13C NMR spectroscopy for determining the acylglycerol positional composition of lampante olive oils. Chemical shift assignments and their dependence on sample concentration

Abstract

Two Lampante olive oils with free acidity expressed as oleic acid of 25.9 and 8.4 g per 100 g were used to carry out the 13C NMR study for the assignment of all resonances of free fatty acid fraction and of the acylglycerol fraction comprising 1(3)-mono- (1(3)-MAG), 1,3-di- (1,3-DAG), 1,2-di- (1,2-DAG) and triacylglycerols (TAG). The glycerol carbon resonances were found to be resolved according to the esterification degree of the glycerol molecule but no data were available on the length and the unsaturation pattern of the acyl chains. It was the spectral region where the C-1 carboxy carbons resonate that enabled the determination of free fatty acids and of different acylglycerol species. Moreover, within each acylglycerol class, the acyl chain distribution between the 1,3- and 2- glycerol positions was also calculated where the saturated, vaccenate, oleate and linoleate chains were detected as baseline resolved resonances in 1(3)-MAG, 1,3-DAG, 1,2-DAG and TAG. The variability of chemical shifts to sample concentration changes was also evidenced in all the frequency ranges being studied where the methylene C-2 and C-3 carbons underwent the most dramatic changes in their resonance patterns which prevented any resonance assignment.