Fluorinated cellobiose and maltose as stand-ins for energy surface calculations

To better understand computational predictions of disaccharide conformations, ϕ,ψ maps were constructed for two analogs in which all hydroxyl groups were replaced with fluorine atoms (F-cellobiose and F-maltose). These molecules do not permit hydrogen bonding but should give better steric representation than analogs in which hydrogen atoms replaced exo-cyclic groups. Hartree Fock and B3LYP density functional quantum mechanics (QM) theory were used. The preferred ring shape for fluorinated glucose depends on the level of QM theory, but over the limited ϕ,ψ space that was studied, the rings remained in the ⁴C₁ form. Also, fluorine atoms are remote enough that they do not affect the torsional energies for the glycosidic bonds. F-Cellobiose maps were predictive of the conformations in crystals, but F-maltose maps were less so. The QM F-cellobiose map and an MM4::QM hybrid map for cellobiose itself were similar. However, the hybrid maltose map had many more experimental conformations within its 2-kcal/mol contour than did the QM F-maltose map. The apparent mean strength of an intra-molecular, inter-residue hydrogen bond is about 3 kcal/mol, based on the energy for many of the hydrogen bonded maltose structures on the F-maltose map. The F-maltose map was similar to a new QM map for an analog of maltose in which all hydroxyl groups were replaced with hydrogen atoms.     

Glucose,not cellobiose,is the repeating unit of cellulose and why that is important

Despite nomenclature conventions of the International Union of Pure and Applied Chemistry and the International Union of Biochemistry and Molecular Biology, the repeating unit of cellulose is often said to be cellobiose instead of glucose. This review covers arguments regarding the repeating unit in cellulose molecules and crystals based on biosynthesis, shape, crystallographic symmetry, and linkage position. It is concluded that there is no good reason to disagree with the official nomenclature. Statements that cellobiose is the repeating unit add confusion and limit thinking on the range of possible shapes of cellulose. Other frequent flaws in drawings with cellobiose as the repeating unit include incorporation of O-1 as the linkage oxygen atom instead of O-4 (the O-1 hydroxyl is the leaving group in glycoside synthesis). Also, n often erroneously represents the number of cellobiose units when n should denote the degree of polymerization i.e., the number of glucose residues in the polysaccharide.     

Young’s modulus calculations for cellulose I<Subscript>β</Subscript> by MM3 and quantum mechanics

Quantum mechanics (QM) and molecular mechanics (MM) calculations were performed to elucidate Young’s moduli for a series of cellulose I_β models. Computations using the second generation empirical force field MM3 with a disaccharide cellulose model, 1,4′-O-dimethyl-β-cellobioside (DMCB), and an analogue, 2,3,6,2′,3′,6′-hexadeoxy-1,4′-O-dimethyl-β-cellobioside (DODMCB), that cannot make hydrogen bonds reveal a considerable contribution of intramolecular hydrogen bonding to the molecular stiffness of cellulose I_β; the moduli for DMCB and DODMCB being 85.2 and 37.6 GPa, respectively. QM calculations confirm this contribution with modulus values of 99.7 GPa for DMCB and 33.0 GPa for DODMCB. However, modulus values for DMCB were considerably lower than values previously reported for cellulose I_β. MM calculations with extended cellulose chains (10–40 glucose units) resulted in modulus values, 126.0–147.5 GPa, more akin to the values reported for cellulose I_β. Comparison of the cellodecaose model, 1,4′-O-dimethyl-β-cellodecaoside (DMCD), modulus with that of its hydrogen bonding-deficient analogue, 2,3,6,2′,3′,6′-hexadeoxy-1,4′-O-dimethyl-β-cellodecaoside (DODMCD), corroborates the observed stiffness conferred by intramolecular hydrogen bonds; the moduli for DMCD and DODMCD being 126.0 and 63.3 GPa, respectively. Additional MM3 determinations revealed that modulus values were not strongly affected by intermolecular hydrogen bonding, with multiple strand models providing values similar to the single strand models; 87.5 GPa for a 7-strand DMCB model and 129.5 GPa for a 7 strand DMCD model.     

Optimization of Growth Medium and Enzyme Assay Conditions for Crude Cellulases Produced by a Novel Thermophilic and Cellulolytic Bacterium, Anoxybacillus sp. 527

A newly isolated Anoxybacillus sp. 527 was found to grow on crystalline cellulose as sole carbon and energy sources. Cellulases secreted by strain 527 were better induced by cellobiose, followed by glucose, lactose, sucrose, and cellulose. Cellulase secretion was enhanced by an optimized medium. Cellulase activity was increased by the addition of Ca²⁺ and NH₄⁺ and achieved maximum as 7.0 FPU ml⁻¹ at 70 °C and pH 6.0. Even at 100 °C, the enzymes were still active, which implies their potential application in large-scale cellulose conversion process.     

