Determination of the amino acid sequence in a cyclic lipopeptide using MS with DHT mechanism.

A TOF MS/MS method to directly determine the amino acid sequence in a cyclic lipopeptide without its hydrolysis is described. The fragments of the peptide and the hydrocarbon chains were identified through comparing the MS of two analogues of the lipopeptide; the connecting relationship of amino acid residues in the lipopeptide was determined based on the difference of mass to charge ratio between peaks in the MS spectra and the amino acid analysis; and finally, according to the mechanism of double hydrogen transfer(DHT) the C-terminal of peptide and hydroxy aliphatic acid in the lipopeptide was directly determined without the hydrolysis. The determined sequence of amino acid residues in the cyclic lipopeptide is also supported by the rest peaks in the MS spectra grounded on simple fragmenting mechanism. This method can be used to determine the amino acid sequence in any aliphatic acid loop-inlaying cyclic lipopeptides.