Reorientational relaxation and rotational–translational coupling in water clusters in a d.c. external electric field

Molecular dynamics simulations have been carried out for small water clusters (N=16, 32, 64) in a d.c. electric field at T=200 K. It was shown that for relatively weak fields, there was a significant decrease of reorientational and structural relaxation times for all cluster sizes examined. Regarding the molecular reorientational motions, in the strong field regime, a decoupling of tumbling and spinning librations was observed. Reorientational relaxation times of the dipole and vector joining the two hydrogen atoms were found to follow different relaxation laws, with the former decreasing and the latter increasing with electric field increase. These trends were qualitatively explained by invoking the Debye model with field-dependent friction for dipole librations and the symmetric double-well for spinning rotations on a plane perpendicular to the field axis. Finally, the interdependence of the reorientation on the translational modes of the cluster was indicated, with the translationally slow molecules being rotationally slow as well and vice versa.     

Polyamide Layer Chromatography. VIII. Preparation of Polyamide Layer on Poly(ethylene terephthalate) Film and its Application to Aromatic Nitro Compounds

Poly(ethylene terephthalate) film was used as supporter for preparation of polyamide layer. The most convenient method of preparation was described. This layer can be cut into any size and its sensitivity is higher than the glass plate supported layer. Twenty aromatic nitro compounds were used to show the behavior of polyester film based polyamide layer.     

Rate constants from the reaction path Hamiltonian. I. Reactive flux simulations for dynamically correct rates

As ab initio electronic structure calculations become more accurate, inherent sources of error in classical transition state theory such as barrier recrossing and tunneling may become major sources of error in calculating rate constants. This paper introduces a general method for diabatically constructing the transverse eigensystem of a reaction path Hamiltonian in systems with many degenerate transverse frequencies. The diabatically constructed reaction path Hamiltonian yields smoothly varying coupling constants that, in turn, facilitate reactive flux calculations. As an example we compute the dynamically corrected rate constant for the chair to boat interconversion of cyclohexane, a system with 48 degrees of freedom and a number of degenerate frequencies. The transmission coefficients obtained from the reactive flux simulations agree with previous results that have been calculated using an empirical potential. Furthermore, the calculated rate constants agree with experimental values. Comparison to variational transition state theory shows that, despite finding the true bottleneck along the reaction pathway, variational transition state theory only accounts for half of the rate constant reduction due to recrossing trajectories.     

Shear-flow-based chromatographic separations as an alternative to pressure-driven liquid chromatography

It is only by developing specially designed injection and detection systems that shear-driven chromatography can become a viable alternative to HPLC. In the present paper, a dedicated zero dead-volume injection procedure is presented with which sample volumes can be injected reproducibly in the required picoliter range. In addition, a transversal detection groove system is designed which should allow to perform on-line UV-VIS absorption measurements with path lengths in the millimeter range, with an acceptable theoretical plate loss (only 20% in a 5 cm long channel) and acting as a nearly perfect wave guide.     

Antihydrophobic solvent effects: an experimental probe for the hydrophobic contribution to enzyme-inhibitor binding

The hydrophobic component to the binding affinities of one acyclic phosphinate (4) and three macrocyclic phosphonamidate inhibitors (1-3) to the zinc peptidase thermolysin was probed by varying the solvent composition. Increasing the percentage of ethanol in the buffer solution over the range 0-9% increases the inhibition constants, K(i), by up to an order of magnitude. This approach represents an experimental method for distinguishing solvation from conformational or other effects on protein-ligand binding. The size of the "antihydrophobic effect" is correlated with the amount of hydrophobic surface area sequestered from solvent on association of the inhibitor and enzyme, although it is attenuated from that calculated from the surface tension of ethanol-water mixtures. The results are consistent with the Lum-Chandler-Weeks explanation for the size dependence of the hydrophobic effect.     

Liquid chromatography-diode array detection-electrospray ionisation mass spectrometry/nuclear magnetic resonance analyses of the anti-hyperglycemic flavonoid extract of Genista tenera. Structure elucidation of a flavonoid-C-glycoside

The anti-hyperglycemic flavonoid extract obtained from Genista tenera was first studied by liquid chromatography (LC)-diode array detection (DAD) which showed the presence of two major compounds. One of them was identified as genistein-7-O-glucoside. Luteolin-7-O-glucoside was detected as a minor constituent, while luteolin-7,3'-di-O-glucoside and rutin were found in trace amounts. LC-DAD-ESI-MS and NMR were used to confirm the structure of these compounds and allowed the elucidation of the structure of the unknown major compound, which is the flavonoid 5,7,4'-trihydroxyisoflavone-8-C-glucoside.     

STUDIES ON THE CAPACITY OF pR IN VITRO TO PHOTOCONVERT TO THE LONG-WVELENGTH Ffr-FORM. A SURVEY FO TEN PLANT SPECIES*

Studies on the capacity of Pr in uitro to photoconvert to the long wavelength in uioo-like Pfr form were performed with extracts from 10 species. Red irradiation, immediately after extraction of crude extracts from 9 species, photoconverted Pr to long-wavelength Pfr with an absorbance maximum around 735 nm. Red irradiation of soybean extracts, however, photoconverted Pr to short-wavelength Pfr, with an absorbance maximum at 725 nm. Red irradiation given later than 1.5-2 h after extraction, to extracts of oats, pea, cucumber, radish, sunflower and soybean, photoconverted Pr to a short-wavelength Pfr species with an absorbance maximum around 725 nm. In crude extracts of barley, corn, wheat and zucchini, red irradiation, even after a long dark-incubation period at 4°C of up to 48 h, photoconverted Pr to long-wavelength Pfr with an absorbance maximum around 735 nm. After incubation at 25°C for 3 h, however, Pr from barley also photoconverted to the short-wavelength form. It is suggested that in the group exemplified by oats, Pr rapidly undergoes an alteration following extraction, which results in the loss of the capacity of Pr to photoconvert to long-wavelength Pfr. In contrast, in extracts from the group exemplified by barley, Pr is much more stable and retains the capacity to photoconvert to long-wavelength Pfr for much longer periods.     

Microwave spectra of transient species — cyclopentadienone

The microwave spectrum of the main isotopic species of cyclopentadienone has been measured and assigned. The rotational constants imply an essentially planar geometry and that the ring is rather “fatter” than the analogous ring in fulvene.The dipole moment, derived from Stark-effect measurements, is 10.4 × 10^?30 C m (3.13 D). A vibrational satel- lite spectrum, which, from its inertial defect, corresponds to an out-of-plane vibration has also been assigned.The life time of cyclopentadienone under our spectroscopic conditions is approximately 15 sec.