Raman spectral analysis for rapid determination of zearalenone and alpha-zearalanol.

Mycotoxins, including zearalenone, are important natural products produced by fungi that occasionally contaminate agricultural commodities and pose serious health risks to consumers of food and feed. Zearalenone and its metabolite, α-zearalanol, are of significant concern due to their estrogenic and anabolic steroid activity. Several governments have regulatory standards and advisory guidelines for zearalenone and α-zearalanol. Raman and ultraviolet spectroscopy were employed with density functional theory methods to evaluate spectroscopic properties to distinguish between zearalenone and α-zearalanol systematically. Raman bands were assigned based on vibrational frequency calculations. A portable Raman spectroscopy instrument (785 nm laser) distinguished between zearalenone and α-zearalanol in a label-free manner. Many vibrational bands of zearalenone and α-zearalanol are similar, including high-intensity peaks at 1315 cm-1 and 1650 cm-1. However, the intensities in the Raman spectra at 1465 cm-1, 1495 cm-1, and 1620 cm-1 enabled the identification of zearalenone. The Raman peak at 1450 cm-1 is associated with α-zearalanol. These vibrational bands serve as spectral indicators to differentiate between the structurally similar zearalenone and α-zearalanol.

Infrared Spectroscopic Analysis of the Adsorption of Pyridine Carboxylic Acids on Colloidal Ceria.

Surface adsorption of a homologous series of pyridine carboxylic acids on a hydrated colloidal cerium dioxide (ceria) film is characterized using the combination of experimental and computationally determined infrared (IR) spectra. Experimental analyses employ attenuated total reflectance (ATR) IR spectroscopy of deposited colloidal ceria thin films equilibrated with three pyridine carboxylic acids at pH 3.0, 5.5, and 8.5. The corresponding computational IR spectra for the energy-minimized intermediate and base forms of the pyridine carboxylic acids use density functional theory calculations at the B3LYP/6-311++G** level of theory. Solvent effects are modeled using both the COSMO implicit solvation model and the inclusion of explicit water molecules. Experimental IR spectra show that the adsorptive interactions between the pyridine carboxylic acids and ceria surface are due to the outer-sphere coordination of cerium ions in the films. Vibrational assignments based on combined experimental and computational results indicate that both pyridyl ring nitrogen and carboxylate functional groups account for the interaction of pyridine carboxylic acids at ceria surfaces. Experimentally determined Langmuir constants point to the intermediate form of picolinic acid (pyridine-2-carboxylic acid) as having the strongest adsorption to ceria compared to the other pyridine carboxylic acids investigated. The enhanced adsorption of picolinic acid is attributed to the adjacency of the protonated pyridyl nitrogen and the carboxylate group relative to nicotinic acid (pyridine-3-carboxylic acid) and isonicotinic acid (pyridine-4-carboxylic acid).

Assessment of the electronic structure and properties of trichothecene toxins using density functional theory.

A comprehensive quantum chemical study was carried out on 35 type A and B trichothecenes and biosynthetic precursors, including selected derivatives of deoxynivalenol and T-2 toxin. Quantum chemical properties, Natural Bond Orbital (NBO) analysis, and molecular parameters were calculated on structures geometry optimized at the B3LYP/6-311+G** level. Type B trichothecenes possessed significantly larger electrophilicity index compared to the type A trichothecenes studied. Certain hydroxyl groups of deoxynivalenol, nivalenol, and T-2 toxin exhibited considerable rotation during molecular dynamics simulations (5 ps) at the B3LYP/6-31G** level in implicit aqueous solvent. Quantitative structure activity relationship (QSAR) models were developed to evaluate toxicity and detection using genetic algorithm, principal component, and multilinear analyses. The models suggest electronegativity and several 2-dimensional topological descriptors contain important information related to trichothecene cytotoxicity, phytotoxicity, immunochemical detection, and cross-reactivity.

Investigation of the mechanism for the preparation of 6-phenyl-2,4-dioxotetrahydropyrans by the potassium carbonate promoted condensation between acetoacetate esters and benzaldehyde.

Treatment of benzaldehyde and an acetoacetate ester with potassium carbonate in an alcohol solvent proceeds via γ-C-alkylation rather than α-C-alkylation resulting in the formation of 6-phenyl-2,4-dioxotetrahydropyran. Based upon results from deuterium exchange experiments, carbon-13 labeling experiments, (1)H NMR monitoring studies, and reactivity studies, our proposed mechanism for this reaction involves deprotonation at the α-carbon, intramolecular proton transfer to form a γ-anion, addition of the resulting γ-anion to the carbonyl carbon of benzaldehyde, and intramolecular transesterification.

