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Hansch model

A fundamental assumption of the approach is that contributions to a from several substituent groups on the same parent compound are additive. The additivity assumption also holds for the more general Hansch model that will be discussed below. [Pg.387]

Frequently, the relationship between biological activity and log P is curved and shows a maximum [ 18]. In that case, quadratic and non-linear Hansch models have been proposed [19]. The parabolic model is defined as ... [Pg.389]

A non-linear Hansch model has been applied to the bactericidal concentrations (O of 17 doubly substituted phenols (Table 37.2) which have been reported by Klarmann et al. [20]. By means of multiple linear regression we obtain the parabolic Hansch model of eq. (37.9) ... [Pg.390]

The coefficients of the regression are all highly significant (p < 0.0001) and the fit of the model to the observed data is shown in Fig. 37.1. Using non-linear regression we obtain the bilinear Hansch model for the bactericidal activities of the 17 phenol analogs (Table 37.2) ... [Pg.390]

Fig. 37.1. Quadratic Hansch model fitted to the bactericidal activities (log 1/C) of 10 doubly substituted phenols in Table 37.2 as a function of lipophilicity (log P) [20]. Fig. 37.1. Quadratic Hansch model fitted to the bactericidal activities (log 1/C) of 10 doubly substituted phenols in Table 37.2 as a function of lipophilicity (log P) [20].
The interpretation of the result of a Free-Wilson analysis is somewhat different from that of a Hansch analysis. The coefficients in the Hansch model represent absolute contributions to the biological activity of a compound from the various... [Pg.395]

The method of PCA can be used in QSAR as a preliminary step to Hansch analysis in order to determine the relevant parameters that must be entered into the equation. Principal components are by definition uncorrelated and, hence, do not pose the problem of multicollinearity. Instead of defining a Hansch model in terms of the original physicochemical parameters, it is often more appropriate to use principal components regression (PCR) which has been discussed in Section 35.6. An alternative approach is by means of partial least squares (PLS) regression, which will be more amply covered below (Section 37.4). [Pg.398]

As an illustration of PLS regression (PLSl) we reconsider the inhibitory potencies of oxidative phosphorylation of 11 doubly substituted salicylanilides [ 17] in Table 37.1. An extended Hansch model is defined by the linear free energy relation ... [Pg.410]

Item d implies that in terms of the Hansch model of Equation 1, partitioning STEP 1 is the primary rate-controlling process characterizing the herbicidal action of the 3-TFMS compounds on Wild Mustard in the presence of Tween 80. Since all the other Hansch relationships in Table XI include fairly significant pa contributions, partitioning as well as other rate processes (possibly more intimately connected with the receptor site within the plant or seed) must be involved in determining overall herbicidal activity in these latter cases. One may speculate that the anomalous observations (a-c) above are the direct consequence of (d)—the lack of Hammett a dependence. If the pa term in the Hansch equation does indeed reflect rate or equilibrium events occurring at or near the herbicidal site of action within the plant or seed (as is often assumed but not... [Pg.246]

The MTD model is obtained by including MTD descriptors in the -f Hansch model ... [Pg.293]

The FW model was proposed in 1964 at the same time as the Hansch model, but is far less widely used. It uses... [Pg.492]

In 1962, Hansch, Maloney and Fujita [Hansch, Maloney et al, 1962] published their study on the structure-activity relationships of plant growth regulators and their dependency on Hammett constants and hydrophobidty. Using the octanol/water system, a whole series of partition coefficients was measured and, thus, a new hydrophobic scale was introduced for describing the inclination of molecules to move through environments characterized by different degrees of hydrophilicity such as blood and cellular membranes. The delineation of Hansch models led to explosive development in QSAR analysis and related approaches [Hansch and Leo, 1995]. This approach known with the name of Hansch analysis became and it still is a basic tool for QSAR modeling. [Pg.1248]

On the other hand, Free-Wilson analysis can be used to ease the derivation of a Hansch model. An inspection of the coefficients of the different terms in Eq. (17) shows that activity increases with increasing lipophilicity of the halogens, that methyl leads to a higher activity than to be expected from its lipophilicity (corresponds to chlorine),... [Pg.544]

In the context of the reinvestigation of Hansch models for the substituted N,N-dimethyl-a-bromophenethylamines, Unger and Hansch [28] formulated recommendations for the proper derivation of extrathermodynamic equations (supplementary comments are given in parentheses) ... [Pg.545]

