A new solution for an old problem: the regiochemical distribution of the acyl chains in galactolipids can be established by electrospray ionization tandem mass spectrometry

Electrospray ionization-quadrupole ion trap mass spectrometry (ESI-QITMS), either in positive- or in negative-ion mode, has been used to establish the chemical structures (chain length, degree of unsaturation, positional distribution) of the fatty acids attached to the primary (sn-1) and secondary (sn-2) hydroxyl groups of the glycerol moiety of natural monogalactosyl- (MGDG) and digalactosyldiacylglycerols (DGDG), isolated from the freshwater dinoflagellate Glenodinium sanguineum and from a marine diatom belonging to the genus Chaetoceros. Fragmentation by collision-induced dissociation of a single component in MGDG and DGDG mixtures, separated by high-performance liquid chromatography (HPLC) and detected on-line by tandem positive-ion ESI-MS, leads to a clear-cut determination of the positional distribution of the sn-glycerol-bound fatty acyl chains. Reversed-phase liquid chromatography allowed a partial resolution of the component mixture before ESI-MS/MS analysis. These results were validated by comparison with ESI-MS data obtained for the sn-2 lysoglyceroglycolipids synthesized via regiospecific enzymatic hydrolysis of the corresponding diacylglycerols by Rhizopus arrhizus lipase.