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Influence of Fatty Acid Desaturation on Spontaneous Acyl Migration in 2-Monoacylglycerols

Compton, David L., Laszlo, Joseph A., Appell, Michael, Vermillion, Karl E., Evans, Kervin O.
journal of the American Oil Chemists' Society 2012 v.89 no.12 pp. 2259-2267
Gibbs free energy, carbon, ethanolysis, fatty acids, spectroscopy, temperature, triacylglycerols
The effect of desaturation from the C9 to the C15 carbon of 2-monoacylglycerol (2-MAG) fatty acids on spontaneous acyl migration is described. Three 2-MAG species, 2-monooleoylglycerol (C18:1 cis-Δ9), 2-monolinoleoylglycerol (C18:2 cis-Δ9,12), and 2-monolinolenoylglycerol (C18:3 cis-Δ9,12,15) were synthesized by lipase-catalyzed ethanolysis of their respective triacylglycerols and isolated in >60 % yield and at 2-MAG purities of >95 % relative to 1-monoacylglycerol (1-MAG). ¹H-NMR spectroscopy was used to monitor the spontaneous acyl migration of the 2-MAG species over a temperature range from 20 to 80 °C. The relative energies of activation calculated from the Arrhenius relationships of the 2-MAG acyl migration rate constants were 73.3, 68.0, and 72.9 kJ mol⁻¹ for the three 2-MAG species, respectively. Density functional calculations performed using the B3LYP functional at the 6-31+G* basis set on the three ketal ring intermediate of the three 2-MAG species followed a similar trend with a lack of relative energetic preference associated with the degree of desaturation. The kinetically determined relative activation energies were approximately twofold higher than the theoretical relative Gibbs free energies of the intermediates, suggesting that other factors influence acyl migration. In general, increasing desaturation after the C9 carbon of 2-MAG fatty acids had no appreciable effect on acyl migration rates.