Density Functional Study of the Optical Rotation of Glucose in Aqueous Solution

Computational NMR of carbohydrates: 1. Glucopyranoses

A high-level calculation of ¹ H and ¹³ C NMR chemical shifts of α- and β-d-glucopyranoses is carried out at the DFT level with taking into account their conformational composition to reveal the most effective computational protocols. A number of dedicated DFT functionals in combination with Jensen's pcS-n (n = 0-4) family of basis sets were applied to evaluate the most reliable combination. It was found that BHandHLYP/pcS-2 provided the most accurate and reliable computational protocol. Based on the performed calculations, the established computational protocol is generally recommended for the calculation of ¹ H and ¹³ C NMR chemical shifts of a wide series of carbohydrates.

The melting properties of D-α-glucose, D-β-fructose, D-sucrose, D-α-galactose, and D-α-xylose and their solubility in water: A revision

Saccharides are still commonly isolated from biological feedstock by crystallization from aqueous solutions. Precise thermodynamic data on solubility are essential to optimize the downstream crystallization process. Solubility modeling, in turn, requires knowledge of melting properties. In the first part of this work, following our previous work on amino acids and peptides, D-α-glucose, D-β-fructose, D-sucrose, D-α-galactose, and D-α-xylose were investigated with Fast Scanning Calorimetry (FSC) in a wide scanning rate range (2000 K·s−1 to 10000 K·s−1). Using the experimental melting properties of saccharides from FSC allowed successfully modeling aqueous solubility for D-sucrose and D-α-galactose with the equation of state PC-SAFT. This provides cross-validation of the measurement methods to determine accurate experimental melting properties with FSC. Unexpectedly, the experimental FSC melting temperatures, extrapolated to zero scanning rates for thermal lag correction, were higher than results determined with DSC and available literature data. To clarify this inconsistency, FSC measurements towards low scanning rates from 10000 K·s−1 to 1 K·s−1 (D-α-glucose, D-β-fructose, D-sucrose) overlapping with the scanning rates of DSC and literature data were combined. At scanning rates below 1000 K·s−1, the melting properties followed a consistent non-linear trend, observed in both the FSC and the literature data. In order to understand the non-linear decrease of apparent melting temperatures with decreasing heating rate, the endothermic peaks were investigated in terms of isoconversional kinetics. The activation energies in the non-linear dependency region are in the range of $$300<{E}_{A}< 600 {\text{kJ}}\bullet {\text{mo}}{\text{l}}^{-1}$$ 300 < E A < 600 kJ ∙ mol - 1 . These values are higher than the enthalpy of sublimation for D-α-glucose, indicating that the non-linear behavior does not have a physical nature but attributes to chemical processes corresponding to the decomposition of molecular compounds within the crystal lattice before melting. The melting properties reported in the literature, commonly determined with conventional methods such as DSC, lead to inaccurate results due to the decomposition of these biomolecules at low heating rates. In addition, the FSC results at lower scanning rates coincide with results from DSC and literature in the overlapping scanning rate range, further validating the accuracy of FSC measurements to determine reliable melting properties of thermally labile biomolecules. The experimental FSC melting properties determined at higher scanning rates are considered as the correct equilibrium melting properties, which are not influenced by any chemical processes. The combination of FSC and PC-SAFT opens the door to model solubility of solid compounds that commonly decompose before melting.

Online analysis of D-glucose and D-mannose aqueous mixtures using Raman spectroscopy: an in silico and experimental approach

