The key role of 13C NMR analysis in the identification of individual components of Polyalthia longifolia leaf oil
Résumé
Polyalthia longifolia produces sesquiterpene-rich essential oils (EOs) whose compositions varied substantially from sample to sample depending on the origin of the plant (Nigeria and Vietnam). Nothing is known about the phytochemistry of Ivoirian P. longifolia. The aim of the present study was to characterize Ivoirian P. longifolia through the chemical composition of the leaf oil and to develop a strategy that allows the identification of minor oxygenated sesquiterpenes whose MS data are not compiled in commercial or laboratory-constructed MS libraries. The EO was submitted to gas chromatography (GC) retention index (RI), GC-mass spectrometry (MS) and 13C nuclear magnetic resonance (NMR) analysis. Then hydrocarbons and oxygenated components were separated and the oxygenated fraction was chromatographed on silica gel. The fractions were analysed by GC(RI) and 13C NMR. Seventy compounds accounting for 91.8% of the EO were identified. Sesquiterpene hydrocarbons, (E)-β-caryophyllene (27.8%), α-zingiberene (20.0%) and allo-aromadendrene (15.0%), were the major components. Various oxygenated sesquiterpenes whose MS data were not compiled in commercial and laboratory-made MS libraries were identified by comparison of their chemical shift values in the spectrum of the fraction of CC with those reported in the literature and compiled in a laboratory-constructed 13C NMR data library. The composition of the investigated Ivoirian P. longifolia oil sample presented similarities and differences with Nigerian and Vietnamese oils. Combined analysis of Ivoirian P. longifolia EO by chromatographic and spectroscopic techniques including 13C NMR without isolation of the components, appeared particularly efficient to identify minor components of EOs, whose MS spectra are insufficiently differentiated or MS data are not compiled in commercial and lab-constructed MS libraries.