Decavanadate effects in biological systems

Vanadium biological studies often disregarded the formation of decameric vanadate species known to interact, in vitro, with high-affinity with many proteins such as myosin and sarcoplasmic reticulum calcium pump and also to inhibit these biochemical systems involved in energy transduction. Moreover, very few in vivo animal studies involving vanadium consider the contribution of decavanadate to vanadium biological effects. Recently, it has been shown that an acute exposure to decavanadate but not to other vanadate oligomers induced oxidative stress and a different fate in vanadium intracellular accumulation. Several markers of oxidative stress analyzed on hepatic and cardiac tissue were monitored after in vivo effect of an acute exposure (12, 24 h and 7 days), to a sub-lethal concentration (5 mM; 1 mg/kg) of two vanadium solutions ("metavanadate" and "decavanadate"). It was observed that "decavanadate" promote different effects than other vanadate oligomers in catalase activity, glutathione content, lipid peroxidation, mitochondrial superoxide anion production and vanadium accumulation, whereas both solutions seem to equally depress reactive oxygen species (ROS) production as well as total intracellular reducing power. Vanadium is accumulated in mitochondria in particular when "decavanadate" is administered. These recent findings, that are now summarized, point out the decameric vanadate species contributions to in vivo and in vitro effects induced by vanadium in biological systems.     

Model of cytosine-, thymine-containing oligodeoxyribonucleotide protonation in solution

In the present study, the experimental data on the pH-induced formation of the i-motif structure in the nucleotide sequence 5′-CCTTTCCTTTTCCTTTCC-3′ (25°C, pH 3.3-8.9) obtained by spectroscopic techniques, such as UV molecular absorption and circular dichroism, has been analyzed using the chemometric soft modeling-based MCR-ALS approach and the hard modeling-based matrix method. Soft modeling using 2 or 3 spectral species correctly reproduced the spectral variations observed experimentally. The use of hard chemical modeling has allowed proposing an equilibrium model, which describes spectral changes as functions of solution acidity. Additionally, the intrinsic protonation constant K _in and the cooperativity parameter ω have been calculated from fitting of the circular dichroism as well as the molecular absorption spectra. The results indicated that folding was accompanied by a cooperative process, i.e. the enhancement of protonated structure stability upon protonation.     

Model of cytosine-, thymine-containing olygodeoxyribonucleotide protonation in solution

In the present study, the experimental data on the pH-induced formation of the i-motif structure in the nucleotide sequence 5'-CCTTTCCTTTTCCTTTCC-3' (25oC, pH 3.3-8.9) obtained by spectroscopic techniques, such as UV molecular absorption and circular dichroism, has been analysed using the chemometric soft modelling-based MCR-ALS approach and the hard modelling-based matrix method. Soft modelling using 2 or 3 spectral species correctly reproduced spectral variations observed experimentally. The use of hard chemical modelling enabled us to propose the equilibrium model, which describes spectral changes as functions of solution acidity. Additionally, the intrinsic protonation constant Kin, and the cooperativity parameter w have been calculated from the fitting procedure of the circular dichroism as well as molecular absorption spectra. The results indicated that folding was accompanied by a cooperative process, i.e. the enhancement of protonated structure stability upon protonation.     

Heparin-polypeptide interactions in aqueous solution

Circular dichroism spectroscopy has been used to study the interactions between heparin and cationic polypeptides in dilute aqueous solution at neutral pH. The results indicate that poly(l-lysine), poly(l-arginine), and poly(l-ornithine) adopt the α-helical conformation in the presence of heparin, rather than the “charged coil” form observed for the polypeptide alone under the same conditions. Maximum interaction for the poly(l-lysine) and poly(l-ornithine) systems occur at an amino acid: disaccharide residue ratio of 2.3 ± 0.1:1, which correlates with the analytical data of 2.3 sulfates per heparin disaccharide. For poly(l-arginine), maximum interaction occurs at a residue ratio of 3.3 ± 0.1:1, and indicates that all the anionic groups (sulfate and carboxyl) of the heparin are involved in this case.The interactions of heparin are analogous to those observed previously for six connective tissue mucopolysaccharides, except that none of the latter had any effect on the conformation of poly(l-ornithine). The poly(l-ornithine)-heparin system shows a thermal “melting” transition at T_m = 56.0 ± 1.0 °C, at which point the polypeptide reverts to the “charged coil” form; the interactions with poly(l-lysine) and poly(l-arginine) are stable up to temperatures > 90 °C. The high thermal stability of these conformation-directing effects indicate a stronger interaction for heparin than the other mucopolysaccharides, which is probably due to the high sulfate content.     

