Solid phases of cyclopentane: combined experimental and simulation study

The phase diagram of cyclopentane has been studied by powder neutron diffraction, providing diffraction patterns for phases I, II, and III, over a range of temperatures and pressures. The putative phase IV was not observed. The structure of the ordered phase III has been solved by single-crystal diffraction. Computational modeling reveals that there are many equienergetic ordered structures for cyclopentane within a small energy range. Molecular dynamics simulations reproduce the structures and diffraction patterns for phases I and III and also show an intermediate disordered phase, which is used to interpret phase II.     

Salarins a and B and tulearin a: new cytotoxic sponge-derived macrolides

Three novel nitrogenous macrolides designated salarin A and B (1 and 2) and tulearin A (3) were isolated from the Madagascar Fascaplysinopsis sp. sponge. The structures of the compounds were elucidated by interpretation of MS and 1D and 2D NMR spectra. Both salarins carry an acetylcarbamate moiety, and in addition, 1 contains a triacylamine group and 2 a methoxymethylketone lactam. Tulearin A carries the naturally rare carbamate ester. The compounds were found to be toxic to brine shrimp larvae, and salarin A and tulearin A were also cytotoxic to leukemia cells.     

Taumycins A and B, two bioactive lipodepsipeptides from the Madagascar sponge Fascaplysinopsis sp

Two closely related lipodepsipeptides, taumycins A and B (1 and 2) have been isolated from the Madagascar sponge Fascaplysinopsis sp. The two compounds have the same 12-membered oxodepsipeptide ring system in common. Both were toxic to brine shrimp larvae, and taumycin A (1 microM), but not taumycin B, inhibited growth of the human UT-7 leukemic cell line. The structure of the two compounds, likely to be derived from microorganisms, was established by MS and 1D and 2D NMR data.     

Synthesis and photochemical properties of a light-activated fluorophore to label His-tagged proteins

Rapid and efficient light-induced fluorescence enhancement is demonstrated on a DMNPB-"caged" coumarin derivative carrying a His-tag recognition motif.     

Electrochemical study of the thermodynamics and kinetics of hydrophilic ion transfers across water | <Emphasis Type="Italic">n</Emphasis>-octanol interface

Thermodynamics and kinetics of hydrophilic ion transfers across water|n-octanol (W|OCT) interface have been electrochemically studied by means of novel three-phase and thin-film electrodes. Three-phase electrodes used for thermodynamics measurements comprise edge plane pyrolytic graphite, the surface of which was partly modified with an ultrathin film of OCT, containing hydrophobic lutetium bis(tetra-tert-butylphthalocyaninato) (Lu[tBu₄Pc]₂) as a redox probe. The transfers of anions and cations from W to OCT were electrochemically driven by reversible redox transformations of Lu[tBu₄Pc]₂ to chemically stable lipophilic monovalent cation and anion , respectively. Upon reduction of Lu[tBu₄Pc]₂, the transfers of alkali metal cations from W to OCT have been studied for the first time, enabling estimation of their Gibbs transfer energies. For kinetic measurements, a thin-film electrode configuration has been used, consisting of the same electrode covered completely with a thin layer of OCT that contained the redox probe and a suitable electrolyte. Combining the fast and sensitive square-wave voltammetry with thin-film electrodes, the kinetics of , , and Cl⁻ transfers have been estimated. Dedicated to Professor Dr. Yakov I. Tur’yan on the occasion of his 85th birthday.     

Modeling equilibrium concentrations of Bjerrum and molecular point defects and their complexes in ice Ih

We present a model for the determination of the thermal equilibrium concentrations of Bjerrum defects, molecular point defects, and their aggregates in ice I(h). First, using a procedure which minimizes the free energy of an ice crystal with respect to the numbers of defect species, we derive a set of equations for the equilibrium concentrations of free Bjerrum and point defects, as well their complexes. Using density-functional-theory calculations, we then evaluate the binding energies of Bjerrum-defect/vacancy and Bjerrum-defect/interstitial complexes. In contrast to the complexes which involve the molecular vacancy, the results suggest that the molecular interstitial binds preferentially to the D-type Bjerrum defect. Using both theoretical binding and formation free energies as well as the available experimental data, we find that the preferential binding and the substantial presence of the interstitial as the predominant point defect in ice I(h) may lead to conditions in which the number of free D defects becomes considerably smaller than that of free L defects. Such a scenario could possibly be involved in the experimentally observed inactivity of D-type Bjerrum defects in the electrical properties of ice I(h).     

Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in NO2

The multiphoton multichannel photodynamics of NO(2) has been studied using femtosecond time-resolved coincidence imaging. A novel photoelectron-photoion coincidence imaging machine was developed at the laboratory in Amsterdam employing velocity map imaging and "slow" charged particle extraction using additional electron and ion optics. The NO(2) photodynamics was studied using a two color pump-probe scheme with femtosecond pulses at 400 and 266 nm. The multiphoton excitation produces both NO(2) (+) parent ions and NO(+) fragment ions. Here we mainly present the time dependent photoelectron images in coincidence with NO(2) (+) or NO(+) and the (NO(+),e) photoelectron versus fragment ion kinetic energy correlations. The coincidence photoelectron spectra and the correlated energy distributions make it possible to assign the different dissociation pathways involved. Nonadiabatic dynamics between the ground state and the A (2)B(2) state after absorption of a 400 nm photon is reflected in the transient photoelectron spectrum of the NO(2) (+) parent ion. Furthermore, Rydberg states are believed to be used as "stepping" states responsible for the rather narrow and well-separated photoelectron spectra in the NO(2) (+) parent ion. Slow statistical and fast direct fragmentation of NO(2) (+) after prompt photoelectron ejection is observed leading to formation of NO(+)+O. Fragmentation from both the ground state and the electronically excited a (3)B(2) and b (3)A(2) states of NO(2) (+) is observed. At short pump probe delay times, the dominant multiphoton pathway for NO(+) formation is a 3x400 nm+1x266 nm excitation. At long delay times (>500 fs) two multiphoton pathways are observed. The dominant pathway is a 1x400 nm+2x266 nm photon excitation giving rise to very slow electrons and ions. A second pathway is a 3x400 nm photon absorption to NO(2) Rydberg states followed by dissociation toward neutral electronically and vibrationally excited NO(A (2)Sigma,v=1) fragments, ionized by one 266 nm photon absorption. As is shown in the present study, even though the pump-probe transients are rather featureless the photoelectron-photoion coincidence images show a complex time varying dynamics in NO(2). We present the potential of our novel coincidence imaging machine to unravel in unprecedented detail the various competing pathways in femtosecond time-resolved multichannel multiphoton dynamics of molecules.     

Synthesis of novel fluorinated 4H-benzo[h]chromen-4-one and 4H-pyrano[3,2-h]quinolin-4-one derivatives

New fluorinated 4H-benzo[h]chromen-4-one and 4H-pyrano[3,2-h]quinolin-4-one derivatives are obtained in moderate to good yields, through a one-pot aldolization-intramolecular S_NAr process, from the tetrakis(dimethylamino)ethylene (TDAE) mediated reductive cleavage of two N,N-dimethylamino-bis-chlorodifluoroacetyl substrates in the presence of heteroaryl aldehydes.     

Using the Relaxation Oscillations Principle for Simple Phonation Modeling

Well-known multimass models of vocal folds are useful to describe main behavior observed in human voicing but their principle of functioning, based on harmonic oscillation, may appear complex. This work is designed to show that a simple one-mass model ruled by laws of relaxation oscillation can also depict main behavior of glottis dynamic. Theory of relaxation oscillation is detailed. A relaxation oscillation model is assessed through a numerical simulation using conventional values for tissue characteristics and subglottal pressure. As expected, raising the mass decreases the fundamental frequency and increases the amplitude of vocal fold vibration: for a mass ranging from 0.01 to 0.4 g, F0 decreased from 297.5 to 42.5 Hz and vibrational amplitude increased from 1.26 to 3.25 mm (for stiffness k=10Nm(-1), damping r=0.015 N s m(-1), and subglottal pressure=1 kPa). Stiffness value has the opposite effect. The subglottal pressure controls the fundamental frequency with a rate ranging from 20 to 50 Hz/kPa. The vibrational amplitude is also controlled linearly by subglottal pressure from 0.22 to 0.26 mm/kPa. The range of phonation threshold pressure (PTP) is close to the values currently proposed, that is, 0.1 to 1 kPa and varies with the fundamental frequency. The relaxation oscillator is a simple and useful tool for modeling vocal fold vibration.