Clathrates hydrophobic effect water

The induction of unconsciousness may be the result of exposure to excessive concentrations of toxic solvents such as carbon tetrachloride or vinyl chloride, as occasionally occurs in industrial situations (solvent narcosis). Also, volatile and nonvolatile anesthetic drugs such as halothane and thiopental, respectively, cause the same physiological effect. The mechanism(s) underlying anesthesia is not fully understood, although various theories have been proposed. Many of these have centered on the correlation between certain physicochemical properties and anesthetic potency. Thus, the oil/water partition coefficient, the ability to reduce surface tension, and the ability to induce the formation of clathrate compounds with water are all correlated with anesthetic potency. It seems that each of these characteristics are all connected to hydrophobicity, and so the site of action may be a hydrophobic region in a membrane or protein. Thus, again, physicochemical properties determine biological activity. 

Host — A - molecular entity that forms an -> inclusion complex with organic or inorganic -> guests, or a - chemical species that can accommodate guests within cavities of its crystal structure. Examples include cryptands and crowns (where there are -> ion-dipole interactions between heteroatoms and positive ions), hydrogen-bonded molecules that form clathrates (e.g., hydroquinone and water), and host molecules of inclusion compounds (e.g., urea or thiourea). The - van der Waals forces and hydrophobic interactions (- hydrophobic effect) bind the guest to the host molecule in clathrates and inclusion compounds. 

If the solute is nonpolar, there is only weak van der Waals attraction with water, and water molecules arrange around the nonpolar solute such that they form the most extensive number of hydrogen bonds, with the ice clathrates (Part IV, Chap. 21) the extreme case. The ordering of water molecules is entropically unfavorable, since they lose orientational and translational freedom. This can be compensated for if the solvated solute molecules aggregate and the ordered water molecules are released from their surface into bulk water, a process which is entropically favorable and the main driving force for the hydrophobic effect [128 to 134]. 

Simulations of solutions have been used to study hydrophobic effects. Thus, Rossky andZicki (1994) found that hydration shells of methane and neon remain intact in mixed solvents this is understandable in terms of clathrate formation—an example of an unusual degree of disordering from the normal structure of water. 

The Hydrophobic Effect - interactions between hydrophobic regions of a protein, which actually increase entropy by destroying the ordered clathrate structures of water around these residues in the unfolded state. The hydrophobic effect is sometimes incorrectly called hydrophobic bonding. Table 6.4 shows numerical values assigned to the relative hydrophobicities of the amino acids. In Table 6.3, the hydrophobic effect can be seen by the more positive AS values for cytochrome c and myoglobin. 

If apolar hydration is characterized by the conditions that AG° > 0, TAS < 0 and AH < 0, then a process which minimizes exposure of apolar groups to water should be a thermodynamically favoured process. Then if two apolar groups of either the same or different molecules come together in water, AS for this process will be positive because some of the structured water is released into the bulk solvent. Such association is called hydrophobic, hydrophobic bonding or hydrophobic interaction (Kauzmann, 1959). The term bond is probably inappropriate because the association is due to entropy rather than to enthalpy effects, a consequence of the disruption of the clathrate structure around the apolar solute (Jolicoeur and Friedman, 1974). Despite the general acceptance of the concept of hydrophobic association, there are different approaches to the problem of understanding this phenomenon. 

By contrast, exposed hydrophobic residues like Val35 may be effectively clathrated or accommodated within a water cavity that introduces a minimal perturbation of the tetrahedral hydrogen bonding network of water. In fact, clathration actually tightens the hydration shell (Figs. 4.1b and 4.2). 

A clathrate hydrate is a crystalline inclusion compound in which small guest molecules, usually hydrophobic, are trapped in polyhedral cages formed by hydrogen-bonded water molecules. True clathrates are formed by guests that interact with the hydrate lattice only by weak, nondirectional forces. In such cases, the water molecules form a completely hydrogen-bonded network, and the inaterials effectively are ices. A number of structures are known for true clathrate hydrates, including the three major families of clathrate hydrate structures that will be discussed later. 

The structure of liquid water was dealt with in detail in Sect. 1.1.2. Once a solute, whether an ion or a neutral solute and whether hydrophilic or hydrophobic, is placed in the water, it is reasonable to expect it to affect the structure around it. The effects may be limited to a hydration shell surrounding the solute that has a structure differing from that in pure water. For instance, around monatomic ions the water molecules in the hydration shell are oriented towards the ion in a more or less spherical symmetry. Around hydrophobic solutes cages of water molecules are formed, that may be icelike but also resemble the structure of clathrates or crystal hydrates. In many cases the effects of the presence of an ion are manifested also beyond the hydration shell or shells. 

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