Carbon and proton magnetic resonance studies on the sites of Mn2+ and Ni2+ ion binding with thiamin and thiamin pyrophosphate and on the solution conformation of the coenzyme

Carbon magnetic resonance was employed to determine the Mn²⁺ and Ni²⁺ binding sites on thiamin and thiamin pyrophosphate and to help elucidate the solution conformation of the latter with Mn²⁺. In comparison with previous proton magnetic resonance studies, the results here presented elucidate direct binding to thiamin even without the presence of the pyrophosphate. In thiamin Mn²⁺ is bound to both the pyrimidine ring and to the oxygen of the β-hydroxyethyl chain, albeit more weakly to the former, Ni²⁺, on the other hand is indicated in binding to the pyrimidine ring only. From observation of water relaxation data, potentiometric determination of the pyrimidinium pK_a with metal ions added and carbon magnetic resonance (competition of thiamin and thiamin pyrophosphate for Mn²⁺) we have very strong evidence to indicate that Mn²⁺ is separated from the pyrimidine ring by a solvent molecule. We were able to estimate association constant between the pyrimidine and Mn²⁺ (50 M^?1) and OH and Mn²⁺ (100 M^?1) in thiamin.In thiamin pyrophosphate both Mn²⁺ and Ni²⁺ much prefer the pyrophosphate as binding sites and at least in the former an inner sphere complex is indicated. Also with Mn²⁺ present the dimethylene pyrophosphate is folded back onto the thiazolium ring according to analysis of proton broadening data and relaxation mechanisms.     

Uptake of cesium, strontium and europium by a poly(sodium acrylate-acrylic acid) hydrogel

The uptake of cesium, strontium and europium from dilute nitric acid solutions by a poly(sodium acrylate-acrylic acid) PAA hydrogel has been investigated. pH variations are consistent with cation exchange processes: COO(-), Na (+)H (+), COO(-), Na (+)M (m+) ( M (m+) = Cs (+)and Sr (2+)) and COOH Eu (3+). Saturation of the gel is achieved for metal/carboxylate ratios R = 0.5. The swelling ratios of gels loaded with metal cations are those of uncharged, shrunk gels (Sr, Eu) or of charged, swollen gels (Cs) in agreement with the formation of uncharged (COO)(2)Sr, (COO)(2)EuX (X = NO(3) or OH) type complexes and (COO(-), Cs(+)) ion pairs. The metal cations are extracted in the gels following the order of their affinities with carboxylic groups Eu(3+) > Sr(2+) > Cs(+). An increase of the ionic strength of the metal aqueous solution up to 0.5M NaNO(3) leads to slightly decrease the europium uptake by the PAA hydrogel, but 0.1M NaNO(3) is sufficient to prevent the Sr and Cs extractions.     

Star-block copolymers as templates for the preparation of stable gold nanoparticles

Gold nanoparticles of improved stability against aggregation were prepared using poly(ethylene oxide)-block-poly(epsilon-caprolactone) (PEO-b-PCL) star-block copolymers. A five-arm star-shaped macroinitiator (PEO) was utilized for the automated parallel controlled ring-opening polymerization of epsilon-caprolactone to prepare a series of PEO-b-PCL star-block copolymers with a constant PEO core linked to PCL blocks of variable length. The PEO core was swelled with KAuCl4 in N,N-dimethylformamide (DMF), and gold nanoparticles were subsequently obtained by reduction with NaBH4. Since the process was always templated by the same PEO core for all investigated polymers, the average dimension of the formed gold nanoparticles was in the same range for all star-block copolymers. In sharp contrast, the size distribution and long-term stability against aggregation of the gold nanoparticles dispersed in DMF were strongly dependent on the PCL block length, confirming the role of PCL blocks as stabilizing blocks for these nanoparticles.     

