Descriptor terms

Statistical parameters, when available, indicating the significance of each of the descriptor s contribution to the final regression equation are listed under its corresponding term in the equation. These include the standard errors written as values, the Student t test values, and the VIF. The significance of the equation will be indicated by the sample size, n the variance explained, r the standard error of the estimate, s the Fisher index, F and the cross-validated correlation coefficient, q. When known, outliers will be mentioned. The equations are followed by a discussion of the physical significance of the descriptor terms. 

Some of these methods are discussed in this chapter. An example using principal component analysis for variable selection is given in Section V. The application of principal component analysis to identify and combine individual correlated properties into new generalized descriptors, termed principal properties, is described in Section IE. 

Equation (8.2) only describes 60 per cent of the variance in PC2 and the high standard error for the shape descriptor term casts some doubt on the predictive ability of the equation. However, it is hoped that these two equations demonstrate the way in which regression models for multivariate dependent data can be generated by means of PCA. 

CAS represents stereochemistry in polymers by text descriptor terms when the necessary information is reported such terms include isotactic, syndiotactic. 

If there is any danger of confusion, descriptor terms are associated with locants. It is important to note, however, that locants do not refer to a single numbering system for the total substance. Instead, the locants for each parent fragment start with one (1). In Figure 8 for example, the locants in la,4ap, 8aa refer to the parent naphthalenyl, while the locants in 2R, 3S refer to the parent oxirane. 

CAS represents stereochemistry in polymers by text descriptor terms when the necessary information is reported such terms include isotactic, syndiotactic, threo-diisotactic, erythro-diisotactic, and disyndiotactic. The term atactic (for a random configuration) is not employed by CAS in indexing specific polymers. In addition to the special terms above, polymer stereochemistry is defined, when appropriate, by the stereo descriptors E, Z, R, S, R, and S. ... 

The two structures in our example are identical and are rotated by only 1 20 h Clearly, rotation of a molecule docs not change its stereochemistry, Thus, the permutation descriptor of both representations should be (+ I). On this basis, we can define an equation where the number of transpositions is correlated with the permutation descriptors in an exponential term (Eq. (9)). 

Usually, the denominator, if present in a similarity measure, is just a normalizet it is the numerator that is indicative of whether similarity or dissimilarity is being estimated, or both. The characteristics chosen for the description of the objects being compared are interchangeably called descriptors, properties, features, attributes, qualities, observations, measurements, calculations, etc. In the formiilations above, the terms matches and mismatches" refer to qualitative characteristics, e.g., binary ones (those which take one of two values 1 (present) or 0 (absent)), while the terms overlap and difference" refer to quantitative characteristics, e.g., those whose values can be arranged in order of magnitude along a one-dimensional axis. 

In this chapter the focus is on structure descriptors. After a definition of this term their properties are described and an oveiwiew is given of some frequently used structure descriptors. 

Because of the fundamental role of graph theory for the understanding of topological descriptors, some terms from graph theory are defined below. Then a selection of topological descriptors are discussed. 

The most widely used descriptor for the hydrophobicity term in toxicology is the distribution coefficient between octanol and water, log Pq< - (the environmental scientists would rather call it log The bulk solvent octanol is of course a... 

After selection of descriptors/NN training, the best networks were applied to the prediction of 259 chemical shifts from 31 molecules (prediction set), which were not used for training. The mean absolute error obtained for the whole prediction set was 0.25 ppm, and for 90% of the cases the mean absolute error was 0.19 ppm. Some stereochemical effects could be correctly predicted. In terms of speed, the neural network method is very fast - the whole process to predict the NMR shifts of 30 protons in a molecule with 56 atoms, starting from an MDL Molfile, took less than 2 s on a common workstation. 

The molecular mechanics force fields available include MM+, OPLS, BIO+, and AMBER. Parameters missing from the force field will be automatically estimated. The user has some control over cutoff distances for various terms in the energy expression. Solvent molecules can be included along with periodic boundary conditions. The molecular mechanics calculations tested ran without difficulties. Biomolecule computational abilities are aided by functions for superimposing molecules, conformation searching, and QSAR descriptor calculation. 

Nutritional Labeling Descriptors. In order to avoid confusion, descriptive terms must be accompanied by definitions which adequately explain the terms. In the case of nutrition-related claims, analytical sampling offers a means of assuring the accuracy of the stated claims. The USDA s FSIS has proposed a Hst of descriptors relevant for meat and poultry products (Table 3). 

The relationship between the stmcture of a molecule and its physical properties can be understood by finding a quantitative stmcture—property relation- ship (QSPR) (10). A basis set of similar compounds is used to derive an equation that relates the physical property, eg, melting poiat or boiling poiat, to stmcture. Each physical property requires its own unique QSPR equation. The compounds ia the basis set used for QSPRs with pyridines have sometimes been quite widely divergent ia respect to stmctural similarity or lack of it, yet the technique still seems to work well. The terms of the equation are composed of a coefficient and an iadependent variable called a descriptor. The descriptors can offer iasight iato the physical basis for changes ia the physical property with changes ia stmcture. 

Transverse Dimensions or Fineness. Historically, the quantity used to describe the fineness or coarseness of a fiber was the diameter. Eor fibers that have irregular cross-sections or that taper along their lengths, the term diameter has no useful meaning. Eor cylindrical fibers, however, diameter is an accurate measurement of the transverse dimension. Though textile fibers can be purchased in a variety of cross-sectional shapes, diameter is stiU a useful descriptor of the transverse dimension. Eiber diameter is important in determining not only the ease with which fibers can be twisted in converting them to yams, but also fiber stiffness, ie, fabric stiffness, and, alternatively, fabric softness and drapeabiHty. 

The lUPAC-IUB Commission on Biochemical Nomenclature (13) recommends that the term vitamin B 2 be used as the genetic descriptor for aU. cottiaoids exhibiting quaUtatively the biological activity of cyanocobalamin. However, because of its commercial importance, cyaaocobalamin is used iaterchangeably with vitamin B 2 heteia. 

In the above narrowed context, prediction refers to the development of system descriptors which can be measured independently in some rather simple and inexpensive way (in terms of time and instrumentation), or it may refer to ab... 

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