Compound characterisation nuclear magnetic resonance

Metabolomics studies the entire metabolism of an organism. It is possible to consider characterising the complex pattern of cellular proteins and metabolites that are excreted in urine. Pattern recognition techniques of nuclear magnetic resonance spectra have been applied to determine the dose-response using certain classical liver and kidney toxicants (Robertson et al, 2000). This could well provide a signature of the functional state of the kidney, and perturbations in the pattern as a result of exposure to a chemical could be observed. But first it would be necessary to understand how compounds with known effects on the kidney affect these processes. 

The application of analytical methods to speciation measurements in complicated systems has remained rather limited, despite the considerable technological progress during the past 25 years. The characterisation methods (e.g. spectroscopy, nuclear magnetic resonance) are often limited to the study of isolated compounds at relatively high concentrations. They, therefore, necessitate the prior employment of sophisticated separation and pre-concentration methods which introduce severe risks of perturbation. The trace analysis methods are often insensitive to the chemical form of the elements measured (e.g. atomic absorption, neutron activation). Those which possess sufficient element specificity (e.g. electron spin resonance, fluorescence, voltammetry) still require significant development before their full potential can be realised. 

Diverse spectroscopic methods have been employed to characterise triterpenes. Ultraviolet (UV) and infrared (IR) spectroscopy are not very useful techniques in elucidating the structure of triterpenes, but the former gives information about compounds with conjugated double bonds and the latter may provide some information about substituents like the hydroxyl group, ester carbonyl group or a,p-unsaturate carbonyl. Other physical data may be of interest to characterise new compounds, but the use of modem spectroscopic methods of nuclear magnetic resonance (NMR) and mass spectroscopy (MS) are essential for the structural determination. 

These workers point out that usually the additive must be separated in a pure state from co-extracted additives usually by thin-layer chromatography (TLC) and then identified by measurement of the UV, IR, nuclear magnetic resonance (NMR) and mass spectra of the compound. This full treatment is required only for new stabilisers - for a characterisation of well known compounds the simplest method is by direct comparison of the UV absorption spectra with those of a series of known stabilisers. For some compounds this will probably be sufficient, but many substituted phenols have similar spectra, and for three of the most frequently used antioxidants the UV spectra are identical. Topanol OC, lonox 330 and Binox M (see Table 2.11 for their chemical constitution) in ethanolic solution all have = 277 nm, with a shoulder at 282 nm. To extend this procedure Ruddle andWilson [66] prepared the spectra of alkaline solutions of the phenols, which were then measured either directly against a solvent blank or as difference spectra measured against the neutral solution. This still gives almost identical spectra for the three compounds mentioned previously. 

Verbruggen and co-workers [13] prepared a model NR gumstock (i.e., cure system only, no fillers or other additives) compound and subjected a thin film (300 pm thick) of it to DPDS at 200 "C. Solvent extraction (with acetone and tetrahydrofuran), swelling experiments (in toluene) and chemical analysis by GPC and nuclear magnetic resonance (NMR) spectroscopy were used to assess the degree of devulcanisation and to characterise the samples produced. It was found that complete network breakdown was obtained with 2.4% w/w of DPDS after 2 h of heating, but that both crosslink scission and main-chain scission had occurred. 

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