Geometric molecular similarity from volume-based distance minimization: Application to saxitoxin and tetrodotoxin

The geometric molecular dissimilarity between two molecules is defined as the difference between the volume of their union minus the volume of their intersection. This dissimilarity has the mathematical properties of a distance. This distance is minimized under all rotations and translations using a discrete Broyden, Fletcher, Goldfarb & Shanno (B.F.G.S.) algorithm. The optimal geometric superimposition of saxitoxin and tetrodotoxin is discussed. © 1995 by John Wiley & Sons, Inc.     

Electron pair localization function: a practical tool to visualize electron localization in molecules from quantum Monte Carlo data

In this work we introduce an electron localization function describing the pairing of electrons in a molecular system. This function, called "electron pair localization function," is constructed to be particularly simple to evaluate within a quantum Monte Carlo framework. Two major advantages of this function are the following: (i) the simplicity and generality of its definition; and (ii) the possibility of calculating it with quantum Monte Carlo at various levels of accuracy (Hartree-Fock, multiconfigurational wave functions, valence bond, density functional theory, variational Monte Carlo with explicitly correlated trial wave functions, fixed-node diffusion Monte Carlo, etc). A number of applications of the electron pair localization function to simple atomic and molecular systems are presented and systematic comparisons with the more standard electron localization function of Becke and Edgecombe are done. Results illustrate that the electron pair localization function is a simple and practical tool for visualizing electronic localization in molecular systems.     

Synthesis and cytotoxic activity of 11-nitro and 11-amino derivatives of acronycine and 6-demethoxyacronycine

Condensation of 2-chloro-3-nitrobenzoic acid with either 5-amino-7-methoxy-2,2-dimethyl-2H-chromene or 5-amino-2,2-dimethyl-2H-chromene afforded diphenylamines 14 and 15. Trifluoroacetic anhydride mediated cyclization gave the corresponding acridones 16 and 17, which were subsequently N-methylated and reduced to 11-aminoacronycine and 11-amino-6-demethoxyacronycine. These two amino compounds, which gave stable water soluble salts, were 2- to 3-fold more potent than acronycine or 6-demethoxyacronycine in inhibiting L1210 cell proliferation.     

"ENERGY"-DEPENDENT QUENCHING OF CHLOROPHYLL FLUORESCENCE and THERMAL ENERGY DISSIPATION IN INTACT LEAVES DURING INDUCTION OF PHOTOSYNTHESIS

Abstract— Intact leaves, previously adapted to darkness for a prolonged period of time, were suddenly illuminated with a strong, photosynthetically saturating, white light (ca 1500 μmol m⁻² s^_1), resulting in the rapid establishment of a large energy-dependent chlorophyll fluorescence quenching (q_E) as shown by in vivo fluorescence measurements with a pulse amplitude modulation technique. Two different photothermal methods, photoacoustic spectroscopy and photothermal deflection spectroscopy, were used to monitor thermal deactivation of excited pigments during the dark-light transitions. The in vivo photothermal signals measured with both techniques were shown to remain constant during induction of photosynthesis under high light conditions, suggesting that, in contrast to current hypotheses, energy-dependent quenching q_E is not associated with significant changes in thermal dissipation of absorbed light energy in the chloroplasts. When photosynthesis was induced with a low-intensity modulated light, a noticeable decrease in the heat emission yield was observed resulting from the progressive activation of the competing photochemical processes.     

Un nouveau type d'alcaloïdes isoquinoleiques,les bisaporphines

The structures of Beccapoline and Beccapolinium, two new isoquinoline alkaloids from $\text{Polyathia beccarii}$, Annonaceae, have been elucidated by spectroscopic analysis. They are the first examples of natural bisaporphines.     

Chemical ionization desorption mass spectrometry of some quaternary ammonium salts

The ammonia chemical ionization desorption spectra of N,N-dimethyl quaternary ammonium iodides in addition to high protonated molecular ion [M + H]⁺ intensity, show signals for an ion radical composed of N-methyl abstracted salt cation and ammonia [C + NH₃? CH₃]^+˙. These ions corresponding to the cation +2 show increased importance in the chemical ionization mode, using the same reagent gas. The technique of chemical ionization desorption appears suitable for the analysis of salts, and thus for the determination of the molecular weight of both anion and cation.     

Utilization of cyclotron produced fast neutrons in the activation analysis for oxygen

The possible application of cyclotron-produced fast neutrons to activation analysis for oxygen based on the¹⁶O(n, p)¹⁶N reaction has been investigated. Neutrons were produced by bombarding a thick beryllium target with 22 to 45 MeV deuterons. It was found that the sensitivity increases rapidly with the energy of the deuterons. Using 45 MeV deuterons and a 10 μA beam current a sensitivity of about 20 counts per 1 μg oxigen could be achieved, enabling the determination of less than 1 μg oxigen. In a direct comparison it was experimentally established that the sensitivity for cyclotron-produced neutrons assuming a deuteron beam of about 10 μA, is up to two orders of magnitude higher than that achievable for 14 MeV neutrons with a flux of about 10¹⁰ n/s. The interference of fluorine is at about the same level for both the cyclotron-produced and 14 MeV neutrons. Using cyclotron-produced fast neutrons in the investigated energy range, sodium and magnesium can also interfere, but only to a very much lower extent.     

Réactivité de spirochromènes et de mérocyanines azahétérocycliques vis-à-vis de quelques agents nucléophiles

Under the action of nucleophile reagents such as water, hydrogen sulfide and sodium borohydride, the saturated azaheterocyclic spirochromenes are opened either on the azaheterocyclic side or on the benzopyran side or on both parts simultaneously. The merocyanines are either partially reduced or decomposed by the sodium borohydride.     

Use of uranium isotopes for the determination of river tributary flow rates: Case of the Var and the Vésubie

When a river flow rate is known on one point, it is possible to determine the one of upstream tributaries. This can be done by measuring the concentrations of a dissolved element (for example Ca²⁺), provided that no chemical precipitations have occurred in mixture waters. In this later case, the determination of flow rate is however possible by isotopic geochemistry, by knowing isotopic ratios of an element, for ex. ²³⁴U/²³⁸U, in each river considered and the ratio of the concentrations in dissolved ²³⁸U in two of them. A study of the Var and its tributary the Vésubie is performed: in January 31st, 2001, the flow rate D₃, downstream the confluence was 35 m³/s. The ratio of the ²³⁸U concentrations in the Vésubie river and in the Var upstream (²³⁸U₁/²³⁸U₂) was 5.5. The isotopic ratios ²³⁴U/²³⁸U measured in the Var, downstream the confluence were R₃ = 1.51, upstream R₂ = 1.75, and in the Vésubie R₁ = 1.08. We have calculated the Vésubie flow rate as D₁ = 3.2 m³/s.