Observation of the terminal methyl group in fatty acids of the linolenic series by a new proton NMR pulse sequence providing spectral editing and solvent suppression. Application to excised frog muscle and rat brain

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Abstract
A new 1H NMR pulse sequence is described that combines water suppression with the selective observation of signals from coupled spin systems. The pulse sequence is easy to set up and compensates for pulse width inhomogeneity in the biological sample. Suppression of the water signal is achieved by pulses that return the water spins to their equilibrium position; spectral editing is based on the J modulation present in spin-echo spectra and its inhibition by coherent decoupling at one of the resonances of the spin system of interest. The pulse sequence, which ws designed for 1H NMR spectroscopy of tissue, was tested at 470 MHz on excised frog muscle and rat brain. The lactate methyl resonance of caffeine-treated frog sartorius muscle was observed selectively by irradiation at the position of its alcoholic proton. The terminal methyl signal of linolenic acid, along with other fatty acids of the linolenic series (first double bond in the .omega.-3 position), was observed selectively by irradiation at the position of its .omega.-1 methylene group. 1H NMR spectra of rat brain were edited to reveal the terminal methyl of either linolenic series or all other fatty acids. The results suggest that the terminal methyl groups of fatty acids of the linolenic series (mostly docosahexaenoic acid, 22:6) have higher mobility than those of all other fatty acids.