Using simple 13C NMR linewidth and relaxation measurements to make detailed chemical shift assignments in triacylglycerols and related compounds.

Two simple experiments measuring the 13C linewidths nu(1/2) and spin-lattice relaxation times T(1) of each of the signals in the spectrum of trilinolein indicate that the nu(1/2) and T(1) values are consistent with the different degrees of motional freedom expected for the various 13C nuclei. However, for each chain, the nu(1/2) and T(1) measurements indicate a small reversal in mobility at C-10 relative to C-9 before motional freedom again steadily increases on each chain starting at C-11. The T(1) experiment allows unambiguous assignments of the C-8 signal and C-14 signal, which differ by only 0.010 ppm. Measurements of 13C nu(1/2) and T(1) values on tripalmitin provide secure assignments for the C-5 and C-6 signals, for which conflicting assignments have been reported. The T(1) measurements also show that among the tightly clustered C-8 through C-12 signals, the C-11 signals are the most downfield, while the C-12 signals are the most upfield, again contrary to a previous report. Similar measurements of 13C nu(1/2) and T(1) values on other triacylglycerols or related compounds may prove equally useful in making chemical shift assignments and detecting any discontinuities in motional freedom along a chain. The benefits and possible limitations of ultrahigh field NMR for studying triacylglycerols and related compounds are discussed.