Raman spectroscopy was applied to an aqueous solution containing D-mannose and D-glucose at a fixed dry matter content. The Raman measurement apparatus was adapted online at the industrial scale to monitor a bioprocess including an epimerization reaction. Online Raman spectroscopy and deconvolution techniques were successfully applied to monitor in real time the D-mannose and D-glucose concentrations using the Raman shifts at 960 cm-1 and 974 cm-1 respectively. The two anomeric forms, α and β of D-mannose in the pyranose conformation were quantified. In silico analysis of vibrational frequencies and Raman intensities of hydrated structure of D-mannose and D-glucose in the pyranose form for α and β anomers were carried out using a two-step procedure. First molecular dynamics was used to generate the theoretical carbohydrates' structures keeping the experimental dry matter content, then quantum mechanics was used to compute the Raman frequencies and intensities. Computed vibrational frequencies are in satisfactory agreement with the experimental spectra considering a hydration shell approach. Raman intensities are qualitatively in accordance with the experimental data. The interpretation of Raman frequencies and intensities led to acceptable results regarding the current possible structures of D-mannose and D-glucose in aqueous solution. Online Raman spectroscopy coupled with in silico approaches such as quantum mechanics and molecular dynamics methodology is proved to be a valuable tool to quantify the carbohydrates and stereoisomers content in complex aqueous mixtures. This methodology offers a new way to monitor any bioprocesses that encounter aqueous mixtures of D-glucose and D-mannose.

Automated Quantum Chemistry-Based Calculation of Optical Rotation for Large Flexible Molecules

The calculation of optical rotation (OR, [α]_D) for nonrigid molecules was limited to small systems due to the challenging problem of generating reliable conformer ensembles, calculating accurate Boltzmann populations and the extreme sensitivity of the OR to the molecules' three-dimensional structure. Herein, we describe and release the crenso workflow for the automated computation of conformer ensembles in solution and corresponding [α]_D values for flexible molecules. A comprehensive set of 28 organic drug molecules (28-144 atoms) with experimentally determined values is used in our assessment. In all cases, the correct OR sign is obtained with an overall mean relative deviation of 72% (mean absolute deviation of 82 °[dm(g/cm³)]^-1 for experimental values in the range -160 to 287 °[dm(g/cm³)]^-1). We show that routine [α]_D computations for very flexible, biologically active molecules are both feasible and reproducible in about a day of computation time on a standard workstation computer. Furthermore, we observed that the effect of energetically higher-lying structures in the ensemble on the OR is often averaged out and that in 23 out of 28 cases, the correct OR sign is obtained by just considering only the lowest free energy conformer. In four example cases, we show that the approach can also describe the OR of pairs of flexible diastereomers properly. In summary, even very sensitive, multifactorial physicochemical properties appear reliably predictable with minimal user input from efficiently automated quantum chemical methods.

Computational NMR of Carbohydrates: Theoretical Background, Applications, and Perspectives

This review is written amid a marked progress in the calculation of NMR parameters of carbohydrates substantiated by a vast amount of experimental data coming from several laboratories worldwide. By no means are we trying to cover in the present compilation a huge amount of all available data. The main idea of the present review was only to outline general trends and perspectives in this dynamically developing area on the background of a marked progress in theoretical and computational NMR. Presented material is arranged in three basic sections: (1)-a brief theoretical introduction; (2)-applications and perspectives in computational NMR of monosaccharides; and (3)-calculation of NMR chemical shifts and spin-spin coupling constants of di- and polysaccharides.

Mutarotation of aldoses: Getting a deeper knowledge of a classic equilibrium enabled by computational analyses

The mutarotation equilibrium, by which reducing carbohydrates exist in solution as the α and β anomers of cyclic (furanoid and pyranoid) structures, along with open-chain (aldehyde and hydrate) forms, and whose ratios are depending on factors such as temperature, pH and solvent, portraits a phenomenon involved in numerous processes of chemical and biological importance. Herein, we have developed a DFT-based rationale that provides a broader landscape for anomerizations and ring-open chain interconversions, together with the pivotal role exerted not only by the aldehyde intermediate (essentially the only acyclic structure taken into account so far), but also the hydrate form (often more abundant at the equilibrium). These calculations reveal a more complex and richer scenario than was thought, and identify different mutarotation mechanisms that hinge on every monosaccharide. It is noteworthy that pyranose-furanose interconversion may actually occur without the intermediacy of open-chain forms. For the aldoses evaluated, namely d-glucose, d-ribose, and d-xylose, all structures involved in mutarotation undergo interconversion pathways, whose energy barriers calculated at the M06-2X/6-311++G(d,p) level, are in good agreement with previous experimental measurements.

