Meta-descriptors

Once FP has been successfully conducted with consumers, researchers have attempted to go beyond sensory description in the strict sense. First, it is observed that when consumers participate in descriptive analysis studies, they tend to use some hedoni-cally orientated terms, as well as composite notions that may refer to benefits, which are consequences of consuming the product (e.g. thirst quenching, filling up) (Veinand et ai, 2011). They may also use attributes that are more integrative than those used by experts. The French adjective onctueux (creamy and smooth) would typically fall in this category. Such attributes may be referred to as meta-descriptors (Fr0st and Janhpj, 2007) or second-order sensory attributes. ... 

A classical Hansch approach and an artificial neural networks approach were applied to a training set of 32 substituted phenylpiperazines characterized by their affinity for the 5-HTiA-R and the generic arAR [91]. The study was aimed at evaluating the structural requirements for the 5-HTiA/ai selectivity. Each chemical structure was described by six physicochemical parameters and three indicator variables. As electronic descriptors, the field and resonance constants of Swain and Lupton were used. Furthermore, the vdW volumes were employed as steric parameters. The hydrophobic effects exerted by the ortho- and meta-substituents were measured by using the Hansch 7t-ortho and n-meta constants [91]. The resulting models provided a significant correlation of electronic, steric and hydro-phobic parameters with the biological affinities. Moreover, it was inferred that the... 

When a novel homology domain has been discovered, it is possible to store the corresponding domain descriptor (profile or HMM) in a number of dedicated domain databases, which can be used to analyze newly identified sequences for their domain content [9, 10]. Several competing domain- and motif-databases exist, including PROSITE, PFAM, SMART, and Superfam, which contain descriptors for most, if not all, of the known domains involved in the ubiquitin system [11-14]. Recently, a new meta-database named INTERPRO has been established, which tries to combine the descriptors of several domain databases under a single user interface [15]. Pointers to the very useful search engines of the domain databases are provided in Table 12.1. 

Fig. 1. Plane of Chirality Enantio-morphous figures and corresponding torsional angles A—X—Y—Z 7). Molecular examples ([n]paracyclophane, [2.2]meta-cyclophane, bridged[10]anulene and ( )-cyclooctene) with descriptors (/ , S)...

The o scale was developed to incorporate the effect of through-resonance. Both types of correlations included ortho- as well as meta- and para-substituted phenoxides, and the only outliers are compounds that exhibit strong intramolecular hydrogen bonding because this effect is not incorporated in most molecular descriptors. The rate constants for oxidation of phenoxide anions give good Hammett correlations to cr constants, as shown in the following equation ... 

A correlation for less structurally related compounds was also developed by Amalric et al. (1996). To develop this correlation, meta- and para-substituted anisoles were studied. These aromatic compounds were substituted with F, Cl, N02, OH, and NH2 groups. The first-order degradation rate constant, kapf was predicted with the octanol/water partition coefficient (log Kow), Brown s constant (o+), and molar refractivity (MR) used as descriptors. The following correlation was developed ... 

Mem-substituted DHPs offered a good model with size and polar interactions descriptors, r2 = 0.859 and q2 = 0.790, while QCT descriptors alone afforded r2 = 0.820 and q2 = 0.583. In contrast to ortho-substituted DHPs, tt was found not to be important at the meta position. 

Figure 7 Localisation of most important descriptor types in terms of substituent position (ortho meta and para)...

Sponsors should not provide the text of a meta-data descriptor that contains direct or implied product claims to a search engine. Keywords and other meta-data tags that refer to competitor products are prohibited. 

Some chemical structures exhibit typical distances that occur independently of secondary features, which mainly affect the intensity distribution. In particular, aromatic systems contain at least a distance pattern of ortho-, meta-, and para-carbon atoms in the aromatic ring. A monocyclic aromatic system shows additionally a typical frequency distribution. Consequently, Cartesian RDF descriptors for benzene, toluene, and xylene isomers show a typical pattern for the three C-C distances of ortho-, meta-, and para-position (1.4, 2.4, and 2.8 A, respectively) within a benzene ring. This pattern is unique and indicates a benzene ring. Additional patterns occur for the substituted derivatives (3.8 and 4.3 A) that are also typical for phenyl systems. The increasing distance of the methyl groups in meta- and para-Xylene is indicated by a peak shift at 5.1 and 5.8 A, respectively. These types of patterns are primarily used in rule bases for the modeling of structures explained in detail in the application for structure prediction with infrared spectra. 

