Ion-pair reversed-phase and low-molecular-weight aqueous gel permeation high-performance liquid chromatography methods for the determination of amoxicillin oligomers

Summary Polymeric substances formed from concentrated sodium amoxicillin in an aqueous solution have been separated using two high-performance liquid chromatographic methods. We used a C₁₈ reversed-phase column with tetrabutylammonium chloride as an ion-pairing agent with an acetonitrile gradient and a TSKgel G2500PWxl column with water as the solvent for gel permeation chromatography. The separated materials, ranging in size from the monomer to the tetramer, have been characterized by functional-group chemical analysis, while the identification of the piperazine-2,5-dione was done using a pure standard. A greater number of peaks which were also better defined were obtained using the ion-pair reversed-phase method, and open and closed beta-lactam ring polymer forms could be distinguished. Using the gel permeation method, only a few monomer, piperazine-2,5-dione, dimer, trimer and combined amoxicillin trimer and tetramer peaks were obtained with water, although those obtained were quite well defined. The data on the time-course of formation of the oligomers and the amoxicillin degradation product were virtually identical by both methods. In some countries, particularly the UK, the name of the compound is amoxycillin.     

Pressure dependence of energy gaps within local density functional formalism

From a precise first principles evaluation of pressure within local density functional formalism a detailed study of the pressure dependence of the energy gaps has been made, for prototype semiconductor Si. It is shown that although the correct gaps for semiconductors cannot be obtained within local-density functional their pressure variation is still well accounted for. In particular $\text{dE}{}_{\mathrm{g}}\text{dP}$ for the minimum gap is very well reproduced. This suggests that the proposed correction Δ=E_g-ε_g, between the minimum gap E_g and the exact density functional Kohn-Sham gap, ε_g, determined by derivative discontinuities of the exchange correlation energy across the gap is pressure independent (assuming that local density functional gives a good approximation to ε_g).     

A concurrent algorithm for parallel calculation of eigenvalues and eigenvectors of real symmetric matrices

Elementary Jacobi Rotations are used as the basic tools to obtain eigenvalues and eigenvectors of arbitrary real symmetric matrices. The proposed algorithm has a complete concurrent structure, that is: every eigenvalue-eigenvector pair can be obtained in any order and in an independent way from the rest. Examples based on diagonally dominant real symmetric matrices are given.     

Ionic transport across lipid bilayers activated by cryptands and alkyl-cryptands

The ionic flux of Na⁺ K⁺ ions across monoolein and lecithin bilayers, separating two aqueous electrolyte solutions, has been measured in the presence of 5-decyl-4,7,13,16,21-pentaoxa-1,10-diazabicyclo-(8,8,5)-tricosane, 5-decyl-4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo(8,8,8)-hexacosane and their parent compounds 4,7,13,16,21-pentaoxa-1,10-diazabicyclo(8,8,5)-tricosane, 4,7,13,16,21,24-hexaoxa-1,10-diaza-bicyclo-(8,8,8)-hexacosane. Under the effects of an electric field, a significant increase of the membrane conductance, proportional to the macrocycle and/or to the electrolyte concentration, was observed. In spite of the high ion selectivity of both classes of cryptands, related to the different stability constants of the metal ion–macrobicyclic receptor complexes, alkyl cryptands show a greater efficiency to activate the ion transport across the lipid membranes than their parent compounds. Such a result has been explained taking into account the different hydrophobicity of the macrobicyclic compounds. Information on the ionic mobility and on the diffusion coefficient of the ionic species moving across the membrane have been obtained analyzing the experimental data in terms of the carrier mechanism based on the Eyring theory.     

Dimeric palladium complexes with bridging aryl groups: when are they stable?

Stable dimeric palladium(II) complexes of general formula [Pd(2)(mu-R)(2)(eta(3)-allyl)(2)] (R=haloaryl, mesityl) have been prepared. Their X-ray crystal structures, determined for some of the complexes, show that the two coordination square planes are usually coplanar. The haloaryl complexes are fluxional in solution, showing exchange between cis and trans isomers (relative to the orientation of the two allyl groups in the dimer) through solvent-assisted associative bridge splitting. A number of other ancillary ligands (O,O, S,S, or C,N donors) failed to stabilize the bridging situation. Also, bridging phenyls were unstable. The reasons for this behavior and the formation of alternative compounds in attempts at synthesizing them are fully analyzed and explained. Stable aryl bridges seem to be favored by a combination of factors: the use of ancillary ligands of small size and lacking electron lone pairs, and the use of aryl ligands reluctant to homo and hetero C--C coupling. These seem to be more important factors in the stabilization of bridging aryl complexes than the strength of the bridges themselves.     

