cellobiose hydrolysis were calculated. The apparent activation energy (Ea=84.6kJ•mol-1) of cellobiose to monosaccharide was evaluated.RSC Advances 12/2014; 5(13). DOI:10.1039/C4RA14521F · 3.71 Impact Factor