On hydrogen bonding in 1,6-anhydro-beta-D-glucopyranose (levoglucosan): X-ray and neutron diffraction and DFT study

Cellulose Fast Pyrolysis Activated by Intramolecular Hydrogen Bonds

The conversion of inedible biomass by fast pyrolysis is a promising route for sustainable production of renewable fuels and value-added chemicals, but low selectivity toward desired products hampers its economic viability. Understanding the molecular-level reaction pathways of biomass fast pyrolysis could be the key to overcoming this challenge. However, the effects of intramolecular and interchain hydrogen bonds near the reaction center have not been thoroughly explored. In this work, the reaction pathways and kinetics of fast pyrolysis of cellulose, a major component of biomass, were investigated using the density functional theory. A new intramolecular hydroxyl-activated mechanism is presented for cellulose activation. Our calculations incorporating noncovalent interactions accurately captured the activation energy of 50.8 kcal mol^-1, agreeable with the apparent activation energy measured experimentally. The findings of cellulose pyrolysis provide insights into the investigation of interactions during real-life biomass pyrolysis.

Quantitative study of the pyrolysis of levoglucosan to generate small molecular gases

In this paper, we studied 23 possible reaction paths for levoglucosan pyrolysis to generate small molecular gases and 51 compounds (including reactants, intermediates, and products), and quantified the 47 transition states involved in the pathway.

Identification, functional characterization, and crystal structure determination of bacterial levoglucosan dehydrogenase

Levoglucosan is the 1,6-anhydrosugar of d-glucose formed by pyrolysis of glucans and is found in the environment and industrial waste. Two types of microbial levoglucosan metabolic pathways are known. Although the eukaryotic pathway involving levoglucosan kinase has been well-studied, the bacterial pathway involving levoglucosan dehydrogenase (LGDH) has not been well-investigated. Here, we identified and cloned the lgdh gene from the bacterium Pseudarthrobacter phenanthrenivorans and characterized the recombinant protein. The enzyme exhibited high substrate specificity toward levoglucosan and NAD+ for the oxidative reaction and was confirmed to be LGDH. LGDH also showed weak activities (∼4%) toward l-sorbose and 1,5-anhydro-d-glucitol. The reverse (reductive) reaction using 3-keto-levoglucosan and NADH exhibited significantly lower Km and higher kcat values than those of the forward reaction. The crystal structures of LGDH in the apo and complex forms with NADH, NADH + levoglucosan, and NADH + l-sorbose revealed that LGDH has a typical fold of Gfo/Idh/MocA family proteins, similar to those of scyllo-inositol dehydrogenase, aldose-aldose oxidoreductase, 1,5-anhydro-d-fructose reductase, and glucose-fructose oxidoreductase. The crystal structures also disclosed that the active site of LGDH is distinct from those of these enzymes. The LGDH active site extensively recognized the levoglucosan molecule with six hydrogen bonds, and the C3 atom of levoglucosan was closely located to the C4 atom of NADH nicotinamide. Our study is the first molecular characterization of LGDH, providing evidence for C3-specific oxidation and representing a starting point for future biotechnological use of LGDH and levoglucosan-metabolizing bacteria.

High-pressure dielectric studies on 1,6-anhydro-β-D-mannopyranose (plastic crystal) and 2,3,4-tri-O-acetyl-1,6-anhydro-β-D-glucopyranose (canonical glass)

Broadband Dielectric Spectroscopy was applied to investigate molecular dynamics of two anhydrosaccharides, i.e., 1,6-anhydro-β-D-mannopyranose, anhMAN (hydrogen-bonded system) and 2,3,4-tri-O-acetyl-1,6-anhydro-β-D-glucopyranose, ac-anhGLU (van der Waals material), at different thermodynamic conditions. Moreover, the reported data were compared with those recently published for two other H-bonded systems, i.e., 1,6-anhydro-β-D-glucopyranose (anhGLU) and D-glucose (D-GLU). A direct comparison of the dynamical behavior of the materials with a similar chemical structure but significantly differing by the degrees of freedom, complexity, and intermolecular interactions made it possible to probe the impact of compression on the fragility, Temperature-Pressure Superpositioning and pressure coefficient of the glassy crystal/glass transition temperatures (dT_gc/dp ; dT_g/dp). Moreover, the correlation between dT_gc/dp determined experimentally from the high-pressure dielectric data and the Ehrenfest equation has been tested for the plastic crystals (anhGLU and anhMAN) for the first time. Interestingly, a satisfactory agreement was found between both approaches. It is a quite intriguing finding which can be rationalized by the fact that the studied materials are characterized by the low complexity (lower degrees of freedom with respect to the molecular mobility) as well as ordered internal structure. Therefore, one can speculate that in contrast to the ordinary glasses the dynamics of the plastic crystals might be described with the use of a single order parameter. However, to confirm this thesis further, pressure-volume-temperature (PVT) experiments enabling calculations of the Prigogine Defay ratio are required.

