Cerium(IV) ammonium nitrate-mediated oxidative rearrangement of cyclobutanes and oxetanes

A facile CAN-mediated oxidative rearrangement of alkoxyaryl cyclobutanes and oxetanes is described.     

HPTLC Method for Quantification of Apigenin in the Dried Root Powder of Gmelina arborea Linn.

JPC – Journal of Planar Chromatography – Modern TLC - A sensitive high-performance thin-layer chromatographic method has been established for quantification of apigenin in dried root...     

Foldamer simulations: novel computational methods and applications to poly-phenylacetylene oligomers

We apply several methods to probe the ensemble kinetic and structural properties of a model system of poly-phenylacetylene (pPA) oligomer folding trajectories. The kinetic methods employed included a brute force accounting of conformations, a Markovian state matrix method, and a nonlinear least squares fit to a minimalist kinetic model used to extract the folding time. Each method gave similar measures for the folding time of the 12-mer chain, calculated to be on the order of 7 ns for the complete folding of the chain from an extended conformation. Utilizing both a linear and a nonlinear scaling relationship between the viscosity and the folding time to correct for a low simulation viscosity, we obtain an upper and a lower bound for the approximate folding time within the range 70 ns相似文献   

Extraction of tri- and tetravalent actinides with dihexyl N,N-diethylcarbamoylmethyl phosphonate (DHDECMP) and TBP from nitric acid solutions

Extraction of trivalent (Pu³⁺, Am³⁺, actinides and Eu³⁺, a representative of lanthanides) and tetravalent (Np⁴⁺ and Pu⁴⁺) actinides has been studied with dihexyl N,N-di-ethylcarbamoylmethyl phosphonate (DHDECMP) in combination with TBP in benzene from 2M nitric acid. The stoichiometries of the species extracted were found to be M(NO₃)₃·(3–n) TBP·n DHDECMP (for trivalent ions) and M(NO₃)₄·(2–n) TBP·n DHDECMP (for tetravalent ions) by the slope ratio method. The extraction constants evaluated (from the distribution data) indicate that for tetravalent ions (with solvation number two) the extraction constant increases when TBP (Kh=0.17) molecules are successively replaced by more basic DHDECMP (Kh=0.34) molecules. However, for trivalent ions (with solvation number three) when TBP molecules are totally replaced by DHDECMP molecules stereochemical factors appear and instead of increase, a substantial decrease in extraction constants is observed for Eu³⁺ and Am³⁺, a lesser decrease being observed for Pu³⁺ (larger ion).     

Methyl rotational tunneling dynamics of p-xylene confined in a crystalline zeolite host

The methyl rotational tunneling spectrum of p-xylene confined in nanoporous zeolite crystals has been measured by inelastic neutron scattering (INS) and proton nuclear magnetic resonance (NMR), and analyzed to extract the rotational potential energy surfaces characteristic of the methyl groups in the host-guest complex. The number and relative intensities of the tunneling peaks observed by INS indicate the presence of methyl-methyl coupling interactions in addition to the methyl-zeolite interactions. The INS tunneling spectra from the crystals (space group P2(1)2(1)2(1) with four crystallographically inequivalent methyl rotors) are quantitatively interpreted as a combination of transitions involving two coupled methyl rotors as well as a transition involving single-particle tunneling of a third inequivalent rotor, in a manner consistent with the observed tunneling energies and relative intensities. Together, the crystal structure and the absence of additional peaks in the INS spectra suggest that the tunneling of the fourth inequivalent rotor is strongly hindered and inaccessible to INS measurements. This is verified by proton NMR measurements of the spin-lattice relaxation time which reveal the tunneling characteristics of the fourth inequivalent rotor.     

