Pressure dependence of hydrophobic interactions

Pressure-induced denaturation of proteins and related problems are possibly linked to hydrophobicity. As a result, there has been considerable interest in studying the pressure dependence of hydrophobic interactions, highlighted by two studies. The first, by Rick, involved calculation of the pairwise 

---

Hummer, G., Garde, S., Garcia, A. E., Paulaitis, M. E., and Pratt, L. R. (1998b). The pressure dependence of hydrophobic interactions is consistent with the observed pressure denaturation of proteins. Proc. Natl. Acad. Sci. USA 95, 1552-1555. Hummer, G., Garde, S., Garcia, A. E., Pohorille, A., and Pratt, L. R. (1996). An information theory model of hydrophobic interactions. Proc. Natl. Acad. Sci. USA 93, 8951-8955. 

Hummer G, Garde S, Garca AE, Paulaitis ME, Pratt LR. The pressure dependence of hydrophobic interactions is consistent with the observed pressure denaturation of proteins. Proc. Natl. Acad. Sci. U.S.A. 1998 95 1552-1555. 

Ghosh T, Garca AE, Garde S. Enthalpy and entropy contributions to the pressure dependence of hydrophobic interactions. J. Chem. Phys. 2002 116 2480-2486. 

In Fig. 10.8 the yield increases rapidly at low reactant concentration and stabilizes at a given value depending on reaction conditions, pressure for example. This can be easily rationalized in terms of hydrophobic interactions which are highest at the saturation limit. Fig. 10.9 shows a different behavior. For diluted solutions of methacrylonitrile the medium is pseudohomogeneous. Again, the best results are obtained at the saturation limit. However, increasing the concentration of reactants beyond this limit lowers the yield of ) -aminoester. 

Salts of fatty acids are classic objects of LB technique. Being placed at the air/water interface, these molecules arrange themselves in such a way that its hydrophilic part (COOH) penetrates water due to its electrostatic interactions with water molecnles, which can be considered electric dipoles. The hydrophobic part (aliphatic chain) orients itself to air, because it cannot penetrate water for entropy reasons. Therefore, if a few molecnles of snch type were placed at the water surface, they would form a two-dimensional system at the air/water interface. A compression isotherm of the stearic acid monolayer is presented in Figure 1. This curve shows the dependence of surface pressure upon area per molecnle, obtained at constant temperature. Usually, this dependence is called a rr-A isotherm. 

The salt effects of potassium bromide and a series office symmetrical tetraalkylammonium bromides on vapor-liquid equilibrium at constant pressure in various ethanol-water mixtures were determined. For these systems, the composition of the binary solvent was held constant while the dependence of the equilibrium vapor composition on salt concentration was investigated these studies were done at various fixed compositions of the mixed solvent. Good agreement with the equation of Furter and Johnson was observed for the salts exhibiting either mainly electrostrictive or mainly hydrophobic behavior however, the correlation was unsatisfactory in the case of the one salt (tetraethylammonium bromide) where these two types of solute-solvent interactions were in close competition. The transition from salting out of the ethanol to salting in, observed as the tetraalkylammonium salt series is ascended, was interpreted in terms of the solute-solvent interactions as related to physical properties of the system components, particularly solubilities and surface tensions. 

It is possible to control the pressures at which the phase transitions occur by fine tuning the strength of intermolecular interactions between the amphiphilic molecules. The interactions between the hydrophobic tails depend on temperature [37], while the interactions between the hydrophilic heads depend on the chemical composition of the subphase, namely its pH and ionic strength [4], For example, the fatty acid molecules in films prepared on subphase with high pH and high concentration of divalent salt, such as CaCl2 or CdCl2, are normal to the surface, i.e. are in solid state, even at low pressures. Pressure-area isotherms of such films are featureless compressed films are stable and easy to transfer [38]. 

Chemical reactions are often highly pressure-dependent. As a matter of fact, high pressure is an elegant way to perturb reversibly chemical equilibria and reactions. Another advantage of using the pressure parameter is that reactions are slowed or accelerated depending on the type of chemical interaction involved. For instance, pressure weakens electrostatic interactions, but stimulates some hydrophobic interactions, such as stacking between aromatic residues. Similarly to the activation enthalpy, obtained from... 

The transport mechanisms through zeolite membranes depend on different variables such as operation conditions (especially temperature and pressure), membrane pore size distribution, characteristics of the pore surface of the zeohtic-channel network (hydrophilicity/hydrophobicity ratio), as well as the characteristics of the crystal boundaries and the characteristics of the permeating molecules (kinetic diameter, molecular weight, vapor pressure, heat of adsorption), and their interactions in the mixture. 

In RPLC, the influence of pressure on the chromatographic behavior is related to the hydrophobic interactions involved in the retention mechanism and to the change upon adsorption in the numbers of acetonitrile and water molecules in the solvent shells of the protein molecule and of the bonded layer. The importance of the changes in the retention factor and the saturation capacity with a change in the average column pressure will thus depend on the retention mode used and will vary with the hydrophobicity of the molecule [128]. In RPLC, it is larger with polymeric than with monomeric bonded phases [126]. 

The surface pressures at the oil and water sides of the interface depend on the interactions of the hydrophobic and hydrophilic potions of the surfactant molecule at both sides, respectively. If the hydrophobic groups are bulky in nature relative to the hydrophihc groups, then for a flat film such hydrophobic groups tend to crowd so as to form a higher surface pressure at the oil side of the interface this results in bending and expansion at the oil side, forming a W/O microemulsion. 

For some proteins, for instance ovalbumin, high pressure may cause irreversible aggregation. Whether it occurs may depend on the rate of pressure increase. Moderately high pressure generally causes dissociation of most quaternary structures. This is not surprising, since the association is generally due to hydrophobic interactions. 

The role of water in the conformation, the activity and the stability of proteins has been investigated with many experimental and theoretical approaches. Because of its importance it has been coined as the 21 amino acid . There is now sufficient experimental evidence for the fact that dry proteins do not unfold by increased temperature or pressure [21]. Low levels of hydration give rise to a glassy state and the temperature of the glass transition depends on the amount of water as observed for synthetic polymers. Water can therefore be considered as a plasticizer of the protein conformation. Whereas hydrophobic interactions have dominated the interpretation of the data, hydrogen bond networks of water may also play a predominant role in water-mediated interactions [48,49]. 

Elevated pressures can induce functional and structural alterations of proteins. The effects of pressure are governed by Le Chatelier s principle. According to this principle, an increase in pressure favours processes which reduce the overall volume of the system, and conversely increases in pressure inhibit processes which increase the volume. The effects of pressure on proteins depend on the relative contribution of the intramolecular forces which determine their stability and functions. Ionic interactions and hydrophobic interactions are disrupted by pressure. On the other hand, stacking interactions between aromatic rings and charge-transfer interactions are reinforced by pressure. Hydrogen bonds are almost insensitive to pressure. Thus, pressure acts on the secondary, tertiary, and quaternary structure of proteins. The extent and the reversibility, or irreversibility, of pressure effects depend on the pressure range, the rate of compression, and the duration of exposure to increased pressures. These effects are also influenced by other environmental parameters, such as the temperature, the pH, the solvent, and the composition of the medium. 

---