Peyssonnosides A-B, Unusual Diterpene Glycosides with a Sterically Encumbered Cyclopropane Motif: Structure Elucidation Using an Integrated Spectroscopic and Computational Workflow

Two sulfated diterpene glycosides featuring a highly substituted and sterically encumbered cyclopropane ring have been isolated from the marine red alga Peyssonnelia sp. Combination of a wide array of 2D NMR spectroscopic experiments, in a systematic structure elucidation workflow, revealed that peyssonnosides A-B (1-2) represent a new class of diterpene glycosides with a tetracyclo [7.5.0.01,10.05,9] tetradecane architecture. A salient feature of this workflow is the unique application of quantitative interproton distances obtained from the rotating frame Overhauser effect spectroscopy (ROESY) NMR experiment, wherein the β-d-glucose moiety of 1 was used as an internal probe to unequivocally determine the absolute configuration, which was also supported by optical rotatory dispersion (ORD). Peyssonnoside A (1) exhibited promising activity against liver stage Plasmodium berghei and moderate antimethicillin-resistant Staphylococcus aureus (MRSA) activity, with no cytotoxicity against human keratinocytes. Additionally, 1 showed strong growth inhibition of the marine fungus Dendryphiella salina indicating an antifungal ecological role in its natural environment. The high natural abundance and novel carbon skeleton of 1 suggests a rare terpene cyclase machinery, exemplifying the chemical diversity in this phylogenetically distinct marine red alga.

Rational Design of Ni(OH)2 Hollow Porous Architecture for High-Sensitivity Enzyme-Free Glucose Sensor

Ni(OH)₂ electrocatalysts have acquired lots of research attentions as ideal substitutes for noble metals. However, their electrocatalytic performance still cannot meet the demands for applications due to the difficulties in electron transfer and mass transport. According to kinetics principle, the construction of hollow structure is regarded as an effective method to achieve outstanding electrocatalytic performance. In this work, Ni(OH)₂ hollow porous architecture (Ni(OH)₂ HPA) was simply synthesized through a coordinating etching and precipitating (CEP) method for the building of enzymatic-free glucose sensors. Ni(OH)₂ HPA presents large specific surface area (SSA), ordered diffusion channels, and structure stability. As a detection electrode for glucose, Ni(OH)₂ HPA exhibits eminent electroactivity in terms of high sensitivity (1843 μA mM^-1 cm^-2), lower detection limit (0.23 μM), and short response time (1.4 s). The results demonstrate that Ni(OH)₂ HPA has practical applications for construction of enzymatic-free electrochemical sensors. The design of hollow structure also provides an effective engineering method for high-performance sensors.

Comparison of measured and predicted specific optical rotation in gas and solution phases: A test for the polarizable continuum model of solvation

A comparative theoretical and experimental study of dispersive optical activity is presented for a set of small, rigid organic molecules in gas and solution phases. Target species were chosen to facilitate wavelength-resolved measurements of specific rotation in rarefied vapors and in organic solvents having different polarities, while avoiding complications due to conformational flexibility. Calculations were performed with two density functionals (B3LYP and CAM-B3LYP) and with the coupled-cluster singles and doubles (CCSD) ansatz, and solvent effects were included through use of the polarizable continuum model (PCM). Across the various theoretical methods surveyed, CCSD with the modified velocity gauge provided the best overall performance for both isolated and solvated conditions. Zero-point vibrational corrections to equilibrium calculations of chiroptical response tended to improve agreement with gas-phase experiments, but the quality of performance realized for solutions varied markedly. Direct comparison of measured and predicted specific-rotation suggests that PCM, in general, is not able to reproduce attendant solvent shifts (neither between gas and solution phases nor among solvents) and fares better in estimating actual medium-dependent values of this property (although the error is rather system dependent). Thus, more elaborate solvation models seem necessary for a proper theoretical description of solvation in dispersive optical activity.

