Structure determination of long-chain polyunsaturated triacylglycerols by high-resolution C-13 nuclear magnetic resonance

The application of C-13 nuclear magnetic resonance to determine the positional distribution of fatty acids on the glycerol backbone has been investigated. A systematic approach and synthetic triacylglycerols were used to measure the effect on the carbonyl chemical shifts of triacylglycerols by the positional distribution on the glycerol backbone and the number and position of the double bonds within the fatty acids. The correlation of C-13 carbonyl chemical shift to the molecular structure of triacylglycerol was delineated. The assignments for the chemical shifts of the carbonyl nuclei of monoacyltriacylglycerol standards were compiled. The resonance from the carbonyl carbons at the 1,3 positions is resolved from that at the 2 position. The C-13 carbonyl chemical shift was more dependent on the position of the double bonds than the degree of unsaturation of the fatty acids. In particular, little effect was observed in the chemical shifts for fatty acids containing more than two double bonds. However, the chemical shifts were influenced significantly by the position of the first double bond. The difference in the chemical shift of the unsaturated species from that of the saturated species was plotted against the position of the first double bond. A natural logarithmic relationship was found between carbon numbers 5 and 11. Inflection points were found outside of this region at carbon numbers 4 and 13. In addition, the resonances from the saturated species, independent of their chainlength, were degenerate in oil systems, even though small differences were observed in the standards. The applicability of this method was demonstrated in the determination of the composition and positional distribution of the fatty acids in borage and evening primrose oils.