Chirality correction

For each asymmetric atom in R-configuration, the vertex degree 5j is substituted with (6 + c) and for each atom in S-configuration with (5j- c). This transformation is equivalent to making main diagonal elements Oji of the —> ai acency matrix A equal to +cor-cforallchiralatomsin R- or S-configuration, respectively. For achiral atom, the chirality correction factor equals zero. 

In general, values of c < 5 were assumed. The chirality correction was also applied to valence vertex degrees from which valence connectivity indices are derived. 

Chirality correction can be a real number (chirality descriptors of class I) or an imaginary number (chirality descriptors of class II). In the latter case, chirality descriptors are complex numbers. 

The chiral connectivity indices were calculated by analogy with the Kier-Hall connectivity indices by using vertex degrees modified by a chirality correction c = 1 [Xu, Zhang et al., 2006]. 

This chirality descriptor is derived from the Ruch s chirality functions applied to the first-order valence connectivity index. Separate values of the valence connectivity index are calculated for the four atoms/substituents a, h, c, and d bonded to the chiral atom [Lukovits and Linert, 2001]. The chirality correction is calculated by the following function F ... 

Relative chirality indices and chirality correction factors forthe two enantiomers shown below, together with valence vertex degrees and first-order valence connectivity index of the functional groups. 

The chirality correction factor for the calculation of the —> Chi chirality index is 0.00029. 

The chirality correction factor c is added the chosen atomic property w. 

Moreover, to account for atom chirality, chirality correction factors were proposed to be added to the vertex degrees [Schultz, Schultz et al, 1995 Golbraikh, Bonchev et al, 2001a], thus allowing calculation of topological chirality descriptors. 

Techniques for determining the absolute configuration of chiral molecules were not developed until the 1950s and so it was not possible for Eischer and his contemporaries to relate the sign of rotation of any substance to its absolute configuration A system evolved based on the arbitrary assumption later shown to be correct that the enantiomers... 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

The angular splitting of the ho.o (or hh.o) reflexions is a measure for the chiral angle T). However the observed splitting depends as the direction of incidence of the electron beam and must thus be corrected for tilt [20,25]. 

Number of entries with data corrected/completed by the authors Number of new chiral separations per update (each 4 months)... 

If some fields may be empty in the sublevels, all the fields in the main level are required for each entry. A new chiral separation record can be added in CHIRBASE solely if the authors correctly identify both sample and CSP. Since the beginning of the project, our policy has been to contact the authors of all publications containing incomplete, ambiguous or inconsistent data and to ask for additional information. Providing the separations with unique case numbers helps us considerably in this essential task, and also facilitates avoiding redundancies in the database. When chiral separations are reported for the second time in a new publication with exactly the same chromatographic conditions, this is stated in a footnote added in the field comments . In this field, miscellaneous information that cannot appear elsewhere are listed (detection limit, description of a reported chromatogram, racemization study, mobile phase limitations, etc.). 

Molecules like lactic acid, alanine, and glyceraldehyde are relatively simple because each has only one chirality center and only two stereoisomers. The situation becomes more complex, however, with molecules that have more than one chirality center. As a general rule, a molecule with n chirality centers can have up to 2n stereoisomers (although it may have fewer, as we ll see shortly). Take the amino acid threonine (2-amino-3-hydroxybutanoic acid), for example. Since threonine has two chirality centers (C2 and C3), there are four possible stereoisomers, as shown in Figure 9.10. Check for yourself that the R,S configurations are correct. 

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