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

Two simple experiments measuring the C-13 linewidths v(1/2) and spin-lattice relaxation times T-1 of each of the signals in the spectrum of trilinolein indicate that the v(1/2) and T-1 values are consistent with the different degrees of motional freedom expected for the various C-13 nuclei. However, for each chain, the v(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 C-13 v(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 C-13 v(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. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.