Catalyst and Process Design for the Continuous Manufacture of Rare Sugar Alcohols by Epimerization-Hydrogenation of Aldoses

Sugar alcohols are applied in the food, pharmaceutical, polymer, and fuel industries and are commonly obtained by reduction of the corresponding saccharides. In view of the rarity of some sugar substrates, epimerization of a readily available monosaccharide has been proposed as a solution, but an efficient catalytic system has not yet been identified. Herein, a molybdenum heteropolyacid-based catalyst is developed to transform glucose, arabinose, and xylose into less-abundant mannose, ribose, and lyxose, respectively. Adsorption of molybdic acid onto activated carbon followed by ion exchange to the cesium form limits leaching of the active phase, which greatly improves the catalyst stability over 24 h on stream. The hydrogenation of mixtures of epimers is studied over ruthenium catalysts, and it is found that the precursor to the desired polyol is advantageously converted with faster kinetics. This is explained by density functional theory on the basis of its more favorable adsorption on the metal surface and the lower energy barrier for the addition of a hydrogen atom to the primary carbon atom. Finally, different designs for a continuous process for the conversion of glucose into mannitol are studied, and it is uncovered that two reactors in series with one containing the epimerization catalyst and the other containing a mixture of the epimerization and hydrogenation catalysts increases the mannitol/sorbitol ratio to 1.5 from 1 for a single mixed-bed reactor. This opens a prospective route to the efficient valorization of renewables to added-value chemicals.

Simulation of Raman optical activity of multi-component monosaccharide samples

Determination of the saccharide structure in solution is a laborious process that can be significantly enhanced by chiral optical spectroscopies.

Specific rotation of monosaccharides: a global property bringing local information

Carbohydrates generally occur in several conformations that may differ among themselves by energy values that are smaller than the accuracy of the most sophisticated theoretical methods used to determine them. In addition, the preferential orientations of the hydroxyl groups of these molecules cannot be identified by any experimental technique. Therefore, a method that is able to validate the absolute conformations (i.e., consisting of the orientations of the hydroxyl groups) of carbohydrates would be helpful to improve our knowledge about monosaccharides. SR has been used for this purpose, and here, we present a test to measure the specific rotation (SR) ability of a molecule that possesses not only many conformations, but also four adjacent chiral centers. The results show that the final SR value is a weighted average of a global property (obtained for each conformation), and the latter by its turn is influenced by each chiral center in a multi chiral system. By comparing the SR values calculated for the most abundant anomers of xylopyranose with those of the corresponding monochiral analogs obtained by saturation of three different chiral centers each time, the influence of each center on the global property is confirmed.

First total synthesis of fuzanins C, D and their analogues as anticancer agents

Total synthesis of fuzanins C, D and their quinoline analogues has been accomplished from readily available starting materials. Synthesis of fuzanin D described here also serves to establish its absolute configuration. All compounds were screened for anticancer activity on four cancer cell lines. The quinoline analogues 4d, 4c, 3c are relatively more potent.

Polarimetry as a tool for the study of solutions of chiral solutes

Optical rotation of aqueous solutions of D-levoglucosan was studied experimentally in the 0.03-4.0 mol L(-1) concentration range and a nonlinear concentration dependence of specific optical rotation (SR) was revealed. Discontinuities observed in the concentration plot of SR (at 0.1, 0.3, 0.5, 1.0, and 2.0 mol L(-1)) are well correlated with those found by static and dynamic light scattering and identify concentration ranges in which different solution domains (supramers) may exist. The average SR experimental value for a D-levoglucosan aqueous solution ([α]D(28) -58.5±8.7 deg dm(-1) cm(-3) g(-1)) was found to be in good agreement with values obtained by theoretical calculation (TD-DFT/GIAO) of SR for 15 different conformers revealed by conformational sampling at the PCM/B3LYP/6-311++G(2d,2p)//B3LYP/6-31+G(d,p) level, which were shown to be strongly affected by the solvation microenvironment (0, 1, 2, and 3 explicit solvent molecules considered) due to local geometrical changes induced in the solute molecule. This exceptionally high sensitivity of SR makes polarimetry a unique method capable of sensing changes in the structure of supramers detected in this study.