Cellulose Hydrolysis by Cellobiohydrolase Cel7A Shows Mixed Hyperbolic Product Inhibition

In order to establish which are the contribution of linear (total), hyperbolic (partial) or parabolic inhibitions by cellobiose, and also a special case of substrate inhibition, the kinetics of cellobiohydrolase Cel7A obtained from Trichoderma reesei was investigated. Values of kinetic parameters were estimated employing integrated forms of Michaelis–Menten equations through the use of non-linear regression, and criteria for selecting inhibition models are discussed. With cellobiose added at the beginning of the reaction, it was found that cellulose hydrolysis follows a kinetic model, which takes into account a mixed hyperbolic inhibition, by cellobiose with the following parameter values: K _m 5.0 mM, K _ic 0.029 mM, K _iu 1.1 mM, k _cat 3.6 h⁻¹ and k _cat′ 0.2 h⁻¹. Cellulose hydrolysis without initial cellobiose added also follows the same inhibition model with similar values (4.7, 0.029 and 1.5 mM and 3.2 and 0.2 h⁻¹, respectively). According to Akaike information criterion, more complex models that take into account substrate and parabolic inhibitions do not increase the modulation performance of cellulose hydrolysis.     

Conformational analysis and molecular dynamics simulation of cellobiose and larger cellooligomers

The conformational behavior of cellobiose (D -glc-ß(1→4)-D -glc), cellotetraose, and cellooctaose was studied by a combination of energy minimization and molecular dynamics simulations in vacuo at 400 K. These diand oligosaccharide models have considerable flexibility and exhibit a variety of different motions in glycosidic and exocyclic torsions. The glycosidic ?, ψ torsions moved frequently between two local minima on the cellobiose energy surface in the region of known crystal structures. Transitions of the hydroxymethyl side chain were observed between gt,gg, and tg conformations accompanied by changes in intramolecular hydrogen bonding patterns. A reasonable fit to the experimental optical rotation and nuclear magnetic resonance vicinal coupling data of cellobiose in solution required a distribution of its conformations. The oligomers, although generally extended, assumed a more coiled or twisted shape than is observed in the crystalline state of cellulose and exhibited considerable backbone motion due to local ring rotations about the glycosidic bonds. Long-lived transitions to structures having torsion angles 180° from the major minima (ring flips) introduced kinks and bends into the tetramer and octamer. While the glucose rings of the structures remained primarily in the ⁴C₁ conformation, twist and boat structures were also observed in the tetramer and octamer structures. Reducing the simulation temperature to 300 K eliminated some of the transitions seen at 400 K. © 1993 John Wiley & Sons, Inc.     

Solvents effects on the mechanism of cellulose hydrolysis: A QM/MM study

This article reports a combined quantum mechanics/molecular mechanics (QM/MM) investigation on the acid hydrolysis of cellulose in water using two different models, cellobiose and a 40‐unit cellulose chain. The explicitly treated solvent molecules strongly influence the conformations, intramolecular hydrogen bonds, and exoanomeric effects in these models. As these features are largely responsible for the barrier to cellulose hydrolysis, the present QM/MM results for the pathways and reaction intermediates in water are expected to be more realistic than those from a former density functional theory (DFT) study with implicit solvent (CPCM). However, in a qualitative sense, there is reasonable agreement between the DFT/CPCM and QM/MM predictions for the reaction mechanism. Differences arise mainly from specific solute–solvent hydrogen bonds that are only captured by QM/MM and not by DFT/CPCM. © 2015 Wiley Periodicals, Inc.     

Free radicals and their electron spin relaxation in cellobiose. X-band and W-band ESR and electron spin echo studies

By use of 9.7 GHz and 94 GHz ESR spectra and electron spin echo (ESE)-detected spectra the six radical centres produced by γ-irradiation of cellobiose were identified. The radicals are localized on different carbon atoms. Use of high-frequency ESR spectra with computer resolution enhancement methods enabled unique radical identification and determination of g-factors and proton hyperfine splitting, A, with high accuracy. For radiation doses below 20 kGy three radicals dominate: on C₁ with isotropic doublet A = 1.8 mT; on C₂, C₃ and C₄ with triplet A = 2.9 mT; and localized on CH₂ with anisotropic triplet. For doses above 100 kGy the radical on C₁ dominates, because of cleavage of the glycosidic bonds. Electron spin–lattice relaxation shows that radiation damage of the cellulose structure around the radical centres is significant and radical molecules do not participate in phonon dynamics of the host lattice. The relaxation is because of tunnelling motions of the ring or OH-groups, with tunnelling splitting 2.4 cm⁻¹. Electron spin echo dephasing results identify cellobiose ring torsions with activation energy 117 cm⁻¹.     

