Electronic Excited States Responsible for Dimer Formation upon UV Absorption Directly by Thymine Strands: Joint Experimental and Theoretical Study

The study addresses interconnected issues related to two major types of cycloadditions between adjacent thymines in DNA leading to cyclobutane dimers (T<>Ts) and (6-4) adducts. Experimental results are obtained for the single strand (dT)(20) by steady-state and time-resolved optical spectroscopy, as well as by HPLC coupled to mass spectrometry. Calculations are carried out for the dinucleoside monophosphate in water using the TD-M052X method and including the polarizable continuum model; the reliability of TD-M052X is checked against CASPT2 calculations regarding the behavior of two stacked thymines in the gas phase. It is shown that irradiation at the main absorption band leads to cyclobutane dimers (T<>Ts) and (6-4) adducts via different electronic excited states. T<>Ts are formed via (1)ππ* excitons; [2 + 2] dimerization proceeds along a barrierless path, in line with the constant quantum yield (0.05) with the irradiation wavelength, the contribution of the (3)ππ* state to this reaction being less than 10%. The formation of oxetane, the reaction intermediate leading to (6-4) adducts, occurs via charge transfer excited states involving two stacked thymines, whose fingerprint is detected in the fluorescence spectra; it involves an energy barrier explaining the important decrease in the quantum yield of (6-4) adducts with the irradiation wavelength.     

Influence of the initial chemical conditions on the rational design of silica particles

The influence of the water content in the initial composition on the size of silica particles produced using the Stöber process is well known. We have shown that there are three morphological regimes defined by compositional boundaries. At low water levels (below stoichiometric ratio of water:tetraethoxysilane), very high surface area and aggregated structures are formed; at high water content (>40?wt%) similar structures are also seen. Between these two boundary conditions, discrete particles are formed whose size are dictated by the water content. Within the compositional regime that enables the classical Stöber silica, the structural evolution shows a more rapid attainment of final particle size than the rate of formation of silica supporting the monomer addition hypothesis. The clearer understanding of the role of the initial composition on the output of this synthesis method will be of considerable use for the establishment of reliable reproducible silica production for future industrial adoption.

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The Charging of Micellar Nanoparticles in Electrospray Ionization

Charging of nanoparticles through electrospray has scarcely been explored. Spherical nanometer‐sized amphiphilic block copolymer nanoparticles with diameters ranging from ～65 to ～150 nm were electrosprayed and analysed by charge detection spectrometry. Herein, we explore the charging of these micellar nano‐objects by conducting a thorough study in different solvents, including pure water, and upon the addition of “supercharging” agents. The charge (z) of micellar nanoparticles electrosprayed from water solution is compared to the Rayleigh’s limiting charge (z_R) of a charged water droplet of the same dimensions. An average ratio (z/z_R) of 0.6–0.65 is observed for the micellar macro‐ions, supporting the charge residue mechanism, where the number of charges available to the micellar macro‐ion is limited by the number of charges on the nanodroplet, which is a function of the surface tension of the solvent. Also we show the possibility of increasing the charging of micellar nanoparticles in the negative mode by adding organic bases (in particular piperidine) to water/methanol solutions.     

Interfacial behaviour of wheat puroindolines: monolayers of puroindolines at the air–water interface

Puroindolines are among the major basic and cysteine-rich lipid binding proteins of wheat seeds. The interfacial properties of puroindoline-a (PIN-a) and puroindoline-b (PIN-b) are important both from a biological and a technological point of view. In the work reported here, the interfacial characteristics of spread monolayers of wheat puroindolines at the air–water interface were studied at varying subphase compositions using a Langmuir–Blodgett film balance. The compression isotherms (π–A _Sp) were recorded at constant barrier speed (3.3 cm/min). It was observed that both PIN-a and PIN-b form stable monolayers at the air–water interface. The stability of the monolayers was found to be dependent on the subphase composition as well as on the concentration of protein in the spreading solution. When the ionic strength of the subphase is below 0.50, the compression isotherms of both PIN-a and PIN-b remains unaffected with the change in the ionic strength of the subphase; however, when the ionic strength is above 0.50, the compression isotherms of both PIN-a and PIN-b undergo significant change with an increase in the ionic strength of the subphase. A gradual increase in the values of the collapse pressure (π_C) and the limiting area (A ₀) was observed due to an increase in the ionic strength of the subphase from 0.5 to 4.0, which may be correlated with the salt-induced conformational changes of the protein molecule. The presence of NaCl and KCl (ionic strength 1.0) in the subphase has a comparable effect on the compression isotherms of both PIN-a and PIN-b; however, the presence of CaCl₂ (ionic strength 1.0) in the subphase leads to an increase in the values of π_C and A ₀. A change in the pH of the subphase from 3.0 to 7.2 was to have a significant effect on the values of π_C and A ₀, which may be due to the pH-induced alteration of the protein conformation. Received: 8 August 2000 Accepted: 15 December 2000     

ChemInform Abstract: Synthesis and Structural Characterization of ACu9Tt4 (A: Ca,Sr, Ba,Eu; Tt: Si,Ge, Sn) — Tetragonally Distorted Ternary Variants of the Cubic NaZn13 Structure Type. Improved Structure Refinement of SrCu2Ge2.

