Single-cell proteins from methanol: elucidation of the structures of the unsaturated fatty acids

Abstract

The unsaturated fatty acids contained in single-cell proteins prepared from Li 70 yeasts grown on methanol were studied in order to ascertain the position of unsaturation and the stereochemistry of the double bonds. The acids were extracted and esterified to the corresponding fatty acid methyl esters (FAMEs) as described in previous reports. The location of the double bonds was obtained by gas-chromatographic-mass-spectrometric analysis of the trimethylsilyloxy derivatives of the FAMEs. The stereochemistry was deduced by nuclear magnetic resonance spectroscopy using an incremental addition study with Eu(fod)₃ on FAMEs and on the corresponding epoxy derivatives. Three model FAME samples, methyl oleate (9c-C_18:1), methyl elaidate (9t-C_18:1) and methyl linoleate (9c, 12c-C_18:2), were compared. Most of the unsaturated fatty acids (about 74%) present in the single-cell proteins have structures identical with those known in animal lipids. The three most abundant unsaturated fatty acids display the following structure: 9c-C_16:1(22.5%), 9c-C_18:1(19.4%) and 9c, 12c-C_18:2(29.2%). All double bonds possess a cis-configuration; the position of the double bond is at C9–C10 for the monounsaturated acids and C9–C10 and C12–C13 for diunsaturated acids.