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Hydrophobic Effects (hydrophobic + effects)
Selected AbstractsImplications of ideas on super-hydrophobicity for water repellent soilHYDROLOGICAL PROCESSES, Issue 17 2007G. McHale Abstract Water repellence is an important factor in soil erosion due to its role in inhibiting the re-establishment of vegetation after fire and due to its enhancement of run-off. Water repellence is studied across a range of diverse disciplines, such as chemistry, materials, textiles and soil and reclamation science. In recent years many basic studies of water repellence of materials have focused on the role of the sub-mm surface topography of a material in modifying the intrinsic hydrophobicity imparted by the surface chemistry to create super-hydrophobicity. In this report, we first illustrate the types of hydrophobic effects created by a suitable coupling of small scale surface topography with surface chemistry using three materials: an etched metal, a foam and a micro-fabricated pillar structure. These experiments demonstrate the general applicability of the ideas and suggest that they could apply to a granular material such as, a fine sandy soil, particularly when the grains have become coated with a hydrophobic layer. This applicability is confirmed by contact angle measurements of droplets of water on hydrophobic sand. A theoretical model describing the application of these ideas in a loose-packed, but regular, array of uniform spherical grains is then presented and discussed. When the grains are in a dry state initially, the effect of the surface is to increase the apparent water repellence as observed through the contact angle. However, when the spaces between the grains are filled with water, the effect is to provide greater wetting. To qualitatively confirm the enhancement of contact angle caused by the granular structure, model surfaces using 600 and 250 µm hydrophobic glass beads were created. On these surfaces, the contact angle of droplets of water was increased from 108° to 126° and 140° , respectively. Copyright © 2007 John Wiley & Sons, Ltd. [source] Cycloadditions in mixed aqueous solvents: the role of the water concentration,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2005Theo Rispens Abstract We examined the kinetics of a series of cycloaddition reactions in mixtures of water with methanol, acetonitrile and poly(ethylene glycol) (MW 1000). The reactions include the Diels,Alder (DA) reaction between cyclopentadiene and N - n -butylmaleimide or acridizinium bromide, the retro-Diels-Alder (RDA) reaction of 1,4,4a,9a-tetrahydro-4a-methyl-(1,,4,,4a,,9a,)-1,4-methaneanthracene-9,10-dione and the 1,3-dipolar cycloaddition of benzonitrile oxide with N - n -butylmaleimide. Plots of logk vs the molar concentration or volume fraction of water are approximately linear, but with a characteristic break around 40,M water. This break, absent for the RDA reaction, is ascribed to hydrophobic effects. Comparison with aqueous mixtures of the more hydrophobic 1-propanol shows that these mixtures induce qualitatively similar effects on the rate, but that preferential solvation effects cause the mixtures of 1-propanol to exhibit a more complex behavior of logk on composition. The results are analyzed using the Abraham,Kamlett,Taft model. The solvent effects in aqueous mixtures are not satisfactorily described by this model. For some cycloadditions, small maxima in rate are observed in highly aqueous mixtures of alcohols. The origin of these maxima and the aforementioned breaks is most likely the same. Copyright © 2005 John Wiley & Sons, Ltd. [source] NMR spectroscopic characterization of the membrane affinity of polyolsMAGNETIC RESONANCE IN CHEMISTRY, Issue 11 2005Daniela Fischer Abstract Residual dipolar couplings (RDCs) are applied here for the analysis of weak, transient binding events between phosphatidylcholine bilayers and polyols. Large signal responses are observed even for low percentages of ,ligand-receptor complexes,' making RDCs a sensitive tool for the analysis of molecular recognition events. The different degree of alignment in solution can be compared as a result of the calculation of the alignment tensor elements. By varying polarity and/or charge of the molecules under investigation, nonspecific hydrophobic effects can be excluded. Copyright © 2005 John Wiley & Sons, Ltd. [source] Structure and implications for the thermal stability of phosphopantetheine adenylyltransferase from Thermus thermophilusACTA CRYSTALLOGRAPHICA SECTION D, Issue 1 2004Phosphopantetheine adenylyltransferase Phosphopantetheine adenylyltransferase (PPAT) is an essential enzyme in bacteria that catalyzes the rate-limiting step in coenzyme A (CoA) biosynthesis by transferring an adenylyl group from ATP to 4,-phosphopantetheine (Ppant), yielding 3,-dephospho-CoA (dPCoA). The crystal structure of PPAT from Thermus thermophilus HB8 (Tt PPAT) complexed with Ppant has been determined by the molecular-replacement method at 1.5,Å resolution. The overall fold of the enzyme is almost the same as that of Escherichia coli PPAT, a hexamer having point group 32. The asymmetric unit of Tt PPAT contains a monomer and the crystallographic triad and dyad coincide with the threefold and twofold axes of the hexamer, respectively. Most of the important atoms surrounding the active site in E. coli PPAT are conserved in Tt PPAT, indicating similarities in their substrate binding and enzymatic reaction. The notable difference between E. coli PPAT and Tt PPAT is the simultaneous substrate recognition by all six subunits of Tt PPAT compared with substrate recognition by only three subunits in E. coli PPAT. Comparative analysis also revealed that the higher stability of Tt PPAT arises from stabilization of each subunit by hydrophobic effects, hydrogen bonds and entropic effects. [source] Modeling H3 histone N-terminal tail and linker DNA interactionsBIOPOLYMERS, Issue 2 2006Giovanni La Penna Abstract Molecular dynamics computer simulations were performed for the 25-residue N-terminal tail of the H3 histone protein in the proximity of a DNA segment of 10 base pairs (bp), representing a model for the linker DNA in chromatin. Several least biased configurations were used as initial configurations. The secondary structure content of