Determination of the structure of the fatty acid chain in a cyclic lipopeptide using GC–MS

A GC–EIMS method to determine the structure of the fatty acid chains in cyclic lipopeptides is described. The structure of the fatty acid chains can be determined by the characteristic peaks of the MS spectrogram according to the fact that the alpha cleavage predominates the MS of a fatty acid with amino and hydroxy groups, while the McLafferty rearrangement predominates the MS of one without amino or hydroxy group. The characteristics of the strongest peaks of 103 and 102 in MS spectrograms due to alpha cleavage represent the β-hydroxy-fatty acid and the β-amino fatty acid, respectively; the strongest peak of 117 due to alpha cleavage and the relatively weak peak of 88 due to McLafferty rearrangement indicate the β-hydroxy-fatty acid with a branched methyl group at its alpha position. The strongest peak of 74 due to McLafferty suggests the fatty acid without hydroxy or amino group. The ratio of relative intensity (I43/I57) characterizes the branches of alkyl chains. The greater I43/I57 corresponds to an iso alkyl, and the smaller I43/I57 corresponds to an anteiso alkyl. This method can be used to determine the full structure of the fatty acid chains in lipopeptides.