Experimental studies on the hydrogen isotopic effects associated with structural changes in pentacyclic triterpenes
Résumé
As other biochemical compounds archived in soils and sediments, pentacyclic triterpenes can undergo several transformations that can alter the original hydrogen isotopic composition they acquired during biosynthesis. These well-known transformations (i.e. Corbet et al., 1980; Rullkötter et al., 1994) may include migration or suppression of methyl groups and double bonds, ring-A opening and degradation, progressive aromatisation, and, eventually, hydrogen exchange with environmental waters. Reversely, the extent to which these can affect the original δD is poorly understood. In order to quantify these isotopic effects, we submitted a series of standard pentacyclic triterpenes of known δD to various conditions and then measured the δD of each by-product. The set of standard pentacyclic triterpenes included triterpene ketones (lupenone, friedelin, β-amyrenone and α-amyrenone) methyl ethers (miliacin, β-amyrin ME and crusgallin), acetates (β- and δ-amyrenyl, taraxeryl, multiflorenyl and glutinyl acetates) and β-amyrin. In the first experiment, we exposed our standards to UV radiation during 24h. As expected (Simoneit et al., 2009), only ketones (lupenone, friedelin, β-amyrenone and α-amyrenone) were significantly affected. The δD values of compounds amenable to GC-irMS resulting from their photochemical degradation (canaric, putranjivic, nyctanthic and roburic acid derivatives, respectively) did not exhibit significant difference with the δD values of original compounds, except for lupenone. In the second experiment, our standard compounds were subjected to a solution of acetic acid/sulfuric acid/D2O (50:1:1 vol.) for 3 days at 50 °C (Pakrashi and Samanta, 1967). The resulting compounds were recovered by liquid extraction, concentrated, and analysed by GC-MS after addition of 5α-cholestane as an internal standard. Except for friedelin, all compounds were partially transformed. Friedo-rearranged compounds of the oleanane series such as multiflorenyl, taraxeryl and glutinyl acetates were transformed into β-amyrenyl (Δ12), δ-amyrenyl (Δ13,18) and germanicyl acetates (Δ18), β-amyrenyl acetate being converted into δ-amyrenyl acetate that is itself converted into germanicyl acetate. These transformations are in agreement with previous results (Chatterjee et al., 1976). They are accompanied by a significant enrichment in deuterium that is sensitive to the mass spectra of resulting compounds as revealed by a noticeable increase in the molecular mass and an increase in the mass of key fragments. The determination of the δD values of the various compounds obtained will allow us to quantify these isotopic effects. In the third experiment, miliacin was mixed with D2O in the presence of clay minerals (montmorillonite, smectite, kaolinite) for 8 months at 80 °C to estimate hydrogen exchange between water, miliacin and clay minerals. REFERENCES Chatterjee, A., Mukhopadhyay, S., Chattopadhyay, K., 1976. Lewis acid catalysed rearrangement of triterpenoids. Tetrahedron 32, 3051-3053. Corbet, B., Albrecht, P.T., Ourisson, G., 1980. Photochemical or photomimetic fossil triterpenoids in sediments and petroleum. J. Amer. Chem. Soc. 102, 1171-l 173. Pakrashi, S.C., Samanta, T.B., 1967. Acid induced epimerisation and rearrangements of Arborinol, the novel triterpene from Glycosmis arborea. Tetrahedron Letters 38, 3679-3684. Rullkötter, J., Peakman, T.M., ten Haven, H.L., 1994. Early diagenesis of terrigenous triterpenoids and its implications for petroleum geochemistry. Organic Geochemistry 3, 215-233. Simoneit, B.T., Xu, Y., Neto, R.R., Cloutier, J.B., Jaffé, R., 2009. Photochemical alteration of 3-oxygenated triterpenoids: Implications for the origin of 3,4-seco-triterpenoids in sediments. Chemosphere 74, 543-550.