A Novel Synthesis of 1‐Aryl‐6‐bromo‐3‐(5‐ethylthio‐4‐phenyl‐4H‐1,2,4‐triazol‐3‐yl)‐1H‐pyrazolo[4,3‐b]quinolin‐9(4H)‐ones via Thermal Cyclization of 4‐Azidopyrazoles

2‐Aryl‐hydrazononitriles 3a , 3b , 3c were prepared by coupling 3‐ethylthio‐5‐cyanomethyl‐4‐phenyl‐1,2,4‐triazole ( 1 ) with diazonium salts 2a , 2b , 2c . Reacting 3a , 3b , 3c with both ethyl bromoacetate ( 4a ) and 4‐bromobenzyl bromide ( 4b ) in DMF, in the presence of K₂CO₃, at 80 °C for 3–4 h, gave the corresponding 4‐amino‐pyrazoles 6a , 6b , 6c , 6d , 6e , 6f . Diazotization of 6a , 6b , 6c , 6d , 6e , 6f , followed by reaction with NaN₃, leads to the formation of 4‐azidopyrazoles 8a , 8b , 8c , 8d , 8e , 8f , a new heterocyclic ring system. Interestingly, fusion of 4‐azidopyrazoles 8d , 8e , 8f at temperature higher than their melting points with 5 °C for 2 min did not give the expected fused pyrazolo[4,3‐c]isoxazoles 9 but furnished instead the novel pyrazolo[4,3‐b]quinolinones 10a , 10b , 10c , in high yields.     

[Et3N-SO3H][MeSO3] as a highly efficient catalyst for the production of pyrido[2,3-d:6,5-d′]dipyrimidines and bis(pyrazolyl)methanes

Research on Chemical Intermediates - In this research, highly efficient protocols for the production of pyrido[2,3-d:6,5-d′]dipyrimidine and bis(pyrazolyl)methane derivatives using an acid...     

Use of 6‐Methylisoxanthopterin,a Fluorescent Guanine Analog,to Probe Fob1‐Mediated Dynamics at the Stalling Fork Barrier DNA Sequences

Fluorescent nucleic acid base mimics serve as excellent site‐specific and real‐time reporters of the local and global dynamics. In this work, using the fluorescent guanine mimic 6‐methylisoxanthopterin (6‐MI), we unravel the differential dynamics of replication fork barrier/terminator sequences (RFB1 and RFB3) mediated by fork blocking protein (Fob1). By strategic and site‐specific incorporation of this probe, we show that 6‐MI is able to capture the changes in global dynamics exhibited by Fob1 and aids in distinguishing between varied architectural forms like double‐stranded DNA versus Holliday junctions (HJs). This is important as these barriers are hotspots for recombination. Fluorescence lifetime and anisotropy decay studies further revealed that Fob1 strongly dampens the dynamics in double‐stranded RFB1, and the sequence inherently possesses lesser flexibility in comparison to RFB3. We show that 6‐MI can probe the differential oligomeric status of Fob1 in response to various architectures, that is, double‐stranded versus HJs. This work highlights the unique advantages of 6‐MI as a probe when incorporated in nucleic acid frameworks.     

Efficient Synthesis of Polyhydroquinolines via the Hantzsch Reaction Using Iron Loaded Mesoporous Materials

A simple and convenient protocol is described for the preparation of polyhydroquinolines via one‐pot, multi component reaction of an aromatic aldehyde, ethylacetoacetate, ammonium acetate and dimedone using iron loaded mesoporous materials as an efficient catalyst in good yields.     

The assessment of π-π selective stationary phases for two-dimensional HPLC analysis of foods: Application to the analysis of coffee

Differences between alkyl, dipole-dipole, hydrogen bonding, and π-π selective surfaces represented by non-resonance and resonance π-stationary phases have been assessed for the separation of ‘Ristretto’ café espresso by employing 2DHPLC techniques with C18 phase selectivity detection. Geometric approach to factor analysis (GAFA) was used to measure the detected peaks (N), spreading angle (β), correlation, practical peak capacity (n_p) and percentage usage of the separations space, as an assessment of selectivity differences between regional quadrants of the two-dimensional separation plane. Although all tested systems were correlated to some degree to the C18 dimension, regional measurement of separation divergence revealed that performance of specific systems was better for certain sample components. The results illustrate that because of the complexity of the ‘real’ sample obtaining a truly orthogonal two-dimensional system for complex samples of natural origin may be practically impossible.