Reactions of 9-substituted guanines with bromomalondialdehyde in aqueous solution predominantly yield glyoxal-derived adducts

Reactions of 9-ethylguanine, 2'-deoxyguanosine and guanosine with bromomalondialdehyde in aqueous buffers over a wide pH-range were studied. The main products were isolated and characterized by (1)H and (13)C NMR and mass spectroscopy. The final products formed under acidic and basic conditions were different, but they shared the common feature of being derived from glyoxal. Among the 1 : 1 adducts, 1,N(2)-(trans-1,2-dihydroxyethano)guanine adduct (6) predominated at pH < 6 and N(2)-carboxymethylguanine adduct (10a,b) at pH > 7. In addition to these, an N(2)-(4,5-dihydroxy-1,3-dioxolan-2-yl)methylene adduct (11a,b) and an N(2)-carboxymethyl-1,N(2)-(trans-1,2-dihydroxyethano)guanine adduct (12) were obtained at pH 10. The results of kinetic experiments suggest that bromomalondialdehyde is significantly decomposed to formic acid and glycolaldehyde under the conditions required to obtain guanine adducts. Glycolaldehyde is oxidized to glyoxal, which then modifies the guanine base more readily than bromomalondialdehyde. Besides the glyoxal-derived adducts, 1,N(2)-ethenoguanine (5a-c) and N(2),3-ethenoguanine adducts (4a-c) were formed as minor products, and a transient accumulation of two unstable intermediates, tentatively identified as 1,N(2)-(1,2,2,3-tetrahydroxypropano)(8) and 1,N(2)-(2-formyl-1,2,3-trihydroxypropano)(9) adducts, was observed.     

INCREASED UV EXPOSURE IN FINLAND IN 1993

Abstract—
Exceptionally low total ozone, up to 40% below the normal level, was measured over Northern Europe during winter and spring in 1992 and 1993. In 1993 the depletion persisted up to the end of May, resulting in a significant increase of biologically effective UV radiation. The increases were significantly smaller in 1992 and 1994 than in 1993. The UV exposure of the Finnish population was evaluated through measurements and theoretical calculations. The increase in measured erythemal (International Lighting Commission) UV falling onto horizontal surfaces on clear days was determined relative to model calculations for an average ozone amount. The increase was on average 10% from April to May 1993, and the maximal measured increase was 34%. Theoretical calculations for both erythemal and carcinogenic (Skin Cancer Utrecht-Philadelphia) UV indicated that in 1993 the theoretical annual increase to a vertical (cylinder) surface ranged from 8 to 13% in Finland. The reflection of UV from snow considerably increases facial UV doses in Northern Finland.     

Extending the language of DNA molecular recognition by polyamides: unexpected influence of imidazole and pyrrole arrangement on binding affinity and specificity

Pyrrole (Py) and imidazole (Im) polyamides can be designed to target specific DNA sequences. The effect that the pyrrole and imidazole arrangement, plus DNA sequence, have on sequence specificity and binding affinity has been investigated using DNA melting (DeltaT(M)), circular dichroism (CD), and surface plasmon resonance (SPR) studies. SPR results obtained from a complete set of triheterocyclic polyamides show a dramatic difference in the affinity of f-ImPyIm for its cognate DNA (K(eq) = 1.9 x 10(8) M(-1)) and f-PyPyIm for its cognate DNA (K(eq) = 5.9 x 10(5) M(-1)), which could not have been anticipated prior to characterization of these compounds. Moreover, f-ImPyIm has a 10-fold greater affinity for CGCG than distamycin A has for its cognate, AATT. To understand this difference, the triamide dimers are divided into two structural groupings: central and terminal pairings. The four possible central pairings show decreasing selectivity and affinity for their respective cognate sequences: -ImPy > -PyPy- > -PyIm- approximately -ImIm-. These results extend the language of current design motifs for polyamide sequence recognition to include the use of "words" for recognizing two adjacent base pairs, rather than "letters" for binding to single base pairs. Thus, polyamides designed to target Watson-Crick base pairs should utilize the strength of -ImPy- and -PyPy- central pairings. The f/Im and f/Py terminal groups yielded no advantage for their respective C/G or T/A base pairs. The exception is with the -ImPy- central pairing, for which f/Im has a 10-fold greater affinity for C/G than f/Py has for T/A.     

