High-performance thin-layer chromatographic fingerprints of isoflavonoids for distinguishing between Radix Puerariae Lobate and Radix Puerariae Thomsonii

The roots of Pueraria lobata (Wild.) Ohwi and Pueraria thomsonii Benth have been officially recorded in all editions of Chinese Pharmacopoeia under the same monograph 'Gegen' (Radix Puerariae, RP). However, in its 2005 edition, the two species were separated into both individual monographs, namely 'Gegen' (Radix Puerariae Lobatae, RPL) and 'Fenge' (Radix Puerariae Thomsonii, RPT), respectively, due to their obvious content discrepancy of puerarin, the major active constituent. In present paper, the fingerprint of high-performance thin-layer chromatography (HPTLC) combining digital scanning profiling was developed to identify and distinguish the both species in detail. The unique properties of the HPTLC fingerprints were validated by analyzing ten batches of Pueraria lobata and P. thomsonii samples, respectively. The common pattern of the HPTLC images of the roots of Pueraria spp. and the respective different ratios of the chemical distribution can directly discern the two species. The corresponding digital scanning profiles provided an easy way for quantifiable comparison among the samples. Obvious difference in ingredient content and HPTLC patterns of the two species questioned their bio-equivalence and explained that recording both species separately in the current edition of Chinese Pharmacopoeia (2005 edition) is reasonable due to not only the content of major constituent, puerarin, but also the peak-to-peak distribution in the fingerprint and integration value of the total components. Furthermore, the HPTLC fingerprint is also suitable for rapid and simple authentication and comparison of the subtle difference among samples with identical plant resource but different geographic locations.     

Halo substituent effects on intramolecular cycloadditions involving furanyl amides

Intramolecular Diels-Alder reactions involving a series of N-alkenyl-substituted furanyl amides were investigated. Stable functionalized oxanorbornenes were formed in high yield upon heating at 80-110 degrees C. The cycloaddition reactions include several bromo-substituted furanyl amides, and these systems were found to proceed at a much faster rate and in higher yield than without substitution. This effect was observed by incorporating a halogen in the 3- or 5-position of the furan ring and appears to be general. The origin of increased cycloaddition rates for halo-substituted furans has been investigated with quantum mechanical calculations. The success of these reactions is attributed to increases in reaction exothermicities; this both decreases activation enthalpies and increases barriers to retrocycloadditions. Halogen substitution on furan increases reactant energy and stabilizes the product, which is attributed to the preference of electronegative halogens to be attached to a more highly alkylated and therefore more electropositive framework.     

Convergent approaches for the synthesis of the antitumoral peptide, Kahalalide F. Study of orthogonal protecting groups

Kahalalide compounds are peptides that are isolated from a Hawaiian herbivorous marine species of mollusc, Elysia rufescens, and its diet, the green alga Bryopsis sp. Kahalalide F and its synthetic analogues are the most promising compounds of the Kahalalide family because they show antitumoral activity. Linear solid-phase syntheses of Kahalalide F have been reported. Here we describe several new improved synthetic routes based on convergent approaches with distinct orthogonal protection schemes for the preparation of Kahaladide analogues. These strategies allow a better control and characterization of the intermediates because more reactions are performed in solution.     

Photodissociation dynamics of the NO dimer. I. Theoretical overview of the ultraviolet singlet excited states

Molecular orbital theory and calculations are used to describe the ultraviolet singlet excited states of NO dimer. Qualitatively, we derive and catalog the dimer states by correlating them with monomer states, and provide illustrative complete active space self-consistent field calculations. Quantitatively, we provide computational estimates of vertical transition energies and absorption intensities with multireference configuration interaction and equations-of-motion coupled-cluster methods, and examine an important avoided crossing between a Rydberg and a valence state along the intermonomer and intramonomer stretching coordinates. The calculations are challenging, due to the high density of electronic states of various types (valence and Rydberg, excimer and charge transfer) in the 6-8 eV region, and the multiconfigurational nature of the ground state. We have identified a bright charge-transfer (charge-resonance) state as responsible for the broadband seen in UV absorption experiments. We also use our results to facilitate the interpretation of UV photodissociation experiments, including the time-resolved 6 eV photodissociation experiments to be presented in the next two papers of this series.     