DFT conformational studies of alpha-maltotriose.

Recent DFT optimization studies on alpha-maltose improved our understanding of the preferred conformations of alpha-maltose. The present study extends these studies to alpha-maltotriose with three alpha-D-glucopyranose residues linked by two alpha-[1-->4] bridges, denoted herein as DP-3's. Combinations of gg, gt, and tg hydroxymethyl groups are included for both "c" and "r" hydroxyl rotamers. When the hydroxymethyl groups are for example, gg-gg-gg, and the hydroxyl groups are rotated from all clockwise, "c", to all counterclockwise, "r", the minimum energy positions of the bridging dihedral angles (phi(H) and psi(H)) move from the region of conformational space of (-, -), relative to (0 degrees , 0 degrees), to a new position defined by (+, +). Further, it was found previously that the relative energies of alpha-maltose gg-gg-c and "r" conformations were very close to one another; however, the DP-3's relative energies between hydroxyl "c" or "r" rotamers differ by more than one kcal/mol, in favor of the "c" form, even though the lowest energy DP-3 conformations have glycosidic dihedral angles similar to those found in the alpha-maltose study. Preliminary solvation studies using COSMO, a dielectric solvation method, point to important solvent contributions that reverse the energy profiles, showing an energy preference for the "r" forms. Only structures in which the rings are in the chair conformation are presented here.

DFT studies of the disaccharide, alpha-maltose: relaxed isopotential maps.

The disaccharide, alpha-maltose, forms the molecular basis for the analysis of the structure of starch, and determining the conformational energy landscape as the molecule oscillates around the glycosidic bonds is of importance. Thus, it is of interest to determine, using density functionals and a medium size basis set, a relaxed isopotential contour map plotted as a function of the phi(H) and psi(H) dihedral angles. The technical aspects include the method of choosing the starting conformations, the choice of scanning step size, the method of constraining the specific dihedral angles, and the fitting of data to obtain well defined contour maps. Maps were calculated at the B3LYP/6-31+G( *) level of theory in 5 degrees intervals around the (phi(H),psi(H))=(0 degrees ,0 degrees ) position, out to approximately +/-30 degrees or greater, for gg-gg'-c, gg-gg'-r, gt-gt'-c, gt-gt'-r, tg-tg'-c, and tg-tg'-r conformers, as well as one-split gg(c)-gg'(r) conformer. The results show that the preferred conformation of alpha-maltose in vacuo depends strongly upon the hydroxyl group orientations ('c'/'r'), but the energy landscape moving away from the minimum-energy position is generally shallow and transitions between conformational positions can occur without the addition of significant energy. Mapped deviations of selected parameters such as the dipole moment; the C1-O1-C4', H1-C1-O1, and H4'-C4'-O1 bond angles; and deviations in hydroxymethyl rotamers, O5-C5-C6-O6, O5'-C5'-C6'-O6', C5-C6-O6-H, and C5'-C6'-O6'-H', are presented. These allow visualization of the structural and energetic changes that occur upon rotation about the glycosidic bonds. Interactions across the bridge are visualized by deviations in H(O2)...O3', H(O3')...O2, and H1...H4' distances and the H(O2)-O2-C2-C1 and H'(O3')-O3'-C3'-C4' hydroxyl dihedral angles.

DFT study of alpha- and beta-D-allopyranose at the B3LYP/6-311++G ** level of theory.

One hundred and two conformations of alpha- and beta-D-allopyranose, the C-3 substituted epimer of glucopyranose, were geometry optimized using the density functional, B3LYP, and the basis set, 6-311++G **. Full geometry optimization was performed on different ring geometries and on the hydroxymethyl rotamers (gg/gt/tg). Analytically derived Hessians were used to calculate zero point energy, enthalpy, and entropy. The lowest energy and free energy conformation found is the alpha-tg(g-)-4C1-c conformation, which is only slightly higher in electronic (approximately 0.2 kcal/mol) and free energy than the lowest energy alpha-D-glucopyranose. The in vacuo calculations showed a small (approximately 0.3 kcal/mol) energetic preference for the alpha- over the beta-anomer for allopyranose in the 4C1 conformation, whereas in the 1C4 conformation a considerable (approximately 1.6 kcal/mol) energetic preference for the beta- over the alpha-anomer for allopyranose was encountered. The results are compared to previous aldohexose calculations in vacuo. Boat and skew forms were found that remained stable upon gradient optimization although many starting boat conformations moved to other skew forms upon optimization. As found for glucose, mannose, and galactose the orientation and interaction of the hydroxyl groups make the most significant contributions to the conformation/energy relationship in vacuo. A comparison of different basis sets and density functionals is made in the Discussion section, confirming the appropriateness of the level of theory used here.