Antifouling eunicellin-type diterpenoids from the gorgonian Astrogorgia sp

Twelve new eunicellin-based diterpenoids, astrogorgins B-M (1-12), were isolated from a Chinese gorgonian Astrogorgia sp., together with ophirin, muricellin, astrogorgin, calicophirins A and B, and 14-deacetoxycalicophirin B. The structures of the new compounds were elucidated by extensive spectroscopic analysis and by comparison with data reported in the literature. Significant antifouling activity was observed for 14-deacetoxycalicophirin B against the larval settlement of the barnacle Balanus amphitrite at nontoxic concentrations with an EC(50) = 0.59 μg/mL, while the other analogues were effective within an EC(50) range of 5.14-17.8 μg/mL.

Specific rotation as a property to validate monosaccharide conformations

Specific rotation ([α](D)) values were calculated for the 15 conformations of xylopyranose that are the most stable in the gas phase and in aqueous solution. The effects of different theoretical descriptions and the medium on the geometry of the conformers and the [α](D) values are evaluated. Differences in [α](D) values found for the same conformer in all descriptions used were smaller than those found between any two different conformers in the same description. The difference between [α](D) values is prominent, even for two conformations that are distinguished from each other only by the orientation of one secondary hydroxyl group. This finding suggests that [α](D) values may potentially be used in the validation of monosaccharide conformations that are theoretically sampled.

Absolute stereochemistry and preferred conformations of urate degradation intermediates from computed and experimental circular dichroism spectra

The enzymatic oxidation of urate leads to the sequential formation of optically active intermediates with unknown stereochemistry: (-)-5-hydroxyisourate (HIU) and (-)-2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU). In accordance with the observation that a defect in HIU hydrolase causes hepatocarcinoma in mouse, a detoxification role has been proposed for the enzymes accelerating the conversion of HIU and OHCU into optically active (+)-allantoin. The enzymatic products of urate oxidation are normally not present in humans, but are formed in patients treated with urate oxidase. We used time-dependent density functional theory (TDDFT) to compute the electronic circular dichroism (ECD) spectra of the chiral compounds of urate degradation (HIU, OHCU, allantoin) and we compared the results with experimentally measured ECD spectra. The calculated ECD spectra for (S)-HIU and (S)-OHCU reproduced well the experimental spectra obtained through the enzymatic degradation of urate. Less conclusive results were obtained with allantoin, although the computed optical rotations in the transparent region supported the original assignment of the (+)-S configuration. These absolute configuration assignments can facilitate the study of the enzymes involved in urate metabolism and help us to understand the mechanism leading to the toxicity of urate oxidation products.

Conformational study of methylphosphocholine: a prototype for phospholipid headgroups in membranes

Phospholipid bilayers constitute the largest structural component of cell membranes, in which choline phospholipids are abundant. In this study, through a theoretical sampling on a methylphosphocholine (MePC) potential energy surface, a set of conformers was selected as a prototype for the membrane phospholipid head. We performed a detailed conformational study of such a prototype, both as an isolated moiety and in a solvated system. We used the polarizable continuum model (PCM) to account for solvation effects. We used a quantum-mechanical methodology based on density functional theory (DFT) and the 6-31G(d,p) basis set for the calculations. Through this methodology we were able to obtain a set of conformations that presented a mirror-image pattern, in good agreement with the experimental geometric values for the different phosphocholine derivatives. Potential curves for the main parameters of the dihedral space of MePC were obtained and are provided to guide future force-field parameterizations.

Conformational preferences of alpha,alpha-trehalose in gas phase and aqueous solution

This work presents an investigation on the conformational preferences of alpha,alpha-trehalose in gas phase and aqueous solution. Eighty-one systematically selected structures were studied at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level, giving rise to 40 unique conformers. The 19 lower energy structures and some selected other were further re-optimized at the B3LYP/6-311++G(d,p) level. The main factors accounting for the conformer's stability were pointed out and discussed. NBO and QTAIM analyses were performed in some selected conformers in order to address the anomeric and exo-anomeric effects as well as intramolecular hydrogen bonding. The effect of solvent water on the relative stability of the conformers was accounted for by applying the conductor-like polarizable continuum model, CPCM.