Derivatized vancomycin stationary phases for LC chiral separations

In this work, synthetic and natural chiral selectors were combined to form two different chiral stationary phases (CSPs). These were made by bonding R- or S-(1-naphthylethyl) carbamate (R-NEC or S-NEC)-derivatized vancomycin molecules to a silica gel support. The two CSPs were evaluated using a set of 60 enantiomeric pairs. The results were compared to the ones obtained with the commercial underivatized vancomycin CSP. Three Chromatographic modes were used: (i) the normal-phase mode using a nonpolar mobile phase with different ratios of hexane and ethanol; (ii) the reversed-phase mode with hydro-organic mobile phases; and (iii) the polar aprotic organic mode with nonaqueous acetonitrile plus small amounts of methanol and an acid and/or base to control retention and selectivity. It is shown that the polarity of the underivatized vancomycin phase is higher than that of the two R- and S-NEC-derivatized CSPs. In the pH range 4-7, there is no ionization change of the chiral selector for the three CSPs. 43% of the studied compounds were resolved by the NEC-derivatized phases when they could not be resolved by the vancomycin CSP. However, the enantiorecognition for 12% of the compounds on the native vancomycin CSP was lost upon NEC derivatization. 45% of the studied compounds were resolved by the NEC-derivatized and native CSPs. The NEC derivatization procedure may block some useful active sites on the vancomycin molecule. Also, the R- and S-NEC moieties are chiral themselves and can contribute additional interaction sites not available on the native vancomycin molecule.     

Synthesis of monodisperse polymers from proteins

Proteins are functional biopolymers; viewed as molecules, they are also monodisperse polyamides with chemically reactive side chains. This paper describes the use of proteins as starting materials for the synthesis of monodisperse polymers with nonbiological functionalities attached to the side chains. It demonstrates the complete derivatization of amine groups (lysine side chains and N-termini) on three different proteins by addition of activated carboxylate reagents in aqueous solutions containing sodium dedecyl sulfate (SDS), under denaturing conditions. Several different acylating reagents were used to generate derivatized proteins; the resulting compounds constitute a new class of monodisperse, semisynthetic polymers, having the potential for wide variation in the structure of the backbone and of the side chains. Modification of lysozyme on a gram scale demonstrated that the method can generate useful quantities of material.     

Thermodynamic Parameters for the Association of Fluorinated Benzenesulfonamides with Bovine Carbonic Anhydrase II

This paper describes a calorimetric study of the association of a series of seven fluorinated benzenesulfonamide ligands (C₆H_nF_5?nSO₂NH₂) with bovine carbonic anhydrase II (BCA). Quantitative structure–activity relationships between the free energy, enthalpy, and entropy of binding and pK_a and log P of the ligands allowed the evaluation of the thermodynamic parameters in terms of the two independent effects of fluorination on the ligand: its electrostatic potential and its hydrophobicity. The parameters were partitioned to the three different structural interactions between the ligand and BCA: the Zn^II cofactor–sulfonamide bond (≈65 % of the free energy of binding), the hydrogen bonds between the ligand and BCA (≈10 %), and the contacts between the phenyl ring of the ligand and BCA (≈25 %). Calorimetry revealed that all of the ligands studied bind in a 1:1 stoichiometry with BCA; this result was confirmed by ¹⁹F NMR spectroscopy and X‐ray crystallography (for complexes with human carbonic anhydrase II).     

Proton‐bifurcated C—H⋯(O,O) hydrogen bonds in 2,3‐dichloro‐6‐nitro­benzyl­aminium chloride

In the crystal structure of the title salt, C₇H₇Cl₂N₂O₂⁺·Cl⁻, the chloride anions participate in extensive hydrogen bonding with the aminium cations and indirectly link the mol­ecules through multiple N⁺—H⋯Cl⁻ salt bridges. There are two independent mol­ecules in the asymmetric unit, related by a pseudo‐inversion center. The direct inter­molecular coupling is established by C—H⋯O, C—H⋯Cl and C—Cl⋯Cl⁻ inter­actions. A rare three‐center (donor bifurcated) C—H⋯(O,O) hydrogen bond is observed between the methyl­ene and nitro groups, with a side‐on intra­molecular component of closed‐ring type and a head‐on inter­molecular component.