Trehalose-protein interaction in aqueous solution

A variety of sugars are known to enhance the stability of biomaterials. Trehalose, a nonreducing disaccharide composed of two alpha, alpha(1 --> 1)-linked D-glucopyranose units, appears to be one of the most effective protectants. Both in vivo and in vitro, trehalose protects biostructures such as proteins and membranes from damage due to dehydration, heat, or cold. However, despite the significant amount of experimental data on this disaccharide, no clear picture of the molecular mechanism responsible for its stabilizing properties has emerged yet. Three major hypotheses (water-trehalose hydrogen-bond replacement, coating by a trapped water layer, and mechanical inhibition of the conformational fluctuations) have been proposed to explain the stabilizing effect of trehalose on proteins. To investigate the nature of protein-trehalose-water interactions in solution at the molecular level, two molecular dynamics simulations of the protein lysozyme in solution at room temperature have been carried out, one in the presence (about 0.5 M) and one in the absence of trehalose. The results show that the trehalose molecules cluster and move toward the protein, but neither completely expel water from the protein surface nor form hydrogen bonds with the protein. Furthermore, the coating by trehalose does not significantly reduce the conformational fluctuations of the protein compared to the trehalose-free system. Based on these observations, a model is proposed for the interaction of trehalose molecules with a protein in moderately concentrated solutions, at room temperature and on the nanosecond timescale.     

Decavanadate inhibits catalysis by ribonuclease A

Pentavalent organo-vanadates have been used extensively to mimic the transition state of phosphoryl group transfer reactions. Here, decavanadate (V(10)O(28)6-) is shown to be an inhibitor of catalysis by bovine pancreatic ribonuclease A (RNase A). Isothermal titration calorimetry shows that the Kd for the RNase A decavanadate complex is 1.4 microM. This value is consistent with kinetic measurements of the inhibition of enzymatic catalysis. The interaction between RNase A and decavanadate has a coulombic component, as the affinity for decavanadate is diminished by NaCl and binding is weaker to variant enzymes in which one (K41A RNase A) or three (K7A/R10A/K66A RNase A) of the cationic residues near the active site have been replaced with alanine. Decavanadate is thus the first oxometalate to be identified as an inhibitor of catalysis by a ribonuclease. Surprisingly, decavanadate binds to RNase A with an affinity similar to that of the pentavalent organo-vanadate, uridine 2',3'-cyclic vanadate.     

Conformation of allantoicase in aqueous solution

1. The separation of 0.9-S and 10.8-S allantoicase with the aid of a 2H2O-H2O gradient was described. The resulting preparations were subjected to sedimentation equilibrium, optical rotatory dispersion (ORD), circular dichroism (CD) and infrared studies. 2. The molecular weight of 0.9-S allantoicase was determined to be about 1.1 x 10(4) g/mole in studies on the sedimentation behavior, the metal content and amino acid composition. The molecular weight of 10.8-S allantoicase was about 15.4 x 10(4) g/mole. 3. Optical rotatory dispersion, circular dichroism and infrared studies indicated that both molecules contain alpha-helix, beta conformation and random coil. A Cotton effect at 418 nm was ascribed to the asymmetric binding of Mn2+ to the enzyme. Competitive inhibitors decreased the absorption and circular dichroism bands at about 280 nm and 418 nm. These phenomena suggested that the aromatic groups may play an essential role in the binding of substrates and inhibitors by the Mn(2+)-enzyme complex. 4. Comparison of alpha-helical contents of metalloallantoicases showed that the enzymes with low helical contents exhibited high enzymic activities. 5. The nearly identical physicochemical behavior and specific enzymic activity of 0.9-S and 10.8-S allantoicase indicated that they are very similar in structure and conformation.     

Equilibria of polyoxometalates in aqueous solution

In most aqueous polyoxometalate systems, numerous, often highly negatively charged species, are formed. To establish the speciation in such complex polyanion systems, many experimental methods and techniques must be used. Moreover, it is of vital importance that the experimental data are of the highest accuracy and that the data collected from different methods are treated simultaneously with an appropriate computer program. In systems containing one or more sensitive NMR nuclei, a combined EMF-NMR method has been shown to be extremely powerful.This article firstly gives some general comments on equilibria in aqueous polyoxometalate systems including ionic medium effects, and then describes an equilibrium EMF-NMR (³¹P and⁵¹V) study of the five component system H⁺-Mo(VI)-V(V)-P(V)-e^–. The study has been focused on so-called Keggin ratio solutions, Mo+V):P=12:1, since these are commonly used in selective oxidation processes. Special attention has been given to Mo₁₀V₂P and Mo₉V₃P solutions, where positional isomers occur. We have been able to identify and characterize all the five possible isomers of -Mo₁₀V₂PO ₄₀ ^5– at 25 and 90 °C. Besides the results from the five component system, some interesting findings from the binary, ternary and quarternary sybsystems are also reported.     