Unusual asymmetry in halobenzenes,a solid-state,gas-phase and theoretical investigation

The molecular structures of 1-Br-4-F-C₆H₄ and 1-Cl-4-F-C₆H₄ have been studied in the gas phase using gas electron diffraction (GED) and ab initio methods. The structure of 1-Cl,4-F-C₆H₄ in the crystalline phase has also been studied, but whilst the gaseous structures were found to possess C _2v symmetry, the solid-state structure was found to be quite distorted, with three molecules in the asymmetric unit. These fragments only possess C _s symmetry in the plane of the molecules, as opposed to the C _2v symmetry observed in the gas phase. The bonding motifs within the solid-state structure are very unusual and unexpected, with quite different C–F bond lengths for the three moieties, and are a result of weak hydrogen-halogen interactions within the structure.     

Water solvent effects using continuum and discrete models: The nitromethane molecule,CH3NO2

The first three valence transitions of the two nitromethane conformers (CH₃NO₂) are two dark n → π* transitions and a very intense π → π* transition. In this work, these transitions in gas‐phase and solvated in water of both conformers were investigated theoretically. The polarizable continuum model (PCM), two conductor‐like screening (COSMO) models, and the discrete sequential quantum mechanics/molecular mechanics (S‐QM/MM) method were used to describe the solvation effect on the electronic spectra. Time dependent density functional theory (TDDFT), configuration interaction including all single substitutions and perturbed double excitations (CIS(D)), the symmetry‐adapted‐cluster CI (SAC‐CI), the multistate complete active space second order perturbation theory (CASPT2), and the algebraic‐diagrammatic construction (ADC(2)) electronic structure methods were used. Gas‐phase CASPT2, SAC‐CI, and ADC(2) results are in very good agreement with published experimental and theoretical spectra. Among the continuum models, PCM combined either with CASPT2, SAC‐CI, or B3LYP provided good agreement with available experimental data. COSMO combined with ADC(2) described the overall trends of the transition energy shifts. The effect of increasing the number of explicit water molecules in the S‐QM/MM approach was discussed and the formation of hydrogen bonds was clearly established. By including explicitly 24 water molecules corresponding to the complete first solvation shell in the S‐QM/MM approach, the ADC(2) method gives more accurate results as compared to the TDDFT approach and with similar computational demands. The ADC(2) with S‐QM/MM model is, therefore, the best compromise for accurate solvent calculations in a polar environment. © 2015 Wiley Periodicals, Inc.     

The role of salt on cellulose dissolution in ethylene diamine/salt solvent systems

Ethylene diamine (EDA)/salt solvent systems can dissolve cellulose without any pretreatment. A comparison of the electrical conductivity of different salts in EDA was made at 25 °C, and conductivity decreased in the order of KSCN>KI>NaSCN at the same molar concentration. Among the salts tested, potassium thiocyanate (KSCN) was capable of dissolving both high molecular weight (DP>1000) and low molecular weight (DP = 210) cellulose, and this was confirmed by polarized light microscopy. ³⁹K and ¹⁴N NMR experiments were conducted at 70 °C as a function of cellobiose concentration with EDA/KSCN as the solvent. The results showed that the K⁺ ion interacts with cellobiose more than the SCN⁻ ion does. Recovered cellulose was studied by infrared spectroscopy (FTIR) and wide angle X-ray diffraction (WAXD). Changes in the FTIR absorption bands at 1,430 and 1,317 cm⁻¹ were associated with a change in the conformation of the C-6CH₂OH group. The changes in positions and/or intensities of absorption bands at 2,900, 1,163, and 8,97cm⁻¹ were related to the breaking of hydrogen bonds in cellulose. X-ray diffraction studies revealed that cellulose, recovered by precipitating cellulose solutions with water, underwent a polymorphic transformation from cellulose I to cellulose II.     

