13C NMR Spectroscopy to Determine Structure and Composition of Waxes and Glycerides

C-13 nuclear magnetic resonance spectroscopy has been successfully applied to determine the structures of natural derivatives of long-chain fatty acids and in particular, waxes and glycerides. The term wax is used to indicate esters of aliphatic acids with alcohols other than glycerol. C-13 chemical shifts of unsaturated carbons in long-chain esters and acids were explained in terms of long-range sigma-inductive interactions through saturated C-C bonds in polymethylene chains. A mathematical model was proposed to predict NMR shifts at sp(2) carbons of mono-and non-conjugated polyenoic acids and esters. C-13 NMR was also applied for studying the structure of glycerides. Unsaturation degree, cis-trans isomerization, and positional isomerism of fatty acids were determined along with their distribution between the 1,3- and 2-positions of glycerol backbone. The compositional data of the two fatty acid pools which esterify 1,3- and 2-glycerol positions, opened a new frontier in the knowledge of glyceride structures. Positional data of triglycerides enabled the set-up of new analytical methodologies which apply quantitative C-13 NMR spectroscopy to determine the composition of natural mixtures of triacylglycerols, i.e. vegetable oils.