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Solvent Water (solvent + water)
Terms modified by Solvent Water Selected AbstractsA simple method to obtain a swollen PVA gel crosslinked by hydrogen bondsJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2009Emiko Otsuka Abstract A simple method to obtain a physically crosslinked poly(vinyl alcohol) (PVA) hydrogel is reported. In this technique, the PVA solution in pure water was simply cast at room temperature without using any additional chemical. The gelation proceeded during the dehydration after casting the PVA solution into a mold. After the completion of gelation, the swelling ratio of the gel in its equilibrium was measured whenever the solvent water was repeatedly exchanged. The weight gradually decreased due to the elution of non-crosslinked polymers into the solvent, and became constant after sufficient water exchange. The measurements using a Fourier Transform infrared spectroscopy and an X-ray diffraction suggested that the crosslinks due to hydrogen bonds and microcrystals were formed during the dehydration process of the PVA solution. We concluded that the sample obtained by the present method is a physically crosslinked polymer network, insoluble in water, i.e., a swollen gel in water. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 [source] Production of Reactive Oxygen Species on Photolysis of Dilute Aqueous Quinone SolutionsPHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2007Shikha Garg ABSTRACT We have examined the generation of the reactive oxygen species (ROS) superoxide and hydrogen peroxide (H2O2) by irradiation of dilute aqueous solutions of disodium anthraquinone-2-6-disulfonate (AQDS) with simulated sunlight. Irradiating a solution of AQDS in 2 mM NaHCO3 and 0.01 M NaCl produced superoxide and H2O2 at nanomolar concentrations. Experiments in which initial concentrations of dioxygen, H2O2, the superoxide radical trap nitroblue tetrazolium and the electron donor dimethyl sulfoxide were varied suggested that the interaction of solvent water with photo-excited quinone moieties produces dioxygen-reducing radicals, and that these are the primary source of ROS in the system. A kinetic model for ROS production is proposed based on our experimental data. [source] Structures of dipeptides: the head-to-tail storyACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2010Carl Henrik Görbitz The hydrogen-bonding patterns in crystal structures of unprotected, zwitterionic dipeptides are dominated by head-to-tail chains involving the N -terminal amino groups and the C -terminal carboxylate groups. Patterns that include two concomitant chains, thus generating a hydrogen-bonded layer, are of special interest. A comprehensive survey shows that dipeptide structures can conveniently be divided into only four distinct patterns, differing by definition in the symmetry of the head-to-tail chains and amide hydrogen-bonding type, but also in other properties such as peptide conformation and the propensity to include solvent water or various organic guest molecules. Upon crystallization, the choice of pattern for a specific dipeptide is not random, but follows from the amino acid sequence. [source] Fusarium oxysporum trypsin at atomic resolution at 100 and 283,K: a study of ligand bindingACTA CRYSTALLOGRAPHICA SECTION D, Issue 1 2001Wojciech R. Rypniewski The X-ray structure of F. oxysporum trypsin has been determined at atomic resolution, revealing electron density in the binding site which was interpreted as a peptide bound in the sites S1, S2 and S3. The structure, which was initially determined at 1.07,Å resolution and 283,K, has an Arg in the S1 specificity pocket. The study was extended to 0.81,Å resolution at 100,K using crystals soaked in Arg, Lys and Gln to study in greater detail the binding at the S1 site. The electron density in the binding site was compared between the different structures and analysed in terms of partially occupied and overlapping components of peptide, solvent water and possibly other chemical moieties. Arg-soaked crystals reveal a density more detailed but similar to the original structure, with the Arg side chain visible in the S1 pocket and residual peptide density in the S2 and S3 sites. The density in the active site is complex and not fully interpreted. Lys at high concentrations displaces Arg in the S1 pocket, while some main-chain density remains in sites S2 and S3. Gln has been shown not to bind. The free peptide in the S1,S3 sites binds in a similar way to the binding loop of BPTI or the inhibitory domain of the Alzheimer's ,-protein precursor, with some differences in the S1 site. [source] Stable conformations of 12-crown-O3N and its Li+ complex in aqueous solutionJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2002Kenzi Hori Abstract Stable conformations of 12-crown-O3N and its Li complexes in aqueous solution were investigated. To calculate the free energy differences of conformers of 12-crown-O3N and its Li+ complex, our procedure was to make use of two programs, CONFLEX and BOSS. The former generates conformers, and the latter calculates the differences in free energy of solvation between two conformers in aqueous solution. It was confirmed that the present procedure is applicable in solving the question of what is the most stable conformation of 12-crown-O3N in aqueous solution. Results of the calculations suggest that the order of stability for conformers in a vacuum is different from that in aqueous solution. It was also confirmed that the coordination geometry of solvent waters to Li+ changes depending on the distance between the cation and the crown ring. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1226,1235, 2002 [source] Computational Study of the Phosphoryl Transfer Catalyzed by a Cyclin-Dependent KinaseCHEMISTRY - A EUROPEAN JOURNAL, Issue 30 2007Marco De, Vivo Dr. Abstract A cyclin-dependent kinase, Cdk2, catalyzes the transfer of the ,-phosphate from ATP to a threonine or serine residue of its polypeptide substrates. Here, we investigate aspects of the reaction mechanism of Cdk2 by gas-phase density functional calculations, classical molecular dynamics, and Car,Parrinello QM/MM simulations. We focus on the role of the conserved Asp127 and on the nature of the phosphoryl transfer reaction mechanism catalyzed by Cdk2. Our findings suggest that Asp127 is active in its deprotonated form by assisting the formation of the near-attack orientation of the substrate serine or threonine. Therefore, the residue does not act as a general base during the catalysis. The mechanism for the phosphoryl transfer is a single SN2-like concerted step, which shows a phosphorane-like transition state geometry. Although the resulting reaction mechanism is in agreement with a previous density functional study of the same catalytic reaction mechanism (Cavalli et,al., Chem. Comm.2003, 1308,1309), the reaction barrier is considerably lower when QM/MM calculations are performed, as in this study (,42,kcal,mol,1 QM vs. ,24,kcal,mol,1 QM/MM); this indicates that important roles for the catalysis are played by the protein environment and solvent waters. Because of the high amino acid sequence conservation among the whole family of cyclin-dependent kinases (CDKs), these results could be general for the CDK family. [source] Telomerization of acrylic acid with mercaptans: Part 2.POLYMER INTERNATIONAL, Issue 4 2001Kinetics of the synthesis of star-shaped macromolecules of acrylic acid Abstract The telomerization of acrylic acid with a polyfunctional transfer agent initiated by 2,2,-azobisisobutyronitrile, was first investigated in an organic medium (THF) at 65,°C. Transfer constants (CT) for three mercaptans, ethylene glycol bisthioglycolate, trimethylolpropane tris(2-mercaptoacetate) and pentaerythritol tetrakis(3-mercaptopropionate) of about 6.1, 6.3 and 7.3, respectively, were determined. From these results, it followed that a well-defined star poly(acrylic acid) could not be synthesized via telomerization of acrylic acid in THF. Nevertheless, the same study was performed with the tetrafunctional transfer agent in water/THF mixtures. This work emphasized that the nature of the solvent plays an important role in determining the transfer constant. Thus, the value of CT for the tetrafunctional transfer agent decreased from 7.3 in THF to 1, the ,ideal' case for telomerization (CT,,,1), for the mixture of solvents water/THF (80%, 20%, v/v). With this route, it seems that acrylic acid star-shaped macromolecules could be synthesized via telomerization. © 2001 Society of Chemical Industry [source] | |||||||||||||