Prediction of the 1H and 13C NMR Spectra of α-d-Glucose in Water by DFT Methods and MD Simulations

We have applied computational protocols based on DFT and molecular dynamics simulations to the prediction of the alkyl 1H and 13C chemical shifts of alpha-d-glucose in water. Computed data have been compared with accurate experimental chemical shifts obtained in our laboratory. 13C chemical shifts do not show a marked solvent effect. In contrast, the results for 1H chemical shifts provided by structures optimized in the gas phase are only fair and point out that it is necessary to take into account both the flexibility of the glucose structure and the strong effect exerted by solvent water thereupon. Thus, molecular dynamics simulations were carried out to model both the internal geometry as well as the influence of solvent molecules on the conformational distribution of the solute. Snapshots from the simulation were used as input to DFT NMR calculations with varying degrees of sophistication. The most important factor that affects the accuracy of computed 1H chemical shifts is the solute geometry; the effect of the solvent on the shielding constants can be reasonably accounted for by self-consistent reaction field models without the need of explicitly including solvent molecules in the NMR property calculation.

Absolute Configuration of Conformationally Flexiblecis-Dihydrodiol Metabolites by the Method of Confrontation of Experimental and Calculated Electronic CD Spectra and Optical Rotations

We have determined the absolute configurations of conformationally flexible cis-dihydrodiol metabolites (cis-1,2-dihydroxy-3,5-cyclohexadienes), bearing different substituents (e.g., Br, F, CF3, CN, Me) in 3- and 5-positions, by the method of confrontation of experimental and calculated electronic CD spectra and optical rotations. Convergent results were obtained by both methods in eight out of ten cases. For the difficult cases, where either conformer population and/or chiroptical properties (calculated rotational strengths of the long-wavelength Cotton effect or optical rotations) of contributing conformers remain inconclusive, the absolute configuration could still be correctly assigned based on one of the biased properties (either ECD or optical rotation). This approach appears well-suited for a broad spectrum of conformationally flexible chiral molecules.

A theoretical study of glucose mutarotation in aqueous solution

In this work the mechanism of glucose mutarotation is investigated in aqueous solution considering the most likely pathways proposed from experimental work. Two mechanisms are studied. The first involves an intramolecular proton transfer as proposed by textbooks of organic chemistry, and the second uses one solvent water molecule to assist proton transfer. Both mechanisms are studied in the gas phase and in aqueous solution with the help of a polarizable continuum model, which is adopted to introduce the electrostatic nonspecific influence of bulk solvent. The structures are fully characterized through the calculation of the corresponding vibrational frequencies. The rate coefficients for each mechanism are calculated following transition-state theory in both the gas phase and in aqueous solution. Values computed for the water-assisted pathway in the continuum solvent agree best with the experimental results.

Time Dependent Density Functional Theory Modeling of Chiroptical Properties of Small Amino Acids in Solution

Time dependent density functional theory (TDDFT) and the conductor-like screening model (COSMO) of solvation were used to model the specific rotation and optical rotatory dispersion (ORD) of alanine, proline and serine solutions. Zwitterionic, cationic and anionic forms of amino acids were investigated and the results compared with experimental literature data obtained in neutral, acidic and basic conditions, respectively. It was found that TDDFT consistently underestimated the electronic excitation energies of the molecules, leading to calculated optical rotations that are of the correct sign but somewhat larger in magnitude than those of experiment. An additional challenge was encountered in the modeling of serine, an amino acid with a strong tendency to form intramolecular hydrogen bonds. The model used overestimated the extent of such hydrogen bonding for the zwitterions while possibly underestimating such bonding for the cationic form. This effect on the calculated mole fractions of the different conformers had an impact on the specific rotation.