Determination of acid-base dissociation constants of amino- and guanidinopurine nucleotide analogs and related compounds by capillary zone electrophoresis

CZE has been applied for determination of acid-base dissociation constants (pKa) of ionogenic groups of newly synthesized amino- and (amino)guanidinopurine nucleotide analogs, such as acyclic nucleoside phosphonate, acyclic nucleoside phosphonate diesters and other related compounds. These compounds bear characteristic pharmacophores contained in various important biologically active substances, such as cytostatics and antivirals. The pKa values of ionogenic groups of the above compounds were determined by nonlinear regression analysis of the experimentally measured pH dependence of their effective electrophoretic mobilities. The effective mobilities were measured by CZE performed in series of BGEs in a broad pH range (3.50-11.25), at constant ionic strength (25 mM) and temperature (25 degrees C). pKa values were determined for the protonated guanidinyl group in (amino)guanidino 9-alkylpurines and in (amino)guanidinopurine nucleotide analogs, such as acyclic nucleoside phosphonates and acyclic nucleoside phosphonate diesters, for phosphonic acid to the second dissociation degree (-2) in acyclic nucleoside phosphonates of amino and (amino)guanidino 9-alkylpurines, and for protonated nitrogen in position 1 (N1) of purine moiety in acyclic nucleoside phosphonates of amino 9-alkylpurines. Thermodynamic pKa of protonated guanidinyl group was estimated to be in the range of 7.75-10.32, pKa of phosphonic acid to the second dissociation degree achieved values of 6.64-7.46, and pKa of protonated nitrogen in position 1 of purine was in the range of 4.13-4.89, depending on the structure of the analyzed compounds.     

Oscillatory reactions involving hydrogen peroxide and thiosulfate-kinetics of the oxidation of tetrathionate by hydrogen peroxide

The reaction between tetrathionate and hydrogen peroxide forms an important part of several pH oscillators based on the oxidation of thiosulfate. The kinetics of this reaction were examined in a batch reactor by measurement of the initial pH values in the range from 8 to 10.5. Experimental data were evaluated by the method of initial reaction rates combined with the assumption of instantaneously equilibrated acid-base reactions. The rate-determining step was found to be of the first order with respect to tetrathionate, hydrogen peroxide, and OH- ions with the value of rate constant k = (1.50 +/- 0.03) x 10(2) (M2 s)(-1) at 25 degrees C. In the alkaline solution, no distinct catalytic effect of Cu2+ was observed in contrast to earlier assumptions. The waveform of measured pH over the course of the reaction indicates that thiosulfate is probably an intermediate because characteristics of the curves are very similar to those for the oxidation of thiosulfate. We also measured the time evolution of concentrations of major components by the attenuated total internal reflectance infrared spectroscopy to further elucidate the underlying reaction mechanism. These measurements confirm the suspected role of thiosulfate as an intermediate in the studied reaction and provide valuable detailed information on the time evolution of thiosulfate, sulfite, sulfate, tetrathionate, and trithionate. These experimental observations are included in a simple mechanism that accurately simulates the reaction course in an alkaline solution. The results provide considerable new insights into the nature of autocatalysis in the hydrogen peroxide-thiosulfate-Cu2+ reaction and suggest that a new role for Cu2+ in the oscillatory dynamics observed in a flow-through reactor needs to be found.     

Deposition of nanosized latex particles onto silica and cellulose surfaces studied by optical reflectometry

Deposition of positively charged nanosized latex particles onto planar silica and cellulose substrates was studied in monovalent electrolyte solutions at pH 9.5. The deposition was probed in situ with optical reflectometry in a stagnation point flow cell. The surface coverage can be estimated reliably with island film theory as well as with a homogeneous film model, as confirmed with atomic force microscopy (AFM). The deposition kinetics on the bare surface was of first order with respect to the particle concentration, whereby the deposition rate was close to the value expected for a perfect collector. The efficiency coefficient, which was defined as the ratio of the experimental and theoretical deposition rate constants, was in the range from 0.3 to 0.7. Subsequently, the surface saturated and a limiting maximum coverage was attained (i.e., blocking). These trends were in qualitative agreement with predictions of the random sequential absorption (RSA) model, where electrostatic interactions between the particles were included. It was observed, however, that the substrate strongly influenced the maximum coverage, which was substantially higher for silica than for cellulose. The major conclusion of this work was that the nature of the substrate played an important role in a saturated layer of deposited colloidal particles.     

Reaction of 3‐aminoquinoline‐2,4‐diones with isocyanates. Synthesis of novel 3‐(3′‐alkyl/arylureido)quinoline‐2,4‐diones and their cyclic carbinolamide isomers

3‐Alkyl/aryl‐3‐amino‐1H,3H‐quinoline‐2,4‐diones react with alkyl/aryl isocyanates to give novel 3‐alkyl/aryl‐3‐ureido‐1H,3H‐quinoline‐2,4‐diones or 3a‐alkyl/aryl‐9b‐hydroxy‐3,3a,5,9b‐tetrahydro‐1H‐imidazo[4,5‐c]quinoline‐2,4‐diones. In some cases, a mixture of both products was obtained and separated by fractional crystallization. All compounds were characterized by their ¹H, ¹³C, ir and ms data and some of them also by ¹⁵N nmr data.     

Molecular rearrangement of 1‐substituted 3‐aminoquinoline‐2,4‐diones in their reaction with urea and nitrourea synthesis and transformations of reaction intermediates

1‐Substituted 3‐alkyl/aryl‐3‐amino‐1H,3H‐quinoline‐2,4‐diones ( 6 ) react with nitrourea to give 3‐ureido‐1H,3H‐quinoline‐2,4‐diones ( 10 ), 9b‐hydroxy‐3,3a,5,9b‐tetrahydro‐1H‐imidazo[4,5‐c]quinoline‐2,4‐diones ( 11 ), and 3,3a‐dihydro‐5H‐imidazo[4,5‐c]quinoline‐2,4‐diones ( 12 ). Compounds 11 were dehydrated to 12 by the action of phosphorus pentoxide. All three types of compounds rearrange in boiling acetic acid to give three different types of products of molecular rearrangement. A proposed reaction mechanism is discussed.     

