Tandem mass spectrometric analysis of fatty acyl groups of galactolipid molecular species from wheat flour

Our previous work demonstrated that the structures of two and one molecular species of digalactosyl and monogalactosyl diacylglycerols from wheat flour, respectively, were determined by collision-induced dissociation (CID) of sodium-adducted molecules ([M + Na](+)) desorbed by fast atom bombardment (FAB). However, many more components in their HPLC separations were identified. Then fractionated components were structurally determined by CID tandem mass spectrometry (MS/MS), including the fatty acid composition and the double-bond positions in the fatty acyl groups. Furthermore, the relative positions of two fatty acid chains on the glycerol backbone could be assigned by the ratio of the intensity of two specific product ions observed in the CID spectra. The product ion ([M + Na-R2COOH](+)) due to the neutral loss of the fatty acyl group at the sn-2 position via free fatty acid was more abundant than the one ([M + Na-R1COOH](+)) due to the loss of the fatty acyl group at the sn-l position. The regiospecificity of two fatty acyl linkages was also confirmed by the results which were obtained from the FAB mass spectra of sn-2 acyl lysogalactolipids. These compounds were synthesized by a specific enzyme, Lipase XI (from Rhizopus arrhizus), which cleaved specifically ester bond between glycerol backbone and sn-l fatty acyl group. (C) 2001 Elsevier Science B.V. All rights reserved.