Covalent Schiff base catalysis and turnover by a DNAzyme: a M2+ -independent AP-endonuclease mimic

A DNAzyme, synthetically modified with both primary amines and imidazoles, is found to act as a M2+ -independent AP lyase-endonuclease. In the course of the cleavage reaction, this DNAzyme forms a covalent Schiff base intermediate with an abasic site on a complementary oligodeoxyribonucleotide. This intermediate, which is inferred from NaCNBH3 trapping as well as cyanide inhibition, does not evidently accumulate because the second step, dehydrophosphorylative elimination, is fast compared to Schiff base formation. The 5'-product that remains linked to the catalyst hydrolyzes slowly to regenerate free catalyst. The use of duly modified DNAzymes to perform Schiff base catalysis demonstrates the value of modified nucleotides for enhancing the catalytic repertoire of nucleic acids. This work suggests that DNAzymes will be capable of catalyzing aldol condensation reactions.     

A simple and versatile method for the synthesis of acetals from aldehydes and ketones using bismuth triflate

Acetals are obtained in good yields by treatment of aldehydes and ketones with trialkyl orthoformate and the corresponding alcohol in the presence of 0.1 mol % Bi(OTf)3.4H2O. A simple procedure for the formation of acetals of diaryl ketones has also been developed. The conversion of carbonyl compounds to the corresponding 1,3-dioxolane using ethylene glycol is also catalyzed by Bi(OTf)3.4H2O (1 mol %). Two methods, both of which avoid the use of benzene, have been developed.     

Solution properties of poly p-isopropyl α-methylstyrene

Two series (S and F) of poly p-isopropyl α-methylstyrene were characterized by viscometry, light scattering (LS), osmometry and gel permeation chromatography (GPC). S-samples were prepared by bulk anionic polymerization whereas F-samples were obtained on fractionation by preparative GPC of a polymer also prepared in bulk but with a different initiator. The K parameter in the Mark-Houwink relation was 5.55 × 10^?4 for series-S and 4.50 × 10^?4 for series-F, whereas the exponent a was 0.55 for both series. The Mark-Houwink equations for series-S and -F hold good at 25 and 30° for both toluene and tetrahydrofuran as solvents and have been corrected for polydispersity. The chain dimensions obtained with LS were much higher than those computed from the viscosity data indicating that the hydrodynamic theories in their present state should not be used for the calculation of chain dimensions. The deviations from the GPC universal calibration curve of polystyrene observed with some samples were attributed to structural differences among the polymers and to their high polydispersity.     

Experimental studies on the applicability of the Kelvin equation to highly curved concave menisci

The thermodynamic properties of liquids trapped in microscopic pores are described in theory by the Kelvin equation, which relates the equilibrium meniscus curvature to the relative vapor pressure. We report here two series of experiments designed to test the validity of the Kelvin equation by direct measurement of the mean radius of curvature of the surface of cyclohexane condensed between crossed mica cylinders. In one series of experiments, the relative vapor pressure of the volatile cyclohexane was controlled by mixing it with a relatively involatile solute (n-dodecane or n-hexadecane). We found that the mean radius of curvature rapidly reached that predicted by the Kelvin equation at each relative vapor pressure of the volatile liquid, but that there was also a slow, but continuous, accumulation of the “involatile” solute at the point of condensation as the system approached true equilibrium. Such accumulation of very low vapor pressure materials may be one factor responsible for the discordant results reported by earlier workers. We find that the process of impurity buildup is complex, and suggest that studies of real porous systems may be affected by accumulation of “involatile” impurities through the vapor phase and by surface diffusion. The other series of experiments was designed to eliminate the impurity problem by maintaining the vapor pressure by temperature control of the pure liquid. The results from this series of experiments were not time dependent, and no evidence of contamination was found. The measured radii were within ±6% of those predicted by the Kelvin equation, for radii in the range 4–20 nm. We conclude that the thermodynamic basis of the Kelvin equation is valid in principle for menisci with radii as low as 4 nm.     

The total space-time of a point charge and its consequences for black holes

Singularities associated with an incomplete space-timeS are not uniquely defined until a boundaryB is attached to it. [The resulting space-time-with-boundary, ¯S S B, will be termed a total space-time (TST).] Since an incomplete spacetime is compatible with a variety of boundaries, it follows thatS does not represent a unique universe, but instead corresponds to a family of universes, one for each of its distinct TSTs. It is shown here that the boundary attached to the Reissner-Nordström space-time for a point charge is invalid forq ²<m ². When the correct boundary is used, the resulting TST is inextendible. This implies that the Graves-Brill black hole cannot be produced by gravitational collapse. The same is true of the Kruskal-Fronsdal black hole for the point mass, and for those black holes which reduce to the latter for special values of their parameters.     

H+, Na+ and K+ modulated lanthanide luminescent switching of Tb(III) based cyclen aromatic diaza-crown ether conjugates in water

The cyclen based aromatic diaza-15-crown-5 and 18-crown-6 ether conjugates 1Tb-4Tb were designed as luminescent switches for sodium and potassium where the delayed Tb(III) emission, occurring as line-like emission bands between 490-622 nm, was 'switched on' upon recognition of these ions in pH 7.4 buffered water solution.     

SOLUBILIZATION OF PHOTOSYNTHETIC PIGMENT-PROTEIN COMPLEXES

Studies utilizing fractionation of photosynthetic pigment-protein complexes from the chloroplast thylakoid membrane often employ dodecylsulfate at a concentration of 10 mg mL^?1 to disrupt membrane structure prior to electrophoretic fractionation of the complexes. We investigated the effect of varying dodecylsulfate concentration on the solution/air interfacial surface tension in the absence and presence of the same concentrations of thylakoid membranes used by four different fractionation systems that have been commonly employed to fractionate photosynthetic pigment-protein complexes. Concentrations of dodecylsulfate in the range5–10 mg mL^?1, normally utilized to treat thylakoids prior to fractionation, were effective in reducing the interfacial surface tension to levels equivalent to control solutions without added thylakoid membranes. However, thylakoid membranes treated with these concentrations of dodecylsulfate are not resolved into discrete pigment-protein complexes when subjected to electrophoresis on an agarose gel, and do not produce significant amounts of pigment-containing complexes with a molecular size < 100 000 as measured by filtration with size-exclusion membranes. We conclude that many surfactant systems empirically developed to fractionate photosynthetic pigment-protein complexes may not fully solubilize the complexes prior to the electrophoretic step.