A hypothesis on the existence of correlations between molecular stmcture and physico-chemical properties was reported in the work of Kbmer [1874], which dealt with the synthesis of disubstituted benzenes and the discovery of ortho, meta, and para derivatives the different colors of disubstituted benzenes were thought to be related to differences in molecular stmcture and the indicator variables for ortho, meta, and para substitution can be considered as the first three molecular descriptors [Kbrner, 1869, 1874]. 

The (7 value of a para substituent is estimated as the sum of the F and R values for the substituent, where a = b=l.Q, and that for a meta substituents as the sum of F plus 0.27 R, i.e. <2=1.0, 6 = 0.27. Hence the contribution of resonance to is only 27% that of Up. The importance of such a separation of field and resonance effects is that one can use these separate values directly in a regression equation without deciding which a value to use. The problem with such an approach is that rather than one a value (or sum of cr values) for the substituent(s) on a molecule, there are now twice as many descriptors as there are positions of substitution. 

These descriptors offered a number of advantages over the substituent constants that were in common use at that time. They could be easily and rapidly calculated for any structure and thus there were never any missing values , as always seemed the case for substituent constants. There was no need for a parent structure and thus they could be applied to any collection of compounds, not just a congeneric series, and there was never any debate about which position a structural fragment occupied. Many of the substituent constants assumed different values depending on whether a substituent was meta or para to the reaction centre of the model system, as discussed earlier for CT. Recognition of substitution position is obvious for simple cyclic molecules but can be much less clear for complex, multi-ring structures. 

In a previous study of 18 aromatic trlfluoroketones, their inhibitory activity against JHE was correlated to the structure using four substituent parameters (H). The descriptors were chosen more or less arbitrarily and the effect of the different substituent positions could be determined seml-emplrlcally, mainly by examining how the various data-polnts fit the equation. Important conclusions could, however, be drawn based on the empirical and intuitive method the authors found that meta- and para-substituted compounds fit the regression line better and offered better selectivity against JHE. 

It Is Interesting that In the Individual substituent positions molar volume (MR) was found to be the relevant parameter rather than ff. Eg or the STERIMOL descriptors. This fact and the positive coefficients for MR suggest that the enzyme-Inhibitor Interaction proceeds via London dispersion forces (31, ) and the binding to the enzyme Is favored If the bulky substituents are In meta or para positions, which Is In accordance with the earlier results (11). The parameter H-DO In position I seems to be Important In the regression. It systematically appears In each equation. Its path coefficient and partial r value, however. Is relatively low compared to those of Jit and MRjjj. In addition, the H-DOj variable Is not very useful because these Indicator parameters are the most poorly defined and the number of proton donor substituents (H-DO-1) Is rather small. 

In addition to charge descriptors, two energy descriptors have been extensively used for QSAR studies. They are the energy difference AE (or enthalpy difference AH) between the protonated and deprotonated forms of the acid, and Ehomo the energy of the highest occupied molecular orbital of the acid. As early as 1979 Catalan and Macias [171] used INDO calculations [226] to establish a relationship between calculated AE values and the gas-phase acidities of meta- and / ara-substituted phenols. In 1985 La Manna, Tschinke, and Paoloni [227] used STO-3G Hartree-Fock calculations to demonstrate a relationship between Ehomo the gas-phase acidities of a series of benzoic acid... 

A comparison of the substituent effects in monosubstituted tetrazole systems and benzene derivatives has been presented (2011T(67)6316). For this purpose the pEDA index (2009JPOC(22)769), defined as the sum of the occupation of 2p orbitals at aU atoms of the ring minus 6, has been used as a substituent descriptor. In aU three cases (substituted benzenes, IH- and 2H-tetrazole derivatives) the pEDA index, which ilLustrates the Tc-electron transfer from the substituent to the ring or vice versa, is weU correlated with constants. A more detailed analysis revealed that the dependence of 2pj, occupancies at the carbon atoms of benzene in either ortho- or para-positions on pEDA values are represented by linear trends with cc= 0.971 and 0.968, respectively. In contrast, the same correlation for the carbon atom in the meta-position is worse cc= —0.791) and with a small... 

These isomers differ from each other in the relative positions of the methyl groups and can be named in two ways (1) using the descriptors ortho, meta, zxvApara or (2) using locants (i.e., 1,3 is the same as meta). Both methods can be used when the parent is a common name ... 

The descriptors ortho, meta, and para cannot be used when naming an aromatic ring bearing three or more substituents. In such a case, locants are required. That is, each substituent is designated with a number to indicate its location on the ring. 

Disubstituted derivatives of benzene can be differentiated by the use of the descriptors ortho, meta, and para or by the use of locants. 

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