Shape descriptors, physical meaning

Indications of Physical Meanings of Shape and Textural Descriptors... 

The accuracy of this system is dependent on the correlation coefficient of a retention description obtained from studies of QSRR, therefore, the selection of descriptors is the most basic and important task to construct RPS. This selection could be done with statistical framework, even if such description is not clearly derived from theories. The retention description obtained from QSRR studies is more effective for a rapid and accurate prediction of retention than that derived from theoretical models, because the former is simple and does not require introduction of a number of physicochemical parameters (they are often not clearly known and are very difficult and time-consuming to determine) for the latter case. By contrast, the consideration of physical meanings of descriptors derived from QSRR studies gave the overview of retention mechanisms in reversed-phase LC (7-10). That is to say, hydrophobicity, size and shape of alkyl-benzenes and PAHs are dominate factors controlling their retention. 

The development of these molecular descriptors have been based on the physical model of transition states in acid-catalyzed esterification reaction of carboxylic acids and alcohols and acid hydrolysis of esters - standard reactions used by Taft for the development of Eg s empirical steric parameter in the frame of LFER (linear free energy relationship). The physical meaning of the (8, G) shape descriptors is depicted in Fig. 15.3. 

Brown and Martin [12] also investigated the extent to which a number different descriptors (2D and 3D) are able to encode information that relates to the interaction forces which must exist if a ligand is to bind to a receptor. Thus, the information content of each descriptor was assessed by its ability to accurately predict values for the physical properties of a structure from the known values for other structures which are shown to be structurally similar to the first using the descriptor in question. The predicted properties included measured log P values and calculated properties that explored the shape and flexibility of the molecules, including the numbers of hydrogen bond donors and acceptors within a molecule. Two methods of predicting properties were used a similarity-based prediction, where the predicted value for a molecule was taken as the mean value of all molecules within a given similarity threshold to it and cluster-based pre-... 

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