Native Electrospray Mass Spectrometry of DNA G-Quadruplexes in Potassium Solution

A commonly used electrolyte in electrospray mass spectrometry (ESI-MS) of biomolecules is ammonium acetate (NH₄OAc). Although some nucleic acid structures such as duplexes require only proper physiological ionic strength (whatever the monovalent ions) to be properly folded in ESI-MS conditions, the folding of some other nucleic acid structures such as DNA G-quadruplexes also depends on direct binding of specific cations. Here, we developed ESI-MS compatible conditions that allow one to observe DNA G-quaduplexes with K⁺ ions specifically bound between G-quartets. NH₄OAc was replaced with trimethylammonium acetate (TMAA), at concentrations up to 150 mM to provide physiological ionic strength, and the solution was doped with KCl at concentrations up to 1 mM. The trimethylammonium ion is too large to coordinate between G-quartets, where only K⁺ ions bind. Compared with the equivalent NH₄OAc/KCl mixtures, the TMAA/KCl mixtures provide cleaner spectra by suppressing the nonspecific adducts, and favor the formation of similar stacking arrangements as in 100 mM KCl (physiologically relevant cation) for the polymorphic human telomeric DNA G-quadruplexes. This new sample preparation method can be exploited to determine the number of potassium binding sites in new sequences, to screen ligand binding to the structures favored in potassium, and to transfer potassium-bound G-quadruplexes to the mass spectrometer for gas-phase structural probing, as illustrated herein with ion mobility spectrometry experiments.

Synthesis of benzocyclobutenes by palladium-catalyzed C-H activation of methyl groups: method and mechanistic study

An efficient catalytic system has been developed for the synthesis of benzocyclobutenes by C-H activation of methyl groups. The optimal conditions employed a combination of Pd(OAc) 2 and P ( t )Bu 3 as catalyst, K 2CO 3 as the base, and DMF as solvent. A variety of substituted BCB were obtained under these conditions with yields in the 44-92% range, including molecules that are hardly accessible by other methods. The reaction was found limited to substrates bearing a quaternary benzylic carbon, but benzocyclobutenes bearing a tertiary benzylic carbon could be obtained indirectly from diesters by decarboxylation. Reaction substrates bearing a small substituent para to bromine gave an unexpected regioisomer that likely arose from a 1,4-palladium migration process. The formation of this "abnormal" regioisomer could be suppressed by introducing a larger subsituent para to bromine. DFT(B3PW91) calculations on the reaction of 2-bromo-tert-butylbenzene with Pd(P ( t )Bu 3) with different bases (acetate, bicarbonate, carbonate) showed the critical influence of the coordination mode of the base to induce both an easy C-H activation and to allow for a pathway for 1,4-palladium migration. Carbonate is shown to be more efficient than the two other bases because it can abstract the proton easily and at the same time maintain kappa (1)-coordination without extensive electronic reorganization.     

Characterization and applications of reversed-phase column selectivity based on the hydrophobic-subtraction model

A total of 371 reversed-phase columns have now been characterized in terms of selectivity, based on five solute-column interactions (the hydrophobic-subtraction model). The present study illustrates the use of these data for interpreting peak-tailing and column stability. New insights are also provided concerning column selectivity as a function of ligand and silica type, and the selection of columns for orthogonal separations is re-examined. Some suggestions for the quality control of reversed-phase columns during manufacture are offered.     

A phosphonated triarylmethyl radical as a probe for measurement of pH by EPR

A new water soluble phosphonated tetrathiatriarylmethyl radical has been synthesized and its application for pH measurement in a physiological range by EPR is reported.     

Synthesis of CeO2@SiO2 core-shell nanoparticles by water-in-oil microemulsion. Preparation of functional thin film

Synthesis of nanoparticles under restricted environment offered by water-in-oil (W/O) microemulsions provides excellent control over particle size and shape, and inter-particle spacing. Such an environment has been involved to synthesize silica nanoparticles with a CeO2 core, so-called CeO2@SiO2. Aqueous fluids made up of ceria nanoparticles with a size close to 5 nm have been used as the water phase component. The starting CeO2 sols and obtained CeO2@SiO2 nanoparticles have been characterized by dynamic light scattering (DLS), X-ray diffraction, scanning and transmission electron microscopy, and specific surface area measurements. The microemulsion process has been characterized by DLS. Preliminary results on CeO2@SiO2 thin films are presented.     

Para-hydroxybenzoic acid as an indirect detection buffer in capillary zone electrophoresis

Summary Para-hydroxybenzoic acid (p-HBA) has been used as indirect UV detection buffer in capillary zone electrophoresis (CZE). Being an UV-absorbing dibasic acid, p-HBA provides both the necessary buffering for pH control over a wide range and UV absorbance for indirect detection. With sodium dodecyl sulfate (SDS) as a probe, a CZE method using p-HBA solution as running buffer was developed to analyze anions, especially ones with low electrophoretic mobilities. The method was used to separate homologous series of sulfonates, SDS in a formulation sample, and SDS in a standard.     

Copper‐Chelating Azides for Efficient Click Conjugation Reactions in Complex Media

The concept of chelation‐assisted copper catalysis was employed for the development of new azides that display unprecedented reactivity in the copper(I)‐catalyzed azide–alkyne [3+2] cycloaddition (CuAAC) reaction. Azides that bear strong copper‐chelating moieties were synthesized; these functional groups allow the formation of azide copper complexes that react almost instantaneously with alkynes under diluted conditions. Efficient ligation occurred at low concentration and in complex media with only one equivalent of copper, which improves the biocompatibility of the CuAAC reaction. Furthermore, such a click reaction allowed the localization of a bioactive compound inside living cells by fluorescence measurements.     

5‐aminothiazolium salts as potential cyclic azomethine ylides: Base‐induced cycloaddition reactions with aryl,alkyl, and benzoyl isothiocyanates

Reactions of the in situ generated thiazoles 2 with aryl and alkyl isothiocyanates appear to be totally regioselective and give the unexpected 5‐(phenylthio)imidazolium‐4‐thiolates 3 . Such rapid interconversion of mesoionic compounds is explained by a 1,3‐dipolar addition to the C=N bond of the heterocumulene followed by tBuNCS elimination. Similar interactions with benzoyl isothiocyanate exclusively proceed on the C=S unsaturation of the heteroallene moiety and produce the 4‐(phenylthio)thiazolium‐5‐amidines 12 . Structural assignment of isolated imidazoles and thiazoles is based on ¹³C NMR data and chemically confirmed by the NaBH₄ reduction of the alkylated derivatives 5 and 13 . Efforts to isomerize the starting mesoionic thiazole 2a without the use of tBuNCS are unsuccessful. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 10:16–26, 2000     

A New Route for the Modification of Halogen End Groups to Amino End‐Functionalized Poly(tert‐butyl acrylate)s

The synthesis of primary amine end‐functional poly(tert‐butyl acrylate)s has been achieved by using the Gabriel reaction. Polymerization of tert‐butyl acrylate was first achieved by atom transfer radical polymerization using ethyl‐2‐bromoisobutyrate or paramethoxyphenyl‐2‐bromoisobutyrate as initiator. Both resulting polymers, with a bromide‐end atom, were converted into phthalimido intermediates which then were successfully hydrolyzed using potassium hydroxide in tert‐butyl alcohol to result in poly(tert‐butyl acrylate)s terminated by a primary amine function. End group interconversions were followed by ¹H NMR, FT‐IR, and MALDI‐TOF MS measurements. All the results proved that quantitative transformations were achieved at each step. Moreover, the method developed is very easy to carry out.