A split microchannel design and analytical model to compensate for electroosmotic instabilities in micro-separations

Organic polymers offer many advantages as materials for the construction of microfluidic devices but suffer frequently from the limitation that the electrodynamic flow they support can exhibit considerable instability. This article describes a split-channel microfluidic device that can be used to compensate for changes in electroosmotic flow. The design of the separation system divides an analyte plug after injection between two separation channels of differing length. The two channels are later recombined for single point detection, eliminating the need for a scanning optical detection system. The utility of this simple design lies in the fact that the migration time of any analyte can be referenced to its twin in the parallel separation channel. This eliminates the need for a separate electroosmotic marker and allows mobilities measured in multiple devices to be compared quantitatively. Using a model adopted from the literature, the data from the split channel system can be used to precisely account for the drift that characterizes electrophoretic separations made in a polymer chip. The relative standard deviations of the analyte mobilities measured for replicate runs on multiple devices were reduced from values as high as 20% to ca. 1% RSD. This internal standardization procedure also appears to address other sources of drift in the electroosmotic flow (EOF) supported by the polymer microchannel, eliminating the need for careful monitoring of either the temperature or reservoir pH between separation runs.     

Semi-rotating cryogenic modulator for comprehensive two-dimensional gas chromatography

A laboratory-made cryogenic modulator for comprehensive two-dimensional gas chromatography (GCxGC) was constructed and evaluated. Analytes were trapped with carbon dioxide and desorbed with heated GC oven air. The GCxGC system included a non-polar first-dimension column and a semi-polar second-dimension column connected to a flame ionisation detector. A laboratory-written Matlab-based program was used for the data analysis. Peak widths at half height for n-alkanes, obtained with use of new modulator, were at narrowest 60 ms.     

The effect of pretreatment of calcite dispersions with anionic sodium polyacrylate on their flocculation behavior induced by cationic starch

The flocculation performance of cationic starches on calcite pretreated with anionic sodium polyacrylate (NaPA) was investigated by measuring the mean particle size and the dynamic mobility of the calcite dispersions. Cationic starches of different molecular weight and degree of substitution were used. By varying the amount of anionic sodium polyacrylate, which has a strong affinity to the calcium carbonate surface, one is able to anionically modify the particles and reverse the charge character of the originally cationic calcium carbonate. By such modification of the charge character of the calcium carbonate dispersion, it is possible to approach the mechanisms of flocculation caused by cationic macromolecules like starch. The importance of different mechanisms of flocculation, such as bridging, charge neutralization, and flocculation induced by polyelectrolyte complexes (PEC), was further investigated in this work. It was found that when the NaPA is completely absorbed at the calcite surfaces the mechanism of the flocculation induced by the starch is mainly bridging flocculation. Excess NaPA in the calcium carbonate dispersion will result in polyelectrolyte complexes formed between the non-absorbed NaPA and the oppositely charged starch polymers. These complexes will in most cases strongly enhance the flocculation due to mainly charge neutralization. Depending on the ratio of non-absorbed NaPA and the starch in the aqueous phase, the calcite dispersion is either re-stabilized or more strongly flocculated due to the formed polyelectrolyte complexes. Both the mobility and the particle size measurements support the mechanisms described. It was further demonstrated that the molecular weight and degree of substitution of the starches might be adjusted to control the flocculation behavior.     

Crystal structure of potassium tris[3-(2-pyridyl)pyrazolyl]hydroborate

The crystal structure of the potassium tris[3-(2-pyridyl)pyrazolyl]hydroborate has been determined. Potassium tris[3-(2-pyridyl)pyrazolyl]hydroborate crystallizes in the orthorhombic space group Pca2₁ with the parameters a=30.7323(5) ?, b=8.3083(1) ? and c=9.2173(2) ?, V=2353.48(7) ?³ with Z=4. The crystal structure reveals the formation of continuous chains where two bidentate arms of the hydroborate ligand are coordinated to one potassium and the third to another cation. The structures of numerous transition metal and actinide salts have been reported during the last decades but no alkali metal complexes were available.     