The parabolic Hansch model is a good approximation of observed nonlinear structure-activity relationships. However, whereas the left and right sides of a parabola are always nonlinear, many nonlinear lipophilicity relationships show linear left and right sides, as also observed for the function describing the rate constants of... [Pg.549]

Often it is convenient to derive first a Hansch model for a small, chemically closely related subset and then to extend this equation to larger sets of compounds. For the papain binding of benzamido-glycine and mesylamido-glycine phenyl esters, with different substituents at the phenyl ester group, Eq. (44) was derived for the benzamides and Eq. (45) for the mesylamides [77] ... [Pg.554]

The stepwise derivation of Hansch models is further illustrated by equations that were derived for the inhibition of chymotrypsin by thiophosphonates. First, Eq. (48) correlated data where chemical variation was performed only in R2 of Me-P( = 0)(OR2)SR3 then further analogs with chemical variation in R3 were included and finally an indicator variable 7 accounted for charged groups in some residues R3 (Eq. (49)) [79] ... [Pg.555]

The average relative error is 3.9%, and no observation has a relative error greater than 10%. These results are significantly better than those given by the Hansch model. Hansch and Lien > found it necessary to delete two observations to achieve r - 0.911,5 = 0.22. When the full data set is used, the statistics are even worse r = 0.878, s = 0.24. The potential value of this total topological index x is yet to be explored. Because it is a very highly discriminating index, there may be a useful role for it in QSAR. [Pg.415]

I had a question the other day, You know what I d like to do, I d like to see a Hansch analysis on a series of compounds and then I d like to see some molecular orbital calculations on the same series. The point is that you don t compare this type of correlation with that type of correlation in this sense. Rather I think it is more important to look at a particular model and then use whatever techniques you need to get the parameters that go into that model, whether you measure the dipole moment or whether you calculate it from wavefunctions or whether you take the charge densities from pK measurements or whether you calculate the charge density. It is not the Hansch model vs. molecular orbitals. That makes no sense to me. Rather it is using molecular orbital calculations to get at fundamental properties of the molecules—the same way that you would get at them if you measured the infrared absorption of... [Pg.196]

A comparison of the results from Hansch and Free Wilson analyses offers some information, whether a certain Hansch model can be considered to be acceptable or not. In most cases the Free Wilson analysis of a data set shows whether a linear additive model is suited for the analysis only in certain cases is a good fit obtained for nonlinear relationships, especially if there are only few degrees of freedom [22, 390, 391]. [Pg.66]

The Bocek Kopecky model is theoretically and practically related to the nonlinear Hansch model [22, 392, 393] and thus to eq. 79. However, due to the large number of variables this modification has never been used. [Pg.67]

Figure 12 Comparison of the parabolic Hansch model (left curve) and Franke s protein binding model (right curve). Log P, is the lipophilicity limit, where steric hindrance or other unfavorable interactions cause a change of the linear lipophilicity-activity relationship to a parabola (reproduced from Figure 9 of ref. [175] with permission from Birkhauser Verlag AG, Basel, Switzerland). Figure 12 Comparison of the parabolic Hansch model (left curve) and Franke s protein binding model (right curve). Log P, is the lipophilicity limit, where steric hindrance or other unfavorable interactions cause a change of the linear lipophilicity-activity relationship to a parabola (reproduced from Figure 9 of ref. [175] with permission from Birkhauser Verlag AG, Basel, Switzerland).
The Free Wilson model never became as popular as the Hansch model, due to the limitations inherent in its applicability and predictive ability. Only a few hundred studies [390, 391] have been published since 1964. [Pg.139]

In science, researchers can claim monopoly on methods, models, and algorithms and view than as their property, and those using them have to acknowledge the source of such. But all topics of science should be open to all. Thus, QSAR, which stands for quantitative struaure-activity relationship, and not for the Hansch model of QSAR, should be a discipline open to all. Plainly speaking, QSAR is not synonym... [Pg.457]


See other pages where Hansch model is mentioned: [Pg.388]    [Pg.389]    [Pg.397]    [Pg.266]    [Pg.267]    [Pg.118]    [Pg.247]    [Pg.247]    [Pg.295]    [Pg.214]    [Pg.70]    [Pg.493]    [Pg.500]    [Pg.543]    [Pg.545]    [Pg.360]    [Pg.304]    [Pg.247]    [Pg.493]    [Pg.458]    [Pg.458]   
See also in sourсe #XX -- [ Pg.410 ]

See also in sourсe #XX -- [ Pg.554 ]




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