Hydrophobicity measurement

Hydrophobic interaction chromatography (HIC) is a column chromatography technique which can determine particle hydrophobicity by interaction with a hydrophobic gel matrix [142,149,150]. Hydrophilic particles pass through the column without interaction, whereas particles with increased hydrophobicity show a retarded elution and are retained by the column. Hydrophobicity measurements are used to determine the hydrophobicity of nanoparticulate carriers and correlate this to their in vivo biodistribution [10, 149]. 

Comparison of protein hydrophobicity measured using different methods is tabulated in Nakai et al. (1996). 

Protein hydrophobicity has been most frequently expressed as relative values measured by the methods used, since no standardized unit has ever been established. These relative values are incorporated directly into correlation studies with protein functions. Therefore, the correlation coefficient of a measured functionality against the counterpart predicted from the measured hydrophobicity is the most reliable parameter to use for comparing different methods for hydrophobicity measurement. 

Hydrophobicity Measurement. A conventional method (25) of using l-anilinonaphthalene-8-sulfonate (ANS) as a semiquantitative hydrophobic probe was applied. To 8 X 10 5Af ANS in 0.1M Tris-HCl (pH 8.5) was added the sample at a concentration of 0.05% (w/w) and the resulting solution measured for the intensity of its emission spectrum... 

Larger molecules such as proteins usually do not fit these predictions, probably because the molecules adopt an ordered three-dimensional structure in which many of the hydrophobic residues are buried within the structure and unavailable for interaction with the reversed phase. As might be expected from the proposed mechanism of separation, the retention of proteins on reversed-phase columns is not related to molecular weight of the sample, but rather the surface polarity of the molecule. Table I shows that there is a correlation of hydrophobicity (measured by mole % of strongly hydrophobic residues) with retention order for seven different proteins. It is unlikely that the retention of all proteins on a reversed-phase column can be correlated in this manner, because many protein structures have few nonpolar residues exposed to the aqueous environment. For example, although the major A and C apolipoproteins are eluted from a ju-Bondapak alkylphenyl column in an order which can be related to the proposed secondary structures, there is little correlation with the content of hydrophobic residues in each protein and the degree of interaction with the stationary phase. A similar lack of correlation be-... 

Hydrophobicity or lipophilicity is understood as a measure of the relative tendency of an analyte to prefer a non-aqueous to an aqueous environment. The partition coefficients of the substances may differ if determined in different organic-water eluent systems but their logarithms are often linearly related [117]. Octanol-water partitioning is a common reference system which provides the most commonly recognized hydrophobicity measurement, the logarithm of the partition coefficient, log P [8,45,46]. 

Lipophilicity represents the affinity of a molecule or a moiety for a lipophilic environment, while hydrophobicity measures the association of non-polar groups or molecules in an aqueous environment which arises from the tendency of water to exclude non-polar molecules. 

Building materials exposed to weathering are corroded by the action of atmospheric influences, especially water destruction is unavoidable in the long term. Water-repellent treatments can certainly not totally stop these harmful processes, but, given adequate envelopment and attachment to the substrate, nanoscale silicone resin networks can retard material decomposition because of their high durability. Since damage caused by hydrophobic measures can be virtually ruled out if the treatments are properly applied, the organosilicon compounds used in masonry protection will become more and more widely used. 

The octanol-water partition coefficient scale may not be the best tool for hydrophobicity evaluation. The ability of MLC for hydrophobicity measurement and some studies on quantitative structure retention activity relationships (QSAR) are described in Chapter 9. Chapters 10 and 11 contain selected examples of applications in the analysis of a variety of samples, especially pharmaceutical preparations and physiological fluids, some of them are taken from the authors own experience. Details on the nature of the sample, stationary phase, mobile phase composition, detection wavelength, and figures of merit, are tabulated at the end of each of these... 

In MLC, the retention is influenced by two competing equilibria of solute interactions with micelles in the mobile phase (controlled by K m) and their partitioning into the stationary plmse (controlled by P s) (see eq. 9.11). Both partitioning processes depend on the hydrophobicity of die solute. The reciprocal of the intercept in eq. 9.11 is the retention factor at zero micelle concentration, ko, which is a parameter similar to, and as shown below, is useful in hydrophobicity measurements. 

Correlation of Octanol-Water Partition Coefficients with Hydrophobicity Measurements obtained by MLC, Anal. Chem., 59 921 (1987). 

Keurentges, JTF Harbrecht, JG Brinkman, D Hanemaaijer, JH Cohen, MA Van t Riet, H. Hydrophobicity measurements of microfiltration and ultrafiltration membranes. 

The problem addressed by the GFA is the development of accurate QSAR models that are assumed to be linear combinations of a set of features chosen from a given larger set. The given set of features could be basic molecular descriptors, principal components or complex non-linear functions of the molecular descriptors. Typically, molecular descriptors could be hydrophobicity measures, partition coefficients etc. The higher-level features which are functions of the molecular features are called basis functions. The effectiveness of the models is measured by their ability to capture parsimoniously the dependence of the desired activity on an extracted set of relevant basis functions. 

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