The fellowship of natural abundance 2H-isotopomers of monoterpenes.
Site-specific natural abundance hydrogen isotope ratios have been measured by deuterium-NMR in a wide variety of monoterpenes from numerous kinds of plants grown in different environments. Once the NMR signals have been assigned to the whole sets of isotopomers in the different molecules and schemes of connections to the parent isotopomers in the geranyl diphosphate (GPP) precursor have been defined, a very consistent set of isotopic profiles is evidenced. The results, which are incompatible with the mevalonate pathway, can be satisfactorily interpreted by considering the deoxyxylulose pathway (DOXP), which is now recognized as the usual route for monoterpene biosynthesis in plants. Strong deuterium depletion at ex-site 2 of GPP, accompanied by high isotope ratio values at site ex-6, are consistent with synthesis of GPP from isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) molecules independently produced by the DOXP pathway. However, for a given molecular species, significant differences are observed as a function of the plant source, in particular at site ex-6 of GPP. Thus, monoterpenes from plants with a C3 metabolism are mostly characterized by relatively high values of (D/H)6, whereas C4 plants tend to show much lower values. This behavior may be attributed to more or less significant contributions of GPP resulting from the condensation of IPP with DMAPP produced by isomerization. The isotopic profile therefore enables the role of physiological and environmental factors on the relative importance of the "independent" and "isomerized" model to be estimated. More generally, isotope ratios at individual sites in geraniol can be traced back to the corresponding sites in GPP, then to sites of the IPP and DMAPP building blocks, then to the pyruvate and glyceraldehyde 3-phosphate DOXP active molecules, and finally to the carbohydrate photosynthetic precursor. Furthermore, the methylenic hydrogen atoms, which are enantiotopic in geraniol, become diastereotopic in chiral, and more specially in cyclic, monoterpenes. This provides an isotopic verification for the complete stereochemical chain of affiliation, and a way of estimating enantiomeric purity and whether intermolecular exchanges have taken place.