Isotopomers, definition

To understand the evaluation of a CLE, we need to introduce some terms The word isotopomer is a combination of the terms isotope, and isomer. An isotopomer is one of the different labeling states in which a particular metabolite can be encountered [248] that is, a molecule with n carbon atoms has 2" isotopomers. These are usually either depicted using outlined and filled circles for unlabeled and labeled atoms, respectively (see Fig. 14), or are described in text format for example, C 010 would be the isotopomer of a three-carbon molecule labeled at the second position. An isotopomer fraction is the percentage of molecules in this specific labeling state. The positional enrichment is the sum of all isotopomer fractions in which a specific carbon atom in a specific metabolite is labeled [248]. Consequently, the usage of isotopomers enables to account for more information While a molecule with n carbon atoms will yield n positional enrichments, there are 2 — 1 isotopomer fractions (the 2"th measurement is redundant as, by definition, isotopomer fractions must sum up to unity) [260],... 

By definition, it is necessary to recognize geometric isomers, isotopomers and stereoisomers as distinct species. Moreover, there is the pragmatic issue that regio-selectivity, isotopic labeling and stereo-chemical investigations are three very important avenues of mechanistic enquiry. 

From what has been shown in the preceding sections (cf. Eqs. 61 and 73, 83), it is possible to present the molecular structure resulting from both the r -fit method and any of the r()-derived methods in a convenient and easily comparable form, as a structural description in both Cartesian and internal coordinates, and with consistent errors and correlations (for small and larger molecules). A detailed comparison would require a sufficiently large SDS to determine a complete molecular structure, but the requirements are still the least restrictive of all methods presented. The input data must include the covariance matrix of the rotational constants or moments. This matrix may have to be adequately modeled to avoid grossly different weighting of isotopomers which is usually not warranted. The definition of the input data set... 

SPisjgO Stands for the site preference, for a definition see the section about isotopomers. 

The natural abundances of the stable isotopes given in Table 6.3 are global average values, whereas the actual exact values are subjected to small local and temporal variations, due to the slightly different behaviour of isotopologue molecules (by lUPAC definition a molecular entity that differs only in isotope composition, means number of isotopic substitutions in contrast, isotopomer [ isotopic isomer ] molecules are isomers, having the same number of each isotopic atom but in different positions) in the course of chemical reactions or physical processes ([8], kinetic and thermodynamic isotope effects, respectively, see below). The corresponding shifts are so small that they caimot be indicated in the atom-% scale therefore they are expressed in 5-values, differences of the isotope ratio R, e.g., ([ C]/[ C]) of the sample and an international standard relative to this international standard ... 

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