Hydrophobic properties of molecules

In spite of this interest of physical and biochemists, prior to the work of Hansch and Fujita there appears to have been no eifort to devise a scale of hydrophobicity analogous to the Hammett scale for electronic effects, or the Taft scale for steric effects. Part of this reluctance was due to the selection of the relevant solvent system for such a scale. In addition, biochemists who were interested in hydrophobic stabilization of protein structures needed only to consider the few hydrophobic amino acid side chains. 

The relationship between logD and logP is given by equation (3.10). 

In equation (10), the term (1 - a) is the fraction of the molecules that are not protonated at the pH of interest. For a mono-basic compound it is calculated according to equation (3.11)  

To use a partitioning experiment to measure only the hydrophobicity of a solute, one adjusts the pH of the aqueous phase so that only the neutral form of the solute is present. This is typically pH 1 for acidic compounds and pH 12 or 13 for basic compounds. The result is a logP value. Clearly there is no such pH at which zwitterionic compounds exist in the uncharged form in such cases the measurements might be made on neutral analogs. 

Although Meyer and Overton used olive oil for their measurements of hydrophobicity, Hansch and Fujita rejected it because the composition varies from lot to lot. Instead they chose to do the measurements in 1-octanol, which is available commercially in pure form. Octanol has the advantage over pure 

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Molecule (a) is compared with (b), the bonding atoms and central atom are the same, but the difference in linking atoms, =S and =0, can result in different degree of hydropho-bization of mineral surfaces (A0 = 6°). The molecules (b) and (c) possess identical linking atom but different bonding atoms, i.e. -S- and -0-, and this results in a significant difference in hydrophobization (A0 = 18°). The difference in the hydrophobization property of molecules between (a) and (c) is greater (A0 = 24°). 

The Dependence of Odor Intensity on the Hydrophobic Properties of Molecules... 

Washing and Cleaning Action. The properties of alkyl ether sulfates, due to the good solubility and the special hydrophilic/hydrophobic properties of the molecule, are of particular practical interest. From the investigations described in sections 2 and 3, it can be concluded that, in addition to the decrease in the Krafft Point, favorable properties for practical applications can be expected as a result of the inclusion of the oxyethylene groups into the hydrophobic part of the molecule. As is true for other anionic surfactants, the electrical double layer will be compressed by the addition of multivalent cations. By this means, the adsorption at the interface is increased, the surface activity is raised, and, furthermore, the critical micelle concentration decreased. In the case of the alkyl ether sulfates, however these effects can be obtained without encountering undesirable salting out effects. 

The apolar chains of the ceramide portion of gangliosides are responsible for the hydrophobic properties of gangliosides and for their availability to hydrophobic interactions. The formation of an hydrogen bond between the 3-hydroxyl group of the sphingo-sine and the carbonyl oxygen of the fatty acid, would tend to spread the two hydrocarbon chains reducing their tendency for mutual association, and thus to promote association of each chain with other molecules ( 3 ). 

Hansch and coworkers used regression analysis and descriptors for the hydrophobic, electronic, and steric properties of molecules to formulate a QSAR... 

At this point, I would like to indicate that taste chemistry is essentially solution chemistry. It is therefore especially significant that the bitterness of many compounds can be directly related to the hydrophobic properties of the molecules. The sour sensation has also been shown to be related to the Br nsted acid... 

As a direct consequence of 1), the adsorption enthalpy and other thermodynamic properties are composite quantities, containing at least four terms. Some of these terms may dominate, and this is not necessarily the enthalpy of the adsorptive-adsorbent interaction. As a typical illustration, binding of hydrophobic (parts of) molecules on hydrophobic adsorbents from aqueous solution is mainly driven by their dislike for water and not by their attraction to the surface. 

Silanes exhibit hydrophilic and hydrophobic properties. The molecule has both a non-polar and a polar component. The non-polar organic alkyl group is the vehicle for the water-repellent action. The effectiveness, durability and stability of the waterproofing agent depends on the size and nature of this group (Huls, 1991). 

Tocopherols and tocotrienols form pale yellow and viscous liquids at room temperature that are soluble in lipids and lipophilic solvents but are insoluble in water. In agreement with the substitution pattern, the hydrophobic properties of tocopherols decrease in the order of a- > 0- > y- > 8-tocopherol (Sliwiok and Kocjan, 1992). The melting point of RRR-a-tocopherol is ca 3°C, while that of RRR-y-tocopherol is at -3° to -2°C (Schudel et al., 1972). The ultraviolet absorption spectra of tocopherols and tocotrienols in ethanol show maxima in the range of 290-298 nm (Schudel et al., 1972 Diplock, 1985). The pure tocopherols isolated from natural sources exhibit relatively small optical rotations tocopherols are dextrorotatory in ethanol, while they are levorotatory in benzene. The free phenolic hydroxyl group of tocopherol molecules mainly determines their chemical properties (Schudel et al., 1972). 

