Specific Substitution Patterns Research Articles

Tetrachlorodibenzo-p-Dioxin Isomer Differentiation by Capillary Gas Chromatography Fourier Transform-Infrared Spectroscopy

Reference infrared vapor-phase spectra of the 22 tetrachlorodibenzo-dioxin (TCDD) isomers were recorded at low microgram concentrations. These reference spectra of synthetic mixture components separated chromatographically or by spectral subtraction exhibit distinct infrared spectra for each isomer. The infrared frequencies are delineated in correlation tables and are interpreted in terms of substitution patterns which determine the strength of the ether linkage. Absorbance values in the 1330–1280 cm−1 (C-O-C asymmetric stretch) region correlate with specific substitution patterns and molecular geometry. Relative electron-withdrawing capacities for chlorinated aromatic rings in TCDD isomers were estimated on the basis of relative capacities determined for model compounds. Qualitative correlations were established between electron-withdrawing capacities and the effects of resonance and field interactions on the ether linkage absorption frequencies of individual TCDD isomers. Gas chromatography Fourier transform infrared (GC/FT-IR) isomer assignments are generally consistent with those obtained by proton Fourier transform nuclear magnetic resonance (1H FT/NMR) and flame ionization gas chromatography (GC/FID). A chromatographically independent method of assigning TCDD isomer structures on the basis of ether linkage asymmetric stretching frequencies was established by utilization of valence-bond approximations. GC/FT-IR assignments for several TCDD isomers differ from isomer assignments in previously published results. A user-generated, vapor-phase reference library, containing individual TCDD spectra and spectra of isomer pairs that are incompletely resolved on chromatographic columns, correctly identified each isomer in variety of mixtures by means of a software algorithm.

Chemical Approach to Genetics

AbstractThe modifications taking place in the substitution pattern of organic molecules during cross breeding by pollination has been investigated. Through the process of hybridisation of different “chemotypes” of Withania somnifera (Solanaceae), changes have been observed among the substituents present in the steroidal lactones of the withanolide type which are characteristic for this and related species. Several reciprocal cross‐pollinations were performed between the types and the structures of the withanolides produced in the leaves were analysed and determined. These studies led to an understanding of the biogenetic processes leading to specific substitution patterns, and in certain cases enabled a genetic control over the compounds produced in the plant. Compounds not present in natural types could be anticipated to be formed, and indeed were present in the hybrids. Through these studies an insight on the reactions governed by the genetic background of the types taking part in the hybridisation was achieved. Groupings, when found in specific locations of the skeleton could be described as present there due to dominant or recessive genetic characters. These conclusions were reached following analysis of a large number of offsprings for several generations. This method enabled the study of the genetic characters of a plant on the chemical level.