The application of the genetic algorithm to the minimization of potential energy functions

We adapted the genetic algorithm to minimize the AMBER potential energy function. We describe specific recombination and mutation operators for this task. Next we use our algorithm to locate low energy conformation of three polypeptides (AGAGAGAGA, A9, and [Met]-enkephalin) which are probably the global minimum conformations. Our potential energy minima are –94.71, –98.50, and –48.94 kcal/mol respectively. Next, we applied our algorithm to the 46 amino acid protein crambin and located a non-native conformation which had an AMBER potential energy 150 kcal/mol lower than the native conformation. This is not necessarily the global minimum conformation, but it does illustrate problems with the AMBER potential energy function. We believe this occurred because the AMBER potential energy function does not account for hydration.     

Bond energies and attachments sites of sodium and potassium cations to DNA and RNA nucleic acid bases in the gas phase

Gas-phase metal affinities of DNA and RNA bases for the Na(+) and K(+) ions were determined at density functional level employing the hybrid B3LYP exchange correlation potential in connection with the 6-311+G(2df,2p) basis set. All the molecular complexes, obtained by the interaction between several low-lying tautomers of nucleic acid bases and the alkali ions on the different binding sites, were considered. Structural features of the sodium and potassium complexes were found to be similar except in some uracil and thymine compounds in which the tendency of potassium ion toward monocoordination appeared evident. B3LYP bond energies for both metal ions were in agreement with the available experimental results in the cases of uracil and thymine for which the most stable complex was obtained starting from the most stable tautomer of the free nucleic acid base. For adenine, although the interaction of the ions with the most stable free tautomer generated the least stable molecular complex, the best agreement with experiment was found in just this case. For the remaining cytosine and guanine bases, our calculations indicated that the metal ion affinity value closest to experiment should be determined taking into account the role played by the different tautomers of the free bases with similar energy and all the possible complexes obtained by them.     

Synthesis,molecular structure and stereoisomerization of 2-phosphinyl and 2-phosphonylethyl diorganotin halides

Functionally substituted triorganotin halides V–IX of type R₂Sn(X)(CH₂)₂P(O)PhR′ (R = Me, t-Bu; Rt? = OEt, t-Bu; X = Cl, Br) have been synthesized by halogen cleavage of the corresponding tetraorganotin compounds R₂R²Sn(CH₂)₂P(O)PhR′ (R² = Me or Ph), I–IV. The solid state structure of Me₂Sn (Br) (CH₂)₂P(O)PhBu-t (IX), determined by X-ray diffraction, shows a distorted trigonal-bipyramidal structure at the tin atom, with intramolecular coordination of the PO group. Spectroscopic data are in agreement with such a structure in solution for compounds V–IX. Upon varying the temperature, concentration or solvent in solutions of compounds V–IX a stereoisomerization is observed. On the basis of NMR ¹H, ¹³C, ³¹P, ¹¹⁹Sn), IR and conductivity studies, it is suggested that this stereoisomerization involves a hexacoordinated transition state at the tin atom.     

Total synthesis of cirsimarin and evidence of its lipolytic and antilipogenic activities on human adipocytes

A straightforward procedure leading to the flavone cirsimaritin and its 4′-O-β-d-glycosylation to afford cirsimarin, its corresponding flavone glucopyranoside, is described. The biological properties of cirsimarin, measured on human adipocytes, showed potent effects on both lipolysis and lipogenesis. Cirsimarin could therefore be considered as an efficient tool in the struggle against excessive adipose tissue deposition and as a potential candidate in the treatment of obesity.     

How to reduce the experimental time in isotopic 2H NMR using the ERETIC method

Deuterium site-specific natural isotope abundance (Ai) is routinely measured by NMR spectroscopy and quantified using an internal reference peak. This study demonstrates that the substitution of the chemical compound used as internal reference by an electronically generated signal (the ERETIC method) allows a dramatic reduction of the experimental time. Measurements of Ai on eight samples of methylsalicylate have been performed using either an internal reference (TMU) or ERETIC. No significant difference in accuracy or precision had been found between the two methods. However, because the method is applicable in partially saturated conditions, the experiment time was divided by a factor of 4 and the drawbacks associated with an internal chemical reference were avoided when ERETIC was used.     

Ueber den Einfluß von Salzen auf den Färbungsvorgang

Ohne Zusammenfassung     

UV-MALDI-TOF mass spectrometry analysis of heparin oligosaccharides obtained by nitrous acid controlled degradation and high performance anion exchange chromatography

Nitrous acid degradation of heparin followed by high-performance anion-exchange chromatography (HPAEC) separation and ultraviolet matrix assisted laser desorption/ionization time-of-flight (UV-MALDI-TOF) analysis led to the structural determination of six sulfated oligosaccharides. Three different matrices (α-cyano-4-hydroxycinnamic acid (CHCA), nor-harmane, and dihydroxybenzoic acid (DHB)) have been used, and the complementary results obtained allowed in most cases to assign the position of sulfate groups. Based on the different cleavages produced on the purified oligosaccharides in source during the MS analysis by the use of the different matrices, this approach provides a new tool for structural analysis.     

The synthesis and spectroscopic characterisation of hydrotalcite formed from aluminate solutions

Raman spectroscopy has been used to characterise nine hydrotalcites prepared from aluminate and magnesium solutions (magnesium chloride and seawater). The aluminate hydrotalcites are proposed to have the following formula Mg(6)Al(2)(OH)(16)(CO(3)(2-))·xH(2)O, Mg(6)Al(2)(OH)(16)(CO(3)(2-),SO(4)(2-))·xH(2)O, and Mg(6)Al(2)(OH)(16)(SO(4)(2-))·xH(2)O. The synthesis of these hydrotalcites using seawater results in the intercalation of sulfate anions into the hydrotalcite interlayer. The spectra have been used to assess the molecular assembly of the cations and anions in the hydrotalcite structures. The spectra have been conveniently subdivided into spectral features based upon the carbonate anion, the hydroxyl units and water units. This investigation has shown the ideal conditions to form hydrotalcite from aluminate solutions is at pH 14 using a magnesium chloride solution at a volumetric ratio of 1:1. Changes in synthesis conditions resulted in the formation of impurity products aragonite, thenardite, and gypsum.     

Rapid communicationPhotoelectron emission in femtosecond laser assisted scanning tunneling microscopy

Direct illumination of the tunneling gap in an ultrahigh vacuum scanning tunneling microscope with ultrashort pump-probe laser pulses may offer ultimate spatial and temporal resolution in surface experiments. The electronic bandwidth of the tunneling gap ( 1 THz) does not limit the time resolution. Our experiments show that multiphoton photoelectron emission from the sample limits the application of this detection scheme at high laser fluence. However, a substrate specific pump-probe effect in the photoelectron yield with femtosecond transients is observed on Tantalum and on GaAs(110) surfaces. Received: 5 November 1996