Descriptor flexibility descriptors

Flexible optimal descriptors have been defined as specific modifications of adjacency matrix, by means of utilization of nonzero diagonal elements (Randic and Basak, 1999, 2001 Randic and Pompe, 2001a, b). These nonzero values of matrix elements change vertex degrees and consequently the values of molecular descriptors. As a rule, these modifications are aimed to change topological indices. The values of these diagonal elements must provide minimum standard error of estimation for predictive model (that is based on the flexible descriptor) of property/activity of interest. 

FHDSA index RSHM index -> charged partial surface area descriptors flexibility index based on path length - flexibility indices flexibility indices... 

The terms similarity and diversity can have quite different meanings in chemical investigations. Describing the diversity of a data collection with a general valid measure is almost impossible. Descriptor flexibility allows the characterization of similarity by means of statistics for different tasks. The statistical evaluation of descriptors shows that it is recommended to interpret correlation coefficients together with the symmetry of distribution. In contrast to correlation coefficients, skewness and kurtosis are sensitive indicators to constitutional and conformational changes in a molecule. This feature allows a more precise evaluation of structural similarity or diversity of molecular data sets. 

The possibility of rotation of some groups of atoms around some chanical bonds is intuitively associated with molecular flexibility. Consequently, the number of rotatable bonds and the percentage of rotatable bonds are the simplest descriptors that measure the molecular flexibility. More complicated formulas calculate flexibility descriptors by weighting some topological descriptors therefore, they consider the shape and size of the molecules, the number of atoms, and the sum of bond orders (Tarko 2004c). 

Tarko, L., 2004c. Molecular flexibility descriptors for QSAR calculations. Rev. Chim. 

Flexible Descriptors Obtained via Optimization of Correlation Weights of Local Graph Invariants... 

Table 14 Statistical characteristics for models obtained using different definitions of flexible descriptors...

Further prerequisites depend on the chemical problem to be solved. Some chemical effects have an undesired influence on the structure descriptor if the experimental data to be processed do not account for them. A typical example is the conformational flexibility of a molecule, which has a profound influence on a 3D descriptor based on Cartesian coordinates. In particular, for the application of structure descriptors with structure-spectrum correlation problems in... 

In general, the described techniques provide an effective, flexible, and relatively fast solution for library design based on analysis of bioscreening data. The quantitative relationships, based on the assessment of contribution values of various molecular descriptors, not only permit the estimation of potential biological activity of candidate compounds before synthesis but also provide information concerning the modification of the structural features necessary for this activity. Usually these techniques are applied in the form of computational filters for constraining the size of virtual combinatorial libraries and... 

Duchowicz PR, Castro EA, Toropov AA, Benfenati E (2006) Applications of Flexible Molecular Descriptors in the QSPR-QSAR Study of Heterocyclic Drugs. 3 1-38... 

Taken globally, the results show a remarkable adaptability of acetylcholine which can be justified considering both its intrinsic flexibility, and the fact that its intramolecular interactions are not very strong and that almost all media can compete with them. Such adaptability finds a noteworthy implication in significant pairwise correlations between physicochemical properties and geometrical descriptors as well as among physicochemical properties. Thus, Fig. 1.6 shows the revealing 3D... 

This also involves flexibility and rigidity descriptors (RB and AP), a size descriptor (MW) and a polarity descriptor (log Pow). It appears that these kinds of descriptors are essential for AGfUs-... 

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