The title compounds are synthesized from the elements using excess Cu and Si, Ge, or Sn as a flux (alumina crucibles, 1100 °C, 2 h).     

Resolution of 8-aminonaphthalene-1,3,6-trisulfonic acid-labeled glucose oligomers in polyacrylamide gel electrophoresis at low gel concentration

A discontinuous Tris-Cl/acetate (OAc) buffer system, unprecedently containing OAc as the trailing constituent, and operative in polyacrylamide gel electrophoresis (PAGE) at low polyacrylamide concentration (T = 4.8%) is described in the paper. The characteristics of the electrophoretic system are illustrated by the resolution of fluorescent 8-aminonaphthalene-1,3,6-trisulfonic acid (ANTS)-labeled malto-oligosaccharides and dextran homopolymers. In this buffer system, the resolving phase is constituted by Tris-OAc behind a moving boundary formed between the leading chloride ion of Tris-HCl gel buffer and the trailing OAc ion provided by a catholyte of NH(4)OAc. In contrast with the results obtained with Tris-CI/glycinate buffer commonly used in electrophoresis, or with Tris-CI/borate, the best resolution of the glucose oligomers containing 1-4 glucose units in Tris-OAc, pH 8.8, ionic strength of 0.08, was obtained at 4.8% polyacrylamide concentration, using 0.5 M NH(4)OAc, pH 9.5 as the catholyte. Under those conditions, the ANTS-glucose oligomers were separated with mobilities decreasing from glucose to maltohexaose. The linear Ferguson plots (log relative mobility, R(f), vs.%T) of the glucose oligomers show that the surface net charge of those oligomers is inversely related to their sizes, given by the slopes, K(R), of the plots. The molecular weight of the oligomers is directly but nonlinearly related to K(R). The novel electrophoretic system illustrated here for separation of short ANTS-saccharides can be potentially applied to the resolution of other biomolecules such as rapidly migrating DNA, peptides or proteins.     

Global approach for the selection of high temperature comprehensive two-dimensional gas chromatography experimental conditions and quantitative analysis in regards to sulfur-containing compounds in heavy petroleum cuts

Extending the knowledge on sulfur-containing compounds is crucial for the petroleum industry because they contribute to atmospheric pollution by combustion. Most of them are concentrated in heavy petroleum cuts, such as vacuum gas oils (VGOs). However, the resolution of the existing analytical methods does not allow a quantitative speciation of S-compounds contained in VGOs. Therefore, a high temperature GC×GC chromatograph hyphenated to a SCD was implemented in this study to obtain a quantitative S-compounds speciation. Firstly, various thermally stable stationary phases, in particular the new ionic liquid IL59 and Mega Wax-HT, were investigated in 1D-GC as a way to reduce the number of columns sets to be used in GC×GC. Consequently, several normal and reversed configurations of these columns were selected and tested in GC×GC. Then, a decision method was applied to facilitate the choice of the best combination of columns. Finally, the most adapted methods led to an innovative group type quantification and to a quantitative distribution of heavy sulfur species contained in a VGO sample. These results represent a major step towards the study of S-compounds in heavy petroleum cuts.     

Liquid chromatography-electrospray ionization mass spectrometry study of the flavonoids of the roots of Astragalus mongholicus and A. membranaceus

High-performance liquid chromatography-electrospray ionization mass spectrometry has been applied to analyze the flavonoids of Huangqi, the roots of Astragalus mongholicus and A. membranaceus. Eight flavonoids were identified as calycosin-7-O-beta-D-glucoside, calycosin-7-O-beta-D-glucoside-6"-O-malonate (2), ononin, (6aR,11aR)-3-hydroxy-9,10-dimethoxypterocarpan-3-O-bet a-D-glucoside, calycosin, (3R)-7,2'-dihydroxy-3',4'-dimethoxyisoflavan-7-O-beta-D-glucoside, formononetin-7-O-beta-D-glucoside-6"-O-malonate and formononetin by direct comparison with the isolated standards from Huangqi. The existence of (6aR,11aR)-3-hydroxy-9,10-dimethoxypterocarpan, (3R)-7,2'-dihydroxy-3',4'-dimethoxyisoflavan, astrapterocarpanglucoside-6'-O-malonate and astraisoflavanglucoside-6'-O-malonate was detected. This is the first report of flavonoid glycoside malonates in these two Astragalus species, and malonate 2 is a structurally completely identified new compound.