High-resolution 13C-NMR olefinic spectra of DHA and EPA acids, methyl esters and triacylglycerols

Abstract

The high-resolution 13C-NMR spectra of the olefinic carbons of two major n-3 polyunsaturated fatty acids of fish oils, DHA (22:6, n-3) and EPA (20:5, n-3), were recorded at 100.64 MHz in chloroform- d solutions on standard free acids, methyl esters, Atlantic tuna ( Thunnus alalunga) triacylglycerols and methyl esters. All olefinic resonances were resolved in DHA and EPA acids and methyl esters, the number of olefinic signals corresponding to that of unsaturated carbons. The order of the spin-lattice relaxation times ( T 1) and the chemical shift differences (Δδ) between signal pairs related to the same double bond led to the complete assignment of DHA and EPA olefinic carbons in acids and methyl esters. The DHA and EPA olefinic shifts of Thunnus alalunga triacylglycerols were also studied and compared with those of methyl esters. The possibility of using 13C-NMR olefinic carbons for the analysis of fish lipids and oils is discussed.