Two-compartment radial diffusive exchange analysis of the NMR lineshape of (129)Xe dissolved in a perfluorooctyl bromide emulsion

Hyperpolarized (129)Xe (xenon) gas dissolved in a perfluorooctyl bromide (PFOB) emulsion stabilized with egg yolk phospholipid (EYP) is a possible contrast agent for quantitative blood flow measurements using magnetic resonance imaging. The NMR line shape of xenon dissolved in PFOB emulsion depends strongly on the exchange of spins between PFOB and water. The exchange in this system depends on three factors: the geometrical factors (i.e., droplet size and surrounding water volume), the permeability of the EYP monolayer surrounding the droplet, and the diffusion coefficients of xenon in the two media. A theoretical model which predicts the line shape of xenon in the emulsion based on the Bloch-Torrey equations is presented. Fitting the full width at half maximum (FWHM) of the theoretical line shapes with the FWHM of the experimental spectra obtained from emulsions with different water dilutions allows estimation of the volume-weighted average diameter of the PFOB droplets (3.5+/-0.8) microm and the permeability of the EYP membrane surrounding the droplet (58+/-14) microm / s.     

Fumarate of (S,S)-alpha,alpha'-bis(trifluoromethyl)-1,8-anthracenedimethanol. A chiral macrocycle for the Diels-Alder reaction with cyclopentadiene

The synthesis and structural study of the fumarate of (S,S)-alpha,alpha'-bis(trifluoromethyl)-1,8-anthracenedimethanol by NMR spectroscopy are reported. The conformational study of the 13-membered macrocycle is presented. The cited alcohol is assayed as a chiral auxiliary in a Diels-Alder reaction with cyclopentadiene, and after methanolysis, provides the norbornene derivative with high enantioselectivity.     

Unprecedented intermolecular C-H bond activation of a solvent toluene molecule leading to a seven-membered platinacycle

A novel platinum-mediated process involving intermolecular activation of a C(aryl)-H bond of toluene, intramolecular activation of an imine C(aryl)-Cl bond and formation of a C-C bond is reported.     

Beta-turn modified gramicidin S analogues containing arylated sugar amino acids display antimicrobial and hemolytic activity comparable to the natural product

This paper describes the design and synthesis of gramicidin S (GS) analogues 10a-c containing arylated sugar amino acids (SAAs) as a replacement of one of the two (D)Phe-Pro beta-turn regions. The cyclic, amphiphilic peptides adopt a beta-sheet conformation featuring an unusual reverse turn induced by the SAAs. The altered turn region induces a slight distortion of the antiparallel beta-sheet, as compared to GS; the overall geometry however closely resembles that of the nonarylated GS analogue 1. GS analogues 10a-c proved to be as active as the parent GS itself as antibacterial agents and are equally efficient in lysing red blood cells. These properties are in sharp contrast to the diminished biological activity displayed by 1. We conclude that the presence of aromaticity in the turn regions of GS derivatives is required for biological activity, whereas the native conformation of the beta-hairpin is not. Our findings may guide future research toward efficient and nonhemolytic GS analogues for combating bacterial infections.     

High yield syntheses of stable, singly bonded Pd2(6+) compounds

A general method for the syntheses of dipalladium compounds having a singly bonded Pd26+ core and the formula R,S-cis-Pd2(C6H4PPh2)2(O2CR)2Cl2 is described. When the alkyl group in the carboxylate ligands is an electron donating group, the compounds are stable and the yields high. The Pd-Pd distances for the diamagnetic compounds with R = CF3 and CMe3 are 2.5434(4) and 2.5241(9) A, respectively. Calculations at the DFT level suggest that the electronic configuration is sigma2pi4delta2delta*2pi*4. These represent rare examples of palladium(III) compounds.     

Tandem mass spectrometry of intact GroEL-substrate complexes reveals substrate-specific conformational changes in the trans ring

It has been suggested that the bacterial GroEL chaperonin accommodates only one substrate at any given time, due to conformational changes to both the cis and trans ring that are induced upon substrate binding. Using electrospray ionization mass spectrometry, we show that indeed GroEL binds only one molecule of the model substrate Rubisco. In contrast, the capsid protein of bacteriophage T4, a natural GroEL substrate, can occupy both rings simultaneously. As these substrates are of similar size, the data indicate that each substrate induces distinct conformational changes in the GroEL chaperonin. The distinctive binding behavior of Rubisco and the capsid protein was further investigated using tandem mass spectrometry on the intact 800-914 kDa GroEL-substrate complexes. Our data suggest that even in the gas phase the substrates remain bound inside the GroEL cavity. The analysis revealed further that binding of Rubisco to the GroEL oligomer stabilizes the chaperonin complex significantly, whereas binding of one capsid protein did not have the same effect. However, addition of a second capsid protein molecule to GroEL resulted in a similar stabilizing effect to that obtained after the binding of a single Rubisco. On the basis of the stoichiometry of the GroEL chaperonin-substrate complex and the dissociation behavior of the two different substrates, we hypothesize that the binding of a single capsid polypeptide does not induce significant conformational changes in the GroEL trans ring, and hence the unoccupied GroEL ring remains accessible for a second capsid molecule.