the protein was increased by the presence of DNA close to it, but the locations of the secondary motifs were different for different initial orientations of the DNA grooves with respect to the protein. As a common feature to all simulations, the electrostatic attraction between negatively charged DNA and positively charged protein was screened by the water solvent and counterbalanced by the intrinsic compaction of the protein due to hydrophobic effects. The protein secondary structure limited the covering of DNA by the protein to 4,5 bp. The degree of compaction and charge density of the bound protein suggests a possible role of H3 tail in a nonspecific bending and plasticity of the linker DNA when the protein is located in the crowded dense chromatin. © 2006 Wiley Periodicals, Inc. Biopolymers 83: 135,147, 2006 This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source] Mapping the Limits of Substrate Specificity of the Adenylation Domain of TycACHEMBIOCHEM, Issue 4 2009Benoit R. M. Villiers Abstract The limits and potential of substrate promiscuity of the adenylation domain of tyrocidine synthetase 1 were systematically explored. Substrate acceptance is governed by hydrophobic effects (as shown by the correlation of kcat/KM and side-chain log,P), shape complementarity and steric exclusion. The quantification of these factors provides ground rules for understanding and possibly evolving substrate specificity in this class of enzymes. The catalytic potential of tyrocidine synthetase 1 (TycA) was probed by the kinetic characterization of its adenylation activity. We observed reactions with 30 substrates, thus suggesting some substrate promiscuity. However, although the TycA adenylation (A) domain was able to accommodate alternative substrates, their kcat/KM values ranged over six orders of magnitude. A comparison of the activities allowed the systematic mapping of the substrate specificity determinants of the TycA A-domain. Hydrophobicity plays a major role in the recognition of substrate analogues but can be combined with shape complementarity, conferring higher activity, and/or steric exclusion, leading to substantial discrimination against larger substrates. A comparison of the kcat/KM values of the TycA A-domain and phenylalanyl-tRNA synthetase showed that the level of discrimination was comparable in the two enzymes for the adenylation reaction and suggested that TycA was also subjected to high selective pressure. The specificity patterns observed and the quantification of alternative activities provide a basis for exploring possible paths for the future directed evolution of A-domain specificity. [source] Design of Original Bioactive Formulations Based on Sugar,Surfactant/Non-steroidal Anti-inflammatory Catanionic Self-Assemblies: A New Way of Dermal Drug Delivery,CHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2007Sabrina Consola Dr. Abstract A new kind of catanionic assembly was developed that associates a sugar-based surfactant with a non-steroidal anti-inflammatory drug (NSAID). Three different assemblies using indomethacin, ibuprofen and ketoprofen as NSAIDs were easily obtained in water by an acid,base reaction. These assemblies formed new amphiphilic entities because of electrostatic and hydrophobic effects in water and led to the spontaneous formation of vesicles. These catanionic vesicles were then tested as potential NSAID delivery systems for dermatological application. The anti-inflammatory activity was evaluated in vivo, and this study clearly showed an improved therapeutic effect for NSAIDs that were formulated as catanionic vesicles. These vesicles ensured a slower diffusion of the NSAID through the skin. This release probably increased the time of retention of the NSAID in the targeted strata of the skin. Thus, the present study suggests that this catanionic bioactive formulation could be a promising dermal delivery system for NSAIDs in the course of skin inflammation treatment. [source] Water Accessibility to the Binding Cleft as a Major Switching Factor from Entropy-Driven to Enthalpy-Driven Binding of an Alkyl Group by Synthetic ReceptorsCHEMISTRY - AN ASIAN JOURNAL, Issue 5 2010Sayaka Matsumoto Abstract Free energy, enthalpy, and entropy changes in the binding of alkyl pyridines to water-soluble zinc porphyrin receptors with varying accessibility of water to the binding cleft were determined to explain why the driving force of hydrophobic effects is enthalpic in some occasions and entropic in others. Zinc porphyrins bearing four alkyl pillars with terminal solubilizing poly(oxyethylene) (POE) chains of molecular weight of 750 (1), with eight alkyl pillars with terminal solubilizing POE chains of molecular weight of 350 (3), and with eight alkyl pillars with POE of molecular weight of 750 (4) had a binding cleft with decreasing water accessibility in this order as revealed by binding selectivity of imidazole/pyridine. Although all these porphyrins showed that the free energy of binding (,,Go) increases linearly as the alkyl group of the guest is lengthened (,,Go per CH2 was 2.6, 2.8, and 2.6,kJ,mol,1 for 1, 3, and 4, respectively), the origin of the free energy gain was much different. Receptor 1 with the most hydrophilic binding site bound the alkyl group by an enthalpic driving force (4-pentylpyridine favored over 4-methylpyridine by ,,Ho=,16.4,kJ,mol,1), while receptor 4 with the most hydrophobic binding site by an entropic driving force (4-pentylpyridine favored over 4-methylpyridine by ,,So=39.6,J,K,1,mol,1). Receptor 3 showed intermediate behavior: both enthalpic and entropic terms drove the binding of the alkyl group with the enthalpic driving force being dominant. The binding site of the four-pillared receptor (1) is open and accessible to water molecules, and is more hydrophilic than that of the eight-pillared receptor (4). We propose that the alkyl chains of 1 are exposed to water to produce a room to accommodate the guest to result in enthalpy-driven hydrophobic binding, whereas 4 can accommodate the guest without such structural changes to lead to entropy-driven hydrophobic binding. Therefore, accessibility of water or exposure of the binding site to the water phase switches the driving force of hydrophobic effects from an entropic force to an enthalpic force. [source] | ||||||||||