Molecular interactions in nanocellulose assembly

The contribution of hydrogen bonds and the London dispersion force in the cohesion of cellulose is discussed in the light of the structure, spectroscopic data, empirical molecular-modelling parameters and thermodynamics data of analogue molecules. The hydrogen bond of cellulose is mainly electrostatic, and the stabilization energy in cellulose for each hydrogen bond is estimated to be between 17 and 30 kJ mol^-1 On average, hydroxyl groups of cellulose form hydrogen bonds comparable to those of other simple alcohols. The London dispersion interaction may be estimated from empirical attraction terms in molecular modelling by simple integration over all components. Although this interaction extends to relatively large distances in colloidal systems, the short-range interaction is dominant for the cohesion of cellulose and is equivalent to a compression of 3 GPa. Trends of heat of vaporization of alkyl alcohols and alkanes suggests a stabilization by such hydroxyl group hydrogen bonding to be of the order of 24 kJ mol^-1, whereas the London dispersion force contributes about 0.41 kJ mol^-1 Da^-1 The simple arithmetic sum of the energy is consistent with the experimental enthalpy of sublimation of small sugars, where the main part of the cohesive energy comes from hydrogen bonds. For cellulose, because of the reduced number of hydroxyl groups, the London dispersion force provides the main contribution to intermolecular cohesion.This article is part of a discussion meeting issue 'New horizons for cellulose nanotechnology'.

High pressure dielectric studies on the structural and orientational glass

High pressure dielectric studies on the H-bonded liquid D-glucose and Orientationally Disordered Crystal (ODIC) 1,6-anhydro-D-glucose (levoglucosan) were carried out. It was shown that in both compounds, the structural relaxation is weakly sensitive to compression. It is well reflected in the low pressure coefficient of the glass transition and orientational glass transition temperatures which is equal to 60 K/GPa for both D-glucose and 1,6-anhydro-D-glucose. Although it should be noted that ∂Tg(0)/∂p evaluated for the latter compound seems to be enormously high with respect to other systems forming ODIC phase. We also found that the shape of the α-loss peak stays constant for the given relaxation time independently on the thermodynamic condition. Consequently, the Time Temperature Pressure (TTP) rule is satisfied. This experimental finding seems to be quite intriguing since the TTP rule was shown to work well in the van der Waals liquids, while in the strongly associating compounds, it is very often violated. We have also demonstrated that the sensitivity of the structural relaxation process to the temperature change measured by the steepness index (mp) drops with pressure. Interestingly, this change is much more significant in the case of D-glucose with respect to levoglucosan, where the fragility changes only slightly with compression. Finally, kinetics of ODIC-crystal phase transition was studied at high compression. It is worth mentioning that in the recent paper, Tombari and Johari [J. Chem. Phys. 142, 104501 (2015)] have shown that ODIC phase in 1,6-anhydro-D-glucose is stable in the wide range of temperatures and there is no tendency to form more ordered phase at ambient pressure. On the other hand, our isochronal measurements performed at varying thermodynamic conditions indicated unquestionably that the application of pressure favors solid (ODIC)-solid (crystal) transition in 1,6-anhydro-D-glucose. This result mimics the impact of pressure on the crystallization of fully disordered supercooled van der Waals liquids.

Redetermination of the borax structure from laboratory X-ray data at 145 K

The title compound, sodium tetraborate decahydrate (mineral name: borax), Na₂[B₄O₅(OH)₄]·8H₂O, has been studied previously using X-ray [Morimoto (1956). Miner. J.2, 1–18] and neutron [Levy & Lisensky (1978). Acta Cryst. B34, 3502–3510] diffraction data. The structure contains tetra­borate anions [B₄O₅(OH)₄]²⁻ with twofold rotation symmetry, which form hydrogen-bonded chains, and [Na(H₂O)₆] octa­hedra that form zigzag chains [Na(H₂O)_4/2(H₂O)_2/1]. The O—H bond distances obtained from the present redetermination at 145 K are shorter than those in the neutron study by an average of 0.127 (19) Å.

Raman spectrum of [Ru(CNBu t)(CO)(eta2-C6H4-2-CHO)(PPh3)2][BF4].2CDCl3 shows that the crystallographically determined bifurcated hydrogen-bonding interaction Cl3CD...F2BF2- is an example of a blue-shifting hydrogen bond

Raman data suggest that a crystallographically determined Cl3CD...F2BF2- interaction in the solid-state structure of [Ru(CNBut)(CO)(eta2-C6H4-2-CHO)(PPh3)2][BF4].2CDCl3 is an example of a blue-shifting bifurcated hydrogen bond. The nu(C-D) band blue-shifts 5 cm-1 to 2269 cm-1 compared to 2264 cm-1 for CDCl3 in the gas phase and 20 cm-1 from frozen CDCl3 at 2249 cm-1. A conventional interpretation of these band shifts would suggest that the CCl2 fragment of DCCl3 is a stronger hydrogen-bond acceptor than the BF2 fragment of a BF4- group.