Interactions, apolar hydrophobic

Generally, the introduction of apolar molecules (such as hydrocarbons or noble gases), or apolar residues in otherwise polar molecules (such as alkyl side chains in biopolymers) into water leads to a reduction of the degrees of freedom (spatial, orientational, dynamic) of the neighbouring water molecules. This effect is called the hydro-phobic effect or hydrophobic hydration [176], Hydrophobic means water-fearing . It should be noted that the interaction between hydrophobic molecules and water molecules is actually attractive because of the dispersion interactions. However, the water/ water interaction is much more attractive. Water molecules simply love themselves too much to let some other compounds get in the way [26b] Therefore, from the point of view of the water molecules, the term hydrophobic is rather a misnomer it would be better to refer to water as being lipophobic . 

On the basis of the study of the solvent, temperature, and pressure effects, we show how the NMR rotational correlation times T2k for a heavy water molecule in neat liquid and organic solvents are cotrelated with the strength of solute-solvent interactions, in particular, H bonds. At room temperature (30 C), the correlation time is 2.1 ps in the random H-bond network in heavy water, whereas it is as small as 0.1 ps in such an apolar, hydrophobic solvent as carbon tetrachlmi because of the absence of the H bonds between water molecules. Pressure distorts H bonds and accelerates the orientational motion of water molecules in neat liquid. I%m evidence is collected for the limitations of the Stdces-Einstein-Debye (SED) law in solution. 

Surface waters are displaced on formation of the protein-ligand complex and thus provide a favorable entropic contribution to the free energy of complex formation. In particular, displacement of the water found interacting on apolar surfaces makes a large contribution to the AG and provides the driving force for many interactions (the hydrophobic effect). 

Amphipathic a-heUces associate via their hydrophobic faces in a knobs-into-holes side-chain arrangement, a-Hehces are characterized by heptad repeats (repeated seven residues, a-g, within heUx). The helices interact via hydrophobic residues between residues a and d to form an apolar stripe along one side and an electrostatic interaction between residues e and g on the other side of each helix. 

One can readily understand how para substituents can be accommodated without steric interference, since the antibody was prepared against a p-phenylazobenzoate group. The reason for the increase in Ao associated with every para substituent tested is less clear. Pressman et al. (3) ascribed such increases in similar systems to nonspecific dispersion forces that act between any two atoms in close proximity in this case the atoms would belong to the hapten and antibody, respectively. It is not certain whether apolar (hydrophobic) forces may also play a role in the interactions taking place in the para region. 

Enhanced catalytic activity of polymeric bound.imidazole has been investigated by numerous researchers who have used various types of macromolecular backbones and imidazole derivatives as biopolymer models. A summary of our work in the area of polymer containing imidazole has been presented in several review articles (6-8). In polymer catalysts containing imidazole there has been defined three important factors which contribute to the overall enhancement of the catalyst s efficiency cooperative interactions, electrostatic interactions and hydrophobic or apolar interactions. 

Menashi et al.153) could confirm the results of Privalov and Tiktopulo152 and inter-prete the described effects as follows In the case of native tropocollagen, the pyrrolidine residues are probably directed away from the fibrillar axis and are mostly coated by water which is structured in the immediate neighbourhood to the pyrrolidine residues. During the denaturation these pyrrolidine residues form hydrophobic bonds with each other or with other apolar residues within the same chain (endothermic interaction) while the structure of water breaks down (increase of entropy). 

Protein molecules contain both polar and apolar groups. For proteins dissolved in water, these apolar groups tend to be buried in the interior of the globular structure, as a result of expulsion by the surrounding water. However, other interactions, as well as geometrical constraints, interfere with the hydrophobic effect, so that a minor fraction of the water-accessible surface of the protein molecule may be apolar. Protein molecules that do not spontaneously aggregate in water do not have pronounced apolar patches at their surfaces. 

The problems of the constancy of a and the site of reaction are closely linked. It is very convenient to assume that the charge on the micellar head groups is extensively neutralized by counterions which bind specifically to the micellar surface. In this way micellar stability is associated with a balance between hydrophobic attractions between apolar groups and coulombic repulsions of the ionic head groups which will be reduced by favorable interactions with the counterions in both the Stem and the diffuse Gouy-Chapman layers. It is the behavior of the counterions which is important in considerations of their chemical reactivity. 

Hydrophobic interaction (HIC) Aqueous, usually buffered Apolar ligand (e.g. octylamino) bonded to support matrix. A form of AC ligand complexes with apolar (hydrocarbon) sites on protein solute. Usually proteins. 

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