Diketopiperazine-mediated peptide formation in aqueous solution

Though diketopiperazines (DKP) are formed in most experiments concerning the prebiotic peptide formation, the molecules have not been paid attention in the studies of chemical evolution. We have found that triglycine, tetraglycine or pentaglycine are formed in aqueous solution of glycine anhydride (DKP) and glycine, diglycine or triglycine, respectively. A reaction of alanine with DKP resulted in the formation of glycylglycylalanine under the same conditions. These results indicate that the formation of the peptide bonds proceeds through the nucleophilic attack of an amino group of the amino acids or the oligoglycines on the DKP accompanied by the ring-opening.The formation of glycine anhydride, di-, tri- and tetraglycine was also observed in a mixed aqueous solution of urea and glycine in an open system to allow the evaporation of ammonia. A probable pathway is proposed for prebiotic peptide formation through diketopiperazine on the primitive Earth.     

Hydrophobic solvation in aqueous trifluoroethanol solution

The titration of an aqueous solution of a de novo designed peptide with trifluoroethanol (TFE) shows complete helix formation with the addition of only 30% TFE. A molecular simulation of the peptide, in which a single shell of TFE molecules initially surrounds the peptide, reveals preferred sites of solvent interaction. The TFE molecules show greater preference for the hydrophobic compared with hydrophilic side chains. The helix-enhancing ability of TFE in aqueous solution may be rationalized in terms of stabilizing the hydrophobic collapse of apolar side chains of the formed helix. © 1996 John Wiley & Sons, Inc.     

Conformation of poly-L-tyrosine in aqueous solution

The conformational transition of poly-L -tyrosine in 0.1M KCl was investigated by ORD and infrared spectroscopy, potentiometric titration, and sedimentation velocity experiments. It is shown that the fully ordered conformer is obtained by slow titration of the random coil with 0.1N HCl at 25°C. The charge-induced transition, at variance with other poly-α-amino acids, is completed in a narrow range of α. An aggregation process was detected both by potentiometric titration and sedimentation velocity. The polyamino acid aggregates around α = 0.7 at 25°C when the conformational transition is almost complete. Infrared spectra, in the region of the amide I band (1650 cm^?1) showed that the transition is a random coil → antiparallel β one. Evidence exists that the form is of the intramolecular type. The foregoing interpretations of ORD and CD spectra in terms of the α-helix conformation are discussed.     

Oxytocin solution structure changes upon protonation of the N-terminus in dimethyl sulfoxide

Summary With the combined use of various two-dimensional (2D) NMR techniques, a complete assignment of the ¹H and ¹³C resonances of oxytocin, , for two molecular states, protonated and unprotonated at the N-terminal group, was performed in dimethyl sulfoxide. A small but distinct change in the backbone conformation of the six-residue cyclic moiety, associated with the protonation, was first suggested from those NMR parameters relevant to conformation, such as change with temperature in the chemical shifts of the peptide amide protons and changes in chemical shifts and homonuclear as well as heteronuclear three-bond coupling constants. The solution structures of oxytocin for the protonated and unprotonated forms were then calculated using distance analysis in dihedral-angle space, based on a relaxation matrix evaluated from quantitative NOE intensities at different mixing times. Total amounts of 93 and 105 distances were determined for the protonated and the unprotonated forms, respectively. There were 25 interresidue distances relevant to the structure of the cyclic moiety for the protonated form of oxytocin and 43 for the unprotonated form. Overall structures with the lowest target penalty function were similar between the two forms, having a -turn structure at the endocyclic residues of the Tyr-Ile-Gln-Asn moiety. The local backbone conformations near the N-terminus, however, were significantly different between the two forms. This was found to be due to a change in the dihedral angle of the disulfide bridge (^ss around C-S-S-C), which closes the ring in the cyclic peptide. The dihedral angle was about +90° for the unprotonated form and an intermediate value of about +45° for the protonated form.     

百菌清(chlorothalonil)在水中的光化学降解

研究了光源种类、溶液pH、水温和表面活性剂对百菌清光解的影响.结果表明,百菌清水溶液在高压汞灯、紫外灯和太阳光照射下的光解半衰期分别为22.4、82.5和123.8 min;在太阳光和高压汞灯照射下,百菌清在碱性溶液中比中性和酸性溶液中光解快;随着水温的升高,百菌清光解速率加快,水温平均每升高10 ℃,光解速率大约增大1倍.表面活性剂十二烷基磺酸钠、Tween 60和Span 20对百菌清的光解表现出显著的光敏化效应,十六烷基三甲基溴化铵对百菌清光解有强烈的光猝灭效应.