Complete 13C assignments and structural elucidation of n-3 polyunsaturated fatty acids by the use of a new 2D NMR technique:: SAPHIR-HSQC

The structure of six synthetic n-3 polyunsaturated fatty acids bearing three to six double bonds has been confirmed by high resolution NMR. Complete assignment of C-13 spectra of isomers of alpha-linolenic acid. EPA and DHA was established by the use of the new SAPHIR-HSQC method developed at the NMR center and compared with the results obtained from spin lattice relaxation time measurements. Sequential Attribution of Proton-carbon pairs from High Resolution (SAPHIR) HSQC, HSQC-TOCSY experiments allow both the precise measurement of olefinic coupling constants and the location of each double bond inside the chain. We have also drawn attention to the effect of an E-double bond in comparison with a Z-olefinic bond to the chemical shifts of neighboring carbon atoms. A set of shift parameters, obtained from our NMR data, allowed the visualization of electric and steric effects of E-olefinic bond in Delta(11) and omega 3 positions of the carbon chain of polyunsaturated fatty acids. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.