Preparation of highly substituted 6-arylpurine ribonucleosides by Ni-catalyzed cyclotrimerization. Scope of the reaction

Transition metal complex catalyzed cocyclotrimerization of protected alkynylpurine ribonucleosides 1 with various diynes 2 gave rise to a series of 6-arylpurine nucleosides 3 that were further deprotected to free nucleosides 4. Generally, the best yields of cyclotrimerizations were obtained with a catalytic system Ni(cod)2/2PPh(3). On the other hand, CoBr(PPh(3))3 proved to be a superior catalyst for cyclotrimerization of 1 with dipropargyl ether 2g. In addition, Ni catalysis is also suitable for direct cyclotrimerization of unprotected alkynylpurine ribonucleosides 5 to the corresponding 6-arylpurinylribosides 4.     

New modular and efficient approach to 6-substituted pyridin-2-yl C-nucleosides

A novel modular, efficient, and practical methodology of preparation of 6-substituted pyridin-2-yl C-nucleosides was developed. An addition of 2-lithio-6-bromopyridine 2b to TBDMS-protected 2-deoxyribonolactone 5 gave aduct 7 as an equilibrium mixture of anomeric hemiketals 1-(6-bromopyridin-2-yl)-1-hydroxynucleosides 7a,b and its open form 7c. Reduction of the adduct 7 with Et3SiH and BF3 x Et2O afforded the desired 6-bromonucleoside 8a as pure beta-anomer in a total yield of 32% over two steps from 5. Intermediate 8a was then subjected to a series of palladium catalyzed cross-coupling reactions and aminations to give a series of protected 1beta-(6-alkyl-, 6-aryl-, and 6-aminopyridin-2-yl)-2-deoxyribonucleosides 9. Catalytic hydrogenation of 8a gave an unsubstituted pyridine C-nucleoside, and diazotative oxodeamination of 6-aminopyridine nucleoside 9f by isopentyl nitrite in acetic acid gave 6-oxopyridine nucleoside 10i. Deprotection of silylated nucleosides 9 by Et3N.3HF gave a series of free C-nucleosides 10.     

Asymmetric modular synthesis of highly functionalized medium-sized carbocycles and lactones via ring-closing metathesis of sulfoximine-substituted trienes

A modular asymmetric synthesis of medium-sized carbocycles and lactones has been developed that affords highly substituted 7-, 9-, and 11-membered rings. The key steps are (1) the highly diastereoselective synthesis of sulfoximine-substituted homoallylic alcohols from allylic sulfoximines and unsaturated as well as saturated aldehydes, (2) an E-stereoselective alkylation and hydroxyalkylation of sulfoximine-substituted alkenyllithium derivatives, (3) the esterification of sulfoximine-substituted homoallylic alcohols, and (4) the ring-closing metathesis reaction of sulfoximine-substituted trienes with the ruthenium catalyst 8. Two examples for the further synthetic elaboration of the sulfoximine-substituted carbocycles are provided. The selective cleavage of the tert-butyldimethylsilyl group of 12 in the presence of the triethylsilyl group afforded the allylic alcohol 18 which was oxidized to enone 19. A cross-coupling reaction of the sulfoximine-substituted carbocycle 9 with LiCuMe2 furnished the methyl-substituted derivative 20.     

What is the smallest possible constant in Céa’s lemma?

We consider finite element approximations of a second order elliptic problem on a bounded polytopic domain in ℝ^d with d ∈ {1, 2, 3, ...} The constant C ⩾ 1 appearing in Céa’s lemma and coming from its standard proof can be very large when the coefficients of an elliptic operator attain considerably different values. We restrict ourselves to regular families of uniform partitions and linear simplicial elements. Using a lower bound of the interpolation error and the supercloseness between the finite element solution and the Lagrange interpolant of the exact solution, we show that the ratio between discretization and interpolation errors is equal to as the discretization parameter h tends to zero. Numerical results in one and two-dimensional case illustrating this phenomenon are presented. This research was supported by Shandong Province Young Scientists Foundation of China 2005BS01008, Institutional Research Plan AV02 101 90503, and by Grant No A 1019201 of the Academy of Sciences of the Czech Republic.     

In vivo monitoring the changes of interstitial pH and FAD/NADH ratio by fluorescence spectroscopy in healing skin wounds

The aim of our study was to evaluate the changes of interstitial pH and flavin adenine dinucleotide (FAD)/reduced nicotinamide adenine dinucleotide (NADH) ratio in healing skin wounds using fluorescence spectroscopy in Sprague Dawley rats. In the experiment, excisional and incisional models of wound healing were used. The florescein as the pH-sensitive probe using excitation spectra (lambda(Em) = 535 nm) was used for the measurement of pH changes, and synchronous fluorescence spectra (Deltalambda = 60 nm) for the monitoring of FAD/NADH ratio changes were measured from the surfaces of healing wounds. Increase of interstitial pH and FAD/NADH ratio was recorded during the time interval from the 15th to the 65th minute after surgery. The decrease of pH between the 48th and the 72nd hour after surgery as well as the increase of FAD/NADH ratio between the 72nd and the 96th hour of wound healing were recorded. The results indicate that the use of fluorescence spectroscopy may be considered as a valuable tool for noninvasive in vivo monitoring of selected redox parameters in the early phases of wound healing.