Structural Studies of Self‐Folding Cavitands

Resorcinarene‐based cavitands 1a – c fold into a deep open‐ended cavity by means of intramolecular hydrogen bonds in both apolar solutions and the solid state. The X‐ray crystal‐structure analysis of cavitand 1a features a seam of secondary amide C=O⋅⋅⋅H−N interactions that bridge adjacent rings and are held in place by intra‐annular hydrogen bonds. This results in a cavity of 9.2×7.0 Å dimensions. The arrangement of the amides in 1a – 1c is cycloenantiomeric, with clock‐ and counterclockwise orientation of the head‐to‐tail amide sequence. Interconversion rates of the two enantiomers are controlled by solvent polarity: the rate is slow on the NMR time‐scale in aromatic solvents and CDCl₃, but fast in (D₆)acetone. The ¹H‐ and ¹³C‐NMR‐spectral analysis is in agreement with the crystallographic data. Chiral cavitand 1b with eight HN−C(O)−C*HMeEt ((+)‐(S)) groups on its upper rim exists as two cyclodiastereoisomers (in a ca. 3 : 1 ratio) in apolar solution. A `library' of 512 diastereoisomeric cavitands 1c is obtained as a mixture by using the corresponding racemic acid chloride.     

A kinetic study of the polymerization of pure meta-divinylbenzene and pure para-divinylbenzene

The kinetics of the polymerization of pure meta-divinylbenzene (DVB) and pure para-divinylbenzene at 70°C have been studied in the presence of toluene and 2-ethylhexanoic acid. The apparent rate constant ratios (k_p/k_t)^1/2 for these systems have been calculated. meta-Divinylbenzene polymerizes at a higher rate than the para-isomer in both toluene and 2-EHA, and the polymerization rates of meta-DVB and para-DVB before the gel point were both higher in the presence of 2-EHA than in toluene. The monomer conversion at the visual gel point is higher for para-DVB than for meta-DVB. The gel point has also been determined indirectly by size exclusion chromatography, and these results are consistent with the gel times observed visually. The conversion of pendant vinyl groups during the polymerization has been determined by bromination. It is found that the homopolymers of poly(para-DVB) have a substantially higher content of pendant vinyl groups than poly(meta-DVB) both during and at the end of the polymerization. The molecular weight distribution (MWD) prior to gelation has been determined by size exclusion chromatography (SEC). Weight average (M̄_w); and number average (M̄_n) molecular weight prior to gelation and of the sol fractions after gelation have also been measured by SEC. There are larger fractions of high molecular weight polymers prior to gelation, when the polymerization was run in the presence of toluene, than in 2-EHA, mainly due to the differences in solvating power of the two diluents. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3345–3359, 1999     

Solid‐state nuclear magnetic resonance relaxation times in crosslinked macroporous polymer particles of divinylbenzene homopolymers

Monodisperse porous particles of poly(divinylbenzene) prepared by the activated swelling method have been investigated by solid‐state ¹³C crosspolarization magic‐angle spinning (CPMAS) nuclear magnetic resonance (NMR) relaxation measurements. Homopolymeric combinations of two porogens (toluene and 2‐ethylhexanoic acid) and two monomers (meta‐ and para‐divinylbenzene) were studied. Residual vinyl groups were systematically reacted with increasing amounts of bromine, producing 20 different polymers samples for which we measured crosspolarization times, T_CH, proton rotating frame spin‐lattice relaxation, T, ¹³C spin‐lattice relaxation, T, and proton spin‐lattice relaxation, T. These parameters were chosen to reflect expected changes in a wide range of frequencies of motion as a function of structure. Relative differences in the molecular mobility of the major functional groups (aromatic, vinyl and aliphatic) is related to initial reactants used, vinyl concentration, relative reactivity of vinyl groups, distribution of vinyl groups, pore structure, and degree of crosslinking. Variable temperature ¹H combined rotation and multiple pulse NMR (CRAMPS) was used to derive activation energies for selected samples via measurement of the proton spin‐lattice relaxation time, T. Irreversible thermal effects were observed in ambient temperature relaxation after heating to temperatures in the range of 393–418 K. Simple univariate statistical analyses failed to reveal consistent correlations among the known variables. However, the application of more sophisticated multivariate and neural network analyses allowed excellent structure–property predictions to be made from the relaxation time data. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1307–1328, 1999