X- and W-band EPR and Q-band ENDOR studies of the flavin radical in the Na+ -translocating NADH:quinone oxidoreductase from Vibrio cholerae

Na(+)-NQR is the entry point for electrons into the respiratory chain of Vibrio cholerae. It oxidizes NADH, reduces ubiquinone, and uses the free energy of this redox reaction to translocate sodium across the cell membrane. The enzyme is a membrane complex of six subunits that accommodates a 2Fe-2S center and several flavins. Both the oxidized and reduced forms of Na(+)-NQR exhibit a radical EPR signal. Here, we present EPR and ENDOR data that demonstrate that, in both forms of the enzyme, the radical is a flavin semiquinone. In the oxidized enzyme, the radical is a neutral flavin, but in the reduced enzyme the radical is an anionic flavin, where N(5) is deprotonated. By combining results of ENDOR and multifrequency continuous wave EPR, we have made an essentially complete determination of the g-matrix and all major nitrogen and proton hyperfine matrices. From careful analysis of the W-band data, the full g-matrix of a flavin radical has been determined. For the neutral radical, the g-matrix has significant rhombic character, but this is significantly decreased in the anionic radical. The out-of-plane component of the g-matrix and the nitrogen hyperfine matrices are found to be noncoincident as a result of puckering of the pyrazine ring. Two possible assignments of the radical signals are considered. The neutral and anionic forms of the radical may each arise from a different flavin cofactor, one of which is converted from semiquinone to flavohydroquinone, while the other goes from flavoquinone to semiquinone, at almost exactly the same redox potential, during reduction of the enzyme. Alternatively, both forms of the radical signal may arise from a single, extremely stable, flavin semiquinone, which becomes deprotonated upon reduction of the enzyme.     

Geranyl Functionalized Materials for Site-Specific Co-Immobilization of Proteins

Different materials containing carboxylic groups have been functionalized with geranyl-amine molecules by using an EDC/NHS strategy. Chemical modification of the support was confirmed by XRD, UV-spectrophotometer, and FT-IR. This geranyl-functionalized material was successfully applied for four different strategies of site-selective immobilization of proteins at room temperature and aqueous media. A reversible hydrophobic immobilization of proteins (lipases, phosphoglucosidases, or tyrosinase) was performed in neutral pH in yields from 40 to >99%. An increase of the activity in the case of lipases was observed from a range of 2 to 4 times with respect to the initial activity in solution. When chemically or genetically functionalized cysteine enzymes were used, the covalent immobilization, via a selective thiol-alkene reaction, was observed in the presence of geranyl support at pH 8 in lipases in the presence of detergent (to avoid the previous hydrophobic interactions). Covalent attachment was confirmed with no release of protein after immobilization by incubation with hydrophobic molecules. In the case of a selenium-containing enzyme produced by the selenomethionine pathway, the selective immobilization was successfully yielded at acidic pH (pH 5) (89%) much better than at pH 8. In addition, when an azido-enzyme was produced by the azide–homoalanine pathway, the selective immobilization was successful at pH 6 and in the presence of CuI for the click chemistry reaction.     

Arrhythmogenesis in the heart: Multiscale modeling of the effects of defibrillation shocks and the role of electrophysiological heterogeneity

The mechanisms of initiation of ventricular arrhythmias as well as those behind the complex spatiotemporal wave dynamics and its filament organization during ventricular fibrillation (VF) are the topic of intense research and debate. Mechanistic inquiry into the various mechanisms that lead to arrhythmia initiation and VF maintenance is hampered by the inability of current experimental techniques to resolve, with sufficient accuracy, electrical behavior confined to the depth of the ventricles. The objective of this article is to demonstrate that realistic 3D simulations of electrical activity in the heart are capable of bringing a new level of understanding of the mechanisms that underlie arrhythmia initiation and subsequent organization. The article does this by presenting the results of two multiscale simulation studies of ventricular electrical behavior. The first study aims to uncover the mechanisms responsible for rendering the ventricles vulnerable to electric shocks during a specific interval of time, the vulnerable window. The second study focuses on elucidating the role of electrophysiological heterogeneity, and specifically, differences in action potential duration in various ventricular structures, in VF organization. Both studies share common multiscale modeling approaches and analysis, including characterization of scroll-wave filament dynamics.     

Synthesis,spectral characterization and cytotoxicity of Ru–bipyridyl complexes containing hexakis(pyrazol-1-yl)benzene (hpzb) as a co-ligand

A novel ruthenium bisbipyridine complex, [Ru(bpy)₂(hpzb)](PF₆)₂ (1) (hpzb = hexakis(pyrazol-1-yl)benzene) was obtained in the reaction between [Ru(bpy)₂Cl₂], the tritopic ligand hpzb and NH₄PF₆. A high selectivity has been found in the reaction and when the hpzb ligand was made to react with more than one ruthenium fragment, it coordinated selectively only to the first metallic fragment, and it was not possible to introduce two or three ruthenium centres. A similar complex with a deuterated bipyridine has also been obtained. The reaction with the methylated ligand hexakis(3,5-dimethylpyrazol-1-yl)benzene does not take place. A complete assignment of all the proton and carbon NMR signals of 1 was carried out. The orientation of the free pyrazolyl groups is also discussed. The redox properties and the anticancer activity of complex 1 have been studied.