The major physical properties affecting biological potency of a molecule are supposed 1 3) to be electronic, steric, and hydrophobic. Electronic properties influence the reactivities of the compounds and steric properties are clearly important in view of the steric selectivity of interactions between the compound and components of the biological system. Hydrophobic properties of a compound appear to affect its distribution throughout the body organs and tissues as well as the interaction of the compound with its target (receptor) in the biological system 12). 

In keeping with the fundamental importance of this subject and the intense recent activity in this area, several excellent reviews have already summarized different aspects of photophysical processes in organized assemblies. The development in this area up to 1991 was summarized in the volume edited by Rama-murthy [2-4]. Subsequently, several authors discussed the effect of cyclodextrins on many photophysical processes [5]. The binding of organic probes with cyclodextrins and other hosts in aqueous medium arises as a result of the hydrophobic effect. The different aspects of hydrophobic binding have also been reviewed extensively [1]. Many organized assemblies involve an interface between two drastically different media. The properties of molecules at various interfaces have been recently studied using a munber of new experimental and theoretical techniques. This aspect is reviewed by Eisenthal [6a], Robinson et al. [6b], and others [6c-e]. 

Reversed-phase chromatography is now widely used for the fractionation of biological molecules. The technique is based on the use of a non-polar stationary phase and a polar mobile phase. The stationary phase is usually made of a hydrocarbonaceous layer, either n-octyl. Cfl, or n-octadecyl, Cib, ligands, chemically bonded to the surface of a silica matrix via siloxane bonds. Separation Is achieved by exploiting the difference in the hydrophobic properties of the molecules. 

In order to reduce the nonspedfic binding of D1 protein to hydrophobic surface of microtiter plate, the polystyrene surface was blocked by bovine semm albumin (BSA). Eu- labelled D1 protein (1 mg/ml) was incubated with different concentrations of atrazine (ITO -ITO M) for 1 h. The plate was washed 8 times by phosphate buffered saline (PBS), pH 7.5, followed by incubation with enhancing solution and measurement of fluorescence. Despite the high hydrophobic properties of D1 protein and Nylon, the adsorption of D1 protein on the Nylon 11 surface was 3-5 times smaller than on Nylon modified with melamine, which suggests that specific matrix contained herbicide molecules vras created. The binding reaction of the Eu- labelled D1 protein to spedfic polymer demonstrated some competitive effect in presence of atrazine but it was not sufficiently sensitive (Fig. 7). The maximum inhibition of binding reaction was in the range of 20%. 

The initial values of the polarizing resistance of the electrode in contact with modified PE films are 1.5-2 times higher than those of unmodified ones and are in agreement with the hydrophobic properties of the Cl. Hydrophoby rises through the series MO, SIM, Ukramin, AKOR. The kinetic curves of Rp —tr show two characteristic sections. The slope in the first second (initial) is dependent upon the PI and Cl content in the film material. The inclination of the second section remains constant for all samples. Consequently [118], Rp attenuation on the first section is the result of the contact area of the electrolyte and metal surface enlarging with time as the adsorbed Cl molecules are removed from it. The variation velocity of Rp in the second section is... 

The various properties of molecules that can be used in a QSAR are often designed to quantitate the tendency of the molecules to participate in one of the fundamental types of intermolecular interactions electrostatic, hydrogen bonding, dispersion forces, and hydrophobic interactions. In addition, the possibility of steric interference with an interaction is considered. Other methods capitalize on the fact that the 2D structure of a molecule indirectly encodes its properties, instead generate descriptors without an explicit relationship to some physical property. 

It reduced the segmental motion of lipid molecules especially of the hydrocarbon chains of the lipidf which brough about an increase in the hydrophobic property of the inner region of the liposome bilayer. This hybrid liposome/heme gave a stable oxygen adduct. The incorporation of natural membrane proteins not only physically reinforces the liposome/heme in vitro but also increases its bio-compatibility. 

The use of the QSAR technique known as the Hansch Approach in the investigation of odor intensity and odorant physico-chemical properties has indicated that hydrophobic properties of homologous series of compounds, not steric or polar properties, are highly correlated to the level of odor intensity. This was shown to be the case for literature odor threshold and suprathreshold data determined at different laboratories using various media. The poor correlation between odor intensity and the steric properties of molecules (Taft Steric Constant) which had been reported earlier by this author (11) have been further verified by the use of Charton and Verloop Sterimol steric parameters. 

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