Tandem mass spectrometry of kahalalides: Identification of two new cyclic depsipeptides, kahalalide R and S from Elysia grandifolia
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Spectra obtained using electrospray ionization mass spectrometry (ESI-MS) of mollusk Elysia grandifolia showed a cluster of molecular ion peaks centered at a molecular mass of 1478 Da (kahalalide F, an anticancer agent). Two new molecules, kahalalide P (m/z 1464) and Q (m/z 1492) were characterized using tandem mass spectrometry. The mass differences of 14 Da suggest that they are homologous molecules. In addition, previously identified kahalalide D and kahalalide G were also reported. While, ESI-MS of its algal diet Bryopsis plumosa showed presence of only kahalalide F. The amino acid sequences of kahalalide P and Q were proposed using collision induced dissociation (CID) experiments of singly and doubly charged molecular ions and by comparison with the amino acid sequence of kahalalide F. The pathway is presented for the loