Antibiosis,Antibiotics, and the Formaldehyde Cycle: The Unique Importance of Planar Chromatographic Techniques to Progress in these Fields

JPC – Journal of Planar Chromatography – Modern TLC - The BioArena system, which integrates the up-to-date methodology and biological results of bioautography with OPLC as an efficient...     

Two Ways to Achieve the Same Goal—Two Validated Quantitative NMR Strategies for a Low-Abundance Natural Product in Standardized Extracts: The Case of Hepatodamianol in Turnera diffusa

The quantification of low-abundance secondary metabolites in plant extracts is an analytical problem that can be addressed by different analytical platforms, the most common being those based on chromatographic methods coupled to a high-sensitivity detection system. However, in recent years nuclear magnetic resonance (NMR) has become an analytical tool of primary choice for this type of problem because of its reliability, inherent simplicity in sample preparation, reduced analysis time, and low solvent consumption. The versatility of strategies based on quantitative NMR (qNMR), such as internal and external standards and electronic references, among others, and the need to develop validated analytical methods make it essential to compare procedures that must rigorously satisfy the analytical well-established acceptance criteria for method validation. In this work, two qNMR methods were developed for the quantification of hepatodamianol, a bioactive component of T. diffusa. The first method was based on a conventional external standard calibration, and the second one was based on the pulse length-based concentration determination (PULCON) method using the ERETIC2 module as a quantitation tool available in TopSpin software. The results show that both procedures allow the content of the analyte of interest in a complex matrix to be determined in a satisfactory way, under strict analytical criteria. In addition, ERETIC2 offers additional advantages such as a reduction in experimental time, reagent consumption, and waste generated.     

The Cyclobutanocucurbit[5–8]uril Family: Electronegative Cavities in Contrast to Classical Cucurbituril while the Electropositive Outer Surface Acts as a Crystal Packing Driver

The structural parameters for the cyclobutanoQ[5–8] family were determined through single crystal X-ray diffraction. It was found that the electropositive cyclobutano methylene protons (CH₂) are important in forming interlinking crystal packing arrangements driven by the dipole–dipole interactions between these protons and the portal carbonyl O of a near neighbor. This type of interaction was observed across the whole family. Electrostatic potential maps also confirmed the electropositive nature of the cyclobutano CH₂ but, more importantly, it was established that the cavities are electronegative in contrast to classical Q[5–8], which are near neutral.     

An ab initio study of crystal field effects,part 3: Solid- and gas-phase geometry of formamide,modeling the changes in a peptide group due to hydrogen bonds

A model of the solid state of formamide is constructed by optimizing a central molecule in an electrostatic field of the proper symmetry. Attention is paid to the way the electrostatic charges are obtained. Point charges obtained from a Mulliken population analysis yield a final set of atomic charges in the central molecule that agree reasonably well with those obtained experimentally after ak-refinement of formamide. Point charges from a so-called stockholder partitioning agree slightly less. Furthermore, the simple crystal field adaptation of standard ab initio methods reproduces within experimental limits the differences in C=O and C-N lengths, observed between the gas-phase and the solid state geometry. Again, a Mulliken field agrees slightly better than a stockholder field, but the difference in performance is statistically insignificant. In a survey of 221 high-quality single-crystal x-ray determinations of compounds containing the peptide group N-C=O, we found evidence supporting quantitatively the conclusion that the increase of C=O and the decrease of C-N bond length in the gas-to-solid transition is dominated by the effects of hydrogen bonding. It was shown that the C=O bond lengthens by about 0.011 å per H-bond it accepts, while the N-C bond diminishes by about 0.015 å per H-bond it donates.Part 2, see Ref. [5].     

The effect of conformational equilibrium on the enantioselectivity of catalysts: a quantitative relationship for the evaluation,design, and prediction of chiral ligands in the addition of diethylzinc to benzaldehydes

A kinetic system correlating the enantioselectivity with a catalyst’s conformational equilibrium, as a theoretical basis for the evaluation, design, and prediction of chiral ligand, is described for the addition of diethylzinc to benzaldehyde, and more importantly, a quantitative relationship between the conformations and the enantioselectivity is derived from this catalytic asymmetric kinetic system, which interprets that the observed enantioselectivity is not a weighted average of the enantioselectivity of the individual conformers.     

The palladium-tributylammonium formate reagent in the stereoselective hydrogenation,and stereo- and regioselective hydroarylation of alkyl 4-hydroxy-2-alkynoates: a route to substituted butenolides

The reaction of aryl iodides with alkyl 4-hydroxy-2-alkynoates in the presence of formic acid, tri-n-butylamine and a palladium(II) catalyst provides a convenient route to functionalized substituted butenolides through a one pot hydroarylation/cyclization reaction. In the presence of formic acid, tri-n-butylamine and a palladium(II) catalyst, alkyl 4-hydroxy-2-alkynoates undergo a one pot hydrogenation/cyclization reaction to the butenolide ring. By increasing the excess of formic acid, direct formation of saturated γ-lactones can be observed. Reactions occur with high stereoselectivity and, in the case of the hydroarylation, with good regioselectivity.     

The classic "brown-ring" reaction in a new medium: kinetics, mechanism, and spectroscopy of the reversible binding of nitric oxide to iron(II) in an ionic liquid

To elucidate the applicability and properties of ionic liquids (ILs) to serve as chemical reaction media for the activation of small molecules by transition-metal complexes, detailed kinetic and mechanistic studies were performed on the reversible binding of NO to FeCl(2) dissolved in the IL 1-ethyl-3-methylimidazolium dicyanamide ([emim][dca]) as a solvent. We report, for the first time, the application of laser flash photolysis at ambient and high pressure to study the kinetics of this reaction in an IL. The kinetic data and activation parameters for the "on" and "off" reactions suggest that both processes follow a limiting dissociative (D) ligand substitution mechanism, in contrast to that reported for the same reaction in aqueous solution, where this well-known "brown-ring" reaction follows an interchange dissociative (I(d)) ligand substitution mechanism. The observed difference apparently arises from the participation of the IL anion as a N-donor ligand, as evidenced by the formation of polymeric [Fe(dca)(3)Cl](x)[emim](2x) chains in the solid state and verified by X-ray crystallography. In addition, infrared (IR), Mo?ssbauer, and EPR spectra were recorded for the monomeric reaction product [Fe(dca)(5)NO](3-) formed in the IL, and the parameters closely resemble those of the {FeNO}(7) unit in other well-characterized nitrosyl complexes. It is concluded that its electronic structure is best described by the presence of a high-spin Fe(III) (S = 5/2) center antiferromagnetically coupled to NO(-) (S = 1), yielding the observed spin quartet ground state (S(t) = 3/2).     

The biosynthesis of sorbicillinoids in Trichoderma sp. USF-2690: prospect for the existence of a common precursor to sorbicillinol and 5-epihydroxyvertinolide, a new sorbicillinoid member

Biosynthetic feeding studies of [1-¹³C], [2-¹³C], and [1,2-¹³C₂]-labeled sodium acetates into 5-epihydroxyvertinolide, a new sorbicillinoid, gives an incorporation pattern that proves the γ-lactone ring formation associated with a ring cleavage reaction of the precursor, a potential intermediate of sorbicillinol biosynthesis.     

On the blue-colored species,possibly assignable to a radical cation,formed by the aromatic hydrocarbon–sulfur dioxide–oxygen system: The polymerization of olefins initiated by aromatic hydrocarbons–oxygen in liquid sulfur dioxide

It was found that styrene in liquid sulfur dioxide polymerizes, giving polystyrene readily and quantitatively by addition of such aromatic hydrocarbons as anthracene and trans- or cis-stilbene in the presence of oxygen, and the polymerization proceeds via a cationic process. The observations on the electronic spectra and kinetics in the system suggested that the polymerization was initiated by an electron transfer from the aromatic hydrocarbon to oxygen, followed by the formation of styrene radical cation. Supporting evidence of the radical cation is that 1,1-diphenylethylene in liquid sulfur dioxide in the presence of oxygen shows a peak at λ_max = 605 mμ and reacts to give benzophenone and 1,1,3,3-tetraphenyl-butene-1, which are eliminated by addition of a radical or cation inhibitor.     

The stability and eventual lone pair distortion of the hexahalide complexes and molecules of the fifth to eighth main-group elements with one lone pair, as isolated entities or in oligomeric clusters: a vibronic coupling and DFT study

The introduced DFT-supported vibronic coupling model together with the hardness rule indicates, for the title compounds, that the tendency toward lone pair (LP) distortions decreases with increasing coordination number (CN) and upon proceeding from fluoride to iodide as the ligands. Thus, only some hexafluoro complexes and molecules are calculated to actually undergo LP deformations; here, from the possible highest-symmetry deformations, those with C(4v) geometry possess the lowest energy, leading to the complete ablation of one ligand and, hence, explaining the nonexistence of the complexes AsF6(3-), SbF6(3-), and SF6(2-). If a lower-symmetry strain is imposed on the octahedral species, for example, induced by the simultaneous presence of terminal and bridging ligands in oligomers, the vibronic energy potential is activated. It may induce pronounced distortions, which are much larger than those of analogous clusters with central ions lacking the LP. Dimers and tetramers with common edges and faces are investigated, with the predicted and calculated deformations of the constituting octahedra again following the stability sequence C(4v) > C(2v) > C(3v). The model nicely accounts for the observed trends, as well as reproduces the experimental structures and energy balances, at least semiquantitatively; its predictive power exceeds that of the valence-shell electron-pair repulsion concept.