Cellulose wet wiper sheets prepared with cationic polymer and carboxymethyl cellulose using a papermaking technique

Carboxymethyl cellulose (CMC)-rich cellulose sheets were prepared with a cationic retention aid, poly[N,N,N-trimethyl-N-(2-methacryloxyethyl)ammonium chloride] (PTMMAC), using a papermaking technique. When 5% PTMMAC and 5% CMC were added to cellulose slurries, approximately 94% of the polymers were retained in the sheets by formation of polyion complexes between the two polymers. When the PTMMAC/CMC/cellulose sheets were soaked in solutions consisting of ethanol, water and calcium chloride (EtOH/H₂O/CaCl₂) with a weight ratio of 75:24:1, almost all PTMMAC and CMC molecules remained in the sheets, forming the structures of PTMMAC-N⁺Cl⁻ and CMC-COO⁻Ca²⁺Cl⁻ without dissolution of these molecules in the soaking solution. Thus, PTMMAC, CMC and calcium contents in the sheets were able to be determined on the basis of these PTMMAC and CMC structures from analytical data such as nitrogen, calcium and chlorine contents. The trade-off properties between sufficient wet strength in use and water-disintegrability after use can be added to the PTMMAC/CMC/cellulose sheets by selecting weight ratios of the EtOH/H₂O/CaCl₂ solution used as the impregnation liquid.     

A vibrational molecular force field of model compounds with biological interest. IV. Parameters for the different glycosidic linkages of oligosaccharides

The force field previously obtained for both anomers of glucose has been applied to six disaccharides that are molecules of _D-glucopyranosyl residues. These six disaccharides have different types of glycosidic linkages—that is, α, α trehalose dihydrate ( 1-1 ), sophorose monohydrate (β, 1-2 ), laminarabiose (β, 1-3 ), maltose monohydrate (α, 1-4 ) and cellobiose (β, 1-4 ), and gentiobiose (β, 1-6 ). From a careful analysis of the infrared and Raman spectra and from harmonic dynamics calculations in the crystalline state, the results show the reliability and the transferability of the set of parameters previously obtained for different carbohydrates. Below 1500 cm^?1, observed data and the corresponding calculated frequencies agreed within 5 cm^?1 for each of the six disaccharides. The vibrational density of states are well reproduced by these calculations for each molecule, particularly for the fingerprint regions. Moreover, as found by other workers who used sophisticated potential energy functions, no additional terms are needed to express the exoanomeric effect. Specific force constants characteristic of each glycosidic linkage have been derived, particularly for the glycosidic angle bending. More interesting are the values of the internal rotation barriers. It is shown that they are of the same size for both sides of the glycosidic linkage: VC₁O₁ = VO₁C_x′ = 3.29 kcal/mol for an alpha residue and 2.64 kcal/mol for a beta unit (x = 1–6 depends on the position of the glycosidic linkages of the considered disaccharide). © 1995 by John Wiley & Sons, Inc.     

Synthesis and thermal properties of poly(methyl methacrylate)-graft-(cellobiosylamine-C15)

Model experiments for synthesis of a comb-shaped copolymer with cellulose side-chains were performed with cellobiose derivatives. A novel cellobiose monomer, N-(15-methacryloyloxypentadecanoyl)-2,3,6-tri-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-β-d-glucopyranosylamine (2) was prepared from heptaacetylcellobiosyl- amine. Homopolymerization of cellobiose monomer 2 and copolymerization of monomer 2 with methyl methacrylate (MMA) were performed using 2,2′-azobis(isobutyronitrile) (AIBN) as an initiator to obtain homopolymers 3-i (i = 1–4) and copolymers 3-i (i = 5–7), poly(methyl methacrylate)-graft-(heptaacetylcellobiosylamine-C15). The size exclusion chromatography—multi-angle laser light scattering (SEC-MALS) measurements revealed that comb-shaped homopolymers 3-i (i = 1–4) had more compact structures compared to copolymers 3-i (i = 5–7) at the same elution volume. Selective deacetylation of polymers 3-i (i = 1–7) gave novel cellobiose polymers 4-i (i = 1–7), poly(methyl methacrylate)-graft-(cellobiosylamine-C15). The amide linkages between cellobiose moiety and long-chain alkyl group, and the ester linkages between PMMA main-chain and long-chain alkyl group remained after deprotection. The differential scanning calorimetry (DSC) measurements revealed that the T _gs of the polymers 4-i (i = 1, 5, 6, 7) increased with increasing cellobiose composition in the polymers. It was indicated that cellobiose moieties of polymers 4-i (i = 1, 5, 6, 7) reduced the mobility of PMMA main-chain.     

