Hydrophobic hydration measurement

In order to see how the hydrophobic hydration breaks down when an aprotic cosolvent is added to water and how it starts to contribute when water is added to the aprotic solvent, we measured the enthalpies of solution of hydrophobic and, for comparison, hydrophilic solutes in mixed solvent systems of water and... 

The term hydrophobic hydration means the first of the above, that is, the interaction between one solute moleeule and the surrounding water molecules. Hydrophobic hydration can be quantified by measuring the free energy of transfer of a non-polar molecule from its neat liquid state to water. It is straightforward to... 

Understanding hydrophobic hydration requires an estimate of the chemical potential of the non-polar solute in water. Actually, one measures the change in chemical potential as the non-polar solute is transferred from its own liquid to water. This quantity is related to the hydropathy scale discussed earlier in the context of protein folding. 

With the protein-based polymer poly [0.8GVGVP),0.2GEGVP)], at low pH when aU 4 of the Glu (E) residues/lOO residues are as COOH, the transition temperature is near 25° C and the heat of the transition, AH, = 0.97kcal/mole-pentamer (see Rgure 5.28). On raising the pH to the point of less than two C00"/100 residues, the heat of the transition has been reduced, and AH, = 0.27kcal/mole pentamers. The preferred interpretation over a decade ago was (1) that the formation of 2 C00 /100 residues structured almost three-fourths of the thermodynamically measured waters of hydrophobic hydration, and (2) that there exists a competition for hydration between apolar and polar residues, referred to as an apolar-polar repulsive free energy of hydration. 

The relationship between the states of the myosin II motor domain, subfragment 1 (SI), and waters of hydrophobic hydration has been directly observed by the same microwave dielectric relaxation measurements as used with the data in Figures 5.24 and 5.25. Suzuki and coworkers have observed the ADP-bound state of SI to have more water of hydrophobic hydration than the ADP Pj state, that is, than the most polar state. Thus, the effects of the additional negative charges are to destroy water of hydrophobic hydration and to raise the value of T, above physiological temperatures. The removal of the phosphate would result in an increase in A and a lowering of T, below the physiological temperature with the consequence of the association of paired... 

Hence, S decreases with increasing p. Thermal expansion, (dVIdT), is less positive for more strongly structured water it implies that (dSldp)f is less negative for water in hydrophobic hydration layer as compared to bulk water. The hydrophobic effect, as measured by the entropy, increases upon hydrophobic dehydration and decreases with increasing pressure. 

The knowledge of reliable thermodynamic data of saturated and aromatic hydrocarbons in water is very useful in several practical and theoretical fields, f.i. it is essential for the developments of the concepts of the hydrophobic hydration and of the hydrophobic interaction largely used for the understanding of some important biological phenomena. So new experimental data of high quality are necessary, having the elaborations of the existing material reached their limits. In the last years instead new solubility data in pure water have been reported only for methane(10,53a) ethane(53), cyclo-propane(54), n-butane(9,53a) and benzene(17). Enthalpies of hydration have been furthermore determined by direct calorimetric measurements only for benzene and some derivatives, cyclohexane, pentane and hexane(29). 

The difference between these two series of amines is likely to arise principally from the hydrophobic hydration effects. If Evans and Hamann s results are correct they suggest that in ethylamine the extent of hydrophobic hydration may not be much greater than in methylamine, or may persist in part in the cations, since the j3-carbon atom is an appreciable distance from the charge centre near the nitrogen atom. Fyfe s measurements, on the contrary, suggest that this hydrophobic hydration atmosphere is dispersed in the cation. Further measurements on these compounds are obviously required. 

The colloid probe technique was first applied to the investigation of surfactant adsorption by Rutland and Senden [83]. They investigated the effect of a nonionic surfactant petakis(oxyethylene) dodecyl ether at various concentrations for a silica-silica system. In the absence of surfactant they observed a repulsive interaction at small separation, which inhibited adhesive contact. For a concentration of 2 X 10 M they found a normalized adhesive force of 19 mN/m, which is small compared to similar measurements with SEA and is probably caused by sufactant adsorption s disrupting the hydration force. The adhesive force decreased with time, suggesting that the hydrophobic attraction was being screened by further surfactant adsorption. Thus the authors concluded that adsorption occurs through... 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

The present example does support some speculation on an outstanding puzzle for our conceptualization of hydrophobic effects. It is known that the sum of the standard hydration entropies of K+(aq) and Cl (aq) is about twice the standard hydration entropy of Ar(aq) [67]. The case of methanol as the solvent is qualitatively different. If hydrophobic effects are conceptualized on the basis of hydration entropies and specific hydration structures then this is paradoxical according to the measured entropies K+(aq) + Cl (aq) is about as hydrophobic as Ar(aq) + Ar(aq), but the hydration structures neighboring K+(aq), Cl (aq), and Ar(aq) should be qualitatively different. This paradox has not been given a satisfactory explanation. 

DNA immobilized by organosilane chemistries has proven to be an effective method for measuring impedance changes upon hybridization using both gold and platinum electrodes [21,50]. The effect of different silane chemistries creates differences in the hydrophobicity and hydration levels of the modified surface. The organosilane treatment along with the ssDNA... 

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