Determination of the preferred conformation of the bicyclic Galerucella pheromone using density functional theory optimization and calculations of chemical shifts.

A pheromone from the beetle, Galerucella calmariensis, was recently isolated and identified (Bartelt, R. J. et al. J. Chem. Ecol. 2006, 32, 693-712) as a 14-carbon, bicyclic dimethylfuran lactone, with the systematic name 12,13-dimethyl-5,14-dioxabicyclo[9.2.1]tetradeca-1(13),11-dien-4-one. The main 12-membered lactone ring is very flexible; as a result, there exist multiple possible conformations. The preferred conformation cannot be deduced solely from room-temperature NMR measurements. Using density functional (DFT) studies, 26 unique conformers with energies within 10.0 kcal/mol of the global minimum-energy structure were found. A mirror-image plane exists so that each conformer has an "inverse" structure with the same energy, for which the dihedral angles around the flexible ring have opposite sign. The isotropic 1H and 13C NMR chemical shifts of the DFT-optimized structures were calculated using the gauge-including atomic orbital (GIAO) method. By considering the relative energies of the conformers and the calculated and observed NMR spectra, we concluded that the molecule exists primarily as a mixture of two distinct conformers at room temperature, each being present with its mirror-image inverse. Structural interconversions among these likely occur on a time scale that is fast compared to the NMR experiments. Using mode-following and dihedral-driving techniques, several potential pathways were found for the conversion of the lowest-energy conformer to its mirror-image structure. Ab initio molecular dynamics (AIMD) using the 4-31G basis set was carried out for 50 ps to test the availability of various low-energy minima and the transition states found from the searches noted above.

DFT study of alpha- and beta-D-galactopyranose at the B3LYP/6-311++G** level of theory.

Forty-one conformations of alpha- and beta-d-galactopyranose were geometry optimized using the B3LYP density functional and 6-311++G** basis set. Full geometry optimization was performed on different ring geometries and different hydroxymethyl rotamers (gg/gt/tg). Analytically derived Hessians were used to calculate zero point energy, enthalpy, and entropy. The lowest energy and free-energy conformation found is the alpha-gg-(4)C(1)-c chair conformation, which is of lower electronic and free energy than the lowest energy alpha-d-glucopyranose conformer because of favorable hydrogen-bonding interactions. The in vacuo calculations showed considerable ( approximately 2.2kcal/mol) energetic preference for the alpha over the beta anomer for galactopyranose in both the (4)C(1) and (1)C(4) chair conformations. Results are compared to glucopyranose and mannopyranose calculations in vacuo. Boat and skew-boat forms were found that remained stable upon gradient optimization, although many starting conformations moved to other boat forms upon optimization. As with glucopyranose and mannopyranose, the orientation and interaction of the hydroxyl groups make the most significant contributions to the conformation-energy relationship in vacuo.

B3LYP/6-311++G** geometry-optimization study of pentahydrates of alpha- and beta-D-glucopyranose.

Five water molecules were placed in 37 different configurations around alpha- and beta-D-glucopyranose in the gt, gg, and tg conformational states, and the glucose-water complexes were geometry optimized using density functionals at the B3LYP/6-311++G** level of theory. The five water molecules were organized in space and energy minimized using an empirical potential, AMB02C, and then further geometry optimized using DFT algorithms to minimum energy positions. Electronic energy, zero point vibrational energy, enthalpy, entropy, stress energy on glucose and the water cluster, hydrogen-bond energy, and relative free energy were obtained for each configuration using thermodynamic procedures and an analytical Hessian program. The lowest energy complex was that of a clustering of water molecules around the 1- and 6-hydroxyl positions of the beta-gt anomer. Configurations in which the water molecules created a favorable network completely around and under glucose were found to have low energy for both alpha and beta anomers. Calculation of the alpha/beta anomeric ratio using the zero point corrected energy gave, approximately 32/68%, highly favoring the beta anomer in agreement with the experimental approximately 36/64% value. This ratio is better than the approximately 50/50% ratio found in our previous monohydrate study. An approximate hydroxymethyl population was obtained by noting average relative energies among the three conformational states, gt, gg, and tg. In the beta anomer complexes the gt conformation was favored over the gg state, while in the alpha anomer complexes the gg state was favored over the gt conformation, with the tg conformations all being of higher energy making little or no contribution to the rotamer population. Some geometry variances, found between glucose in vacuo and glucose after interaction with water molecules, are described and account for some observed C-5-C-6 bond length anomalies reported by us previously for the vacuum glucose structures.