BIODEGRADATION OF REGENERATED CELLULOSE FILMS BY FUNGI

The biodegradability of Aspergillus niger (A. niger), Mucor (M-305) and Trichoderma(T-311) strains on regenerated cellulose films in media was investigated. The results showedthat T-311 strain isolated from soil adhered on the cellulose film fragments has strongerdegradation effect on the cellulose film than A. niger strain. The weights, molecular weightsand tensile strengths of the cellulose films in both shake culture and solid media decreasedwith incubation time, accompanied by producing CO_2 and saccharides. HPLC, IR and re-leased CO_2 analysis indicated that the biodegradation products of the regenerated cellulosefilms mainly contain oligosaccharides, cellobiose, glucose, arabinose, erythrose, glycerose,glycerol, ethanal, formaldehyde and organic acid, the end products were CO_2 and water.After a month, the films were completely decomposed by fungi in the media at 30℃.     

A Multistage Process to Enhance Cellobiose Production from Cellulosic Materials

Cellobiose, a disaccharide, is a valuable product that can be obtained from cellulose hydrolysis. In this study, a simple methodology is presented to enhance the production and improve the selectivity of cellobiose during enzymatic hydrolysis of cellulose. The approach consisted of a multistage removal of filtrate via vacuum filtration and resuspension of the retentate. By this process, the remaining solid was further hydrolyzed without additional enzyme loading. Compared to the continuous hydrolysis process, the production of cellobiose increased by 45%. Increased selectivity of cellobiose is due to the loss of β-glucosidases in the filtrate, while enhanced productivity is likely due to mitigated product inhibition.     

Selectivity of vibronic coupling in complex molecules with benzene fragments

This paper deals with the mechanisms of localization of Franck-Condon vibronic coupling of πσ^*- or πlπ^*-orbital type in a few vibrational modes, (LVM) in excited electronic states of polyatomic molecules. The analysis of vibronic coupling uses highly symmetric basis sets (for representing MO structures and normal coordinates ξ_R) as well as simplified models that relate the shift Δ_R of the electron potential minima along the normal coordinates to the MO structure and to ξ_R in the form of analytical expressions. The modes that are active in LVM are determined from the experimental luminescence spectra. These ideas about approximately high local symmetry of vibronic coupling in benzene fragments as well as the estimates of Δ_R depending on variations in the MO structure explain the experimental results. L. Ya. Karpov Physicochemical Scientific Research Institute. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 286–291, March–April, 1995. Translated by L. Smolina     

Theoretical simulation of CdTe nanocrystals in aqueous synthesis

At the B3LYP/LANL2DZ theoretical level, Cd₃Te₃, (Cd₃Te₃)₂, (Cd₃Te₃)₃, Cd₄Te₄, and Te–Cd–ligand clusters were optimized. Firstly, hexagon Cd₃Te₃ and tetrahedron Cd₄Te₄ structures (with T_D symmetry) may be the minimum units of CdTe nanocrystals. They have similar conformations with the experimental wurtzite and zinc blende structures, respectively. Secondly, the frequencies of calculated Raman peaks of four clusters appear in about 140 cm⁻¹, which is close to the experimental data. Following, analysis of Te–Cd–ligand molecules elucidates that all our ligands have similar effect to CdTe structure, because the main influence of ligands comes from thiol, which is also the result of experiment. Finally, considering the influence of solvent and ligand, we believe that our wavelengths of absorption peaks which are calculated using the time-dependent density functional theory are perfectly identical with those of CdTe nanocrystals, according to quantum size effect. Moreover, we have testified that all these absorption peaks are the transition from d to p orbitals.     

Synthesis of 1‐C‐Glycoside‐Linked Lipid II Analogues Toward Bacterial Transglycosylase Inhibition

Preparation of Lipid II analogues containing an enzymatically uncleavable 1‐C‐glycoside linkage between the disaccharide moiety and the pyrophosphate‐ or pyrophosphonate‐lipid moiety is described. The synthesis of a common 1‐C‐vinyl disaccharide intermediate has been developed that allows easy preparation of both an elongated sugar‐phosphate bond and a sugar‐phosphonate moiety, which are coupled with the polyprenyl phosphate to give the desired molecules. Inhibition studies show how a subtle structural modification results in dramatically different potency toward bacterial transglycosylase (TGase), and the results identify Lipid II‐C‐O‐PP (IC₅₀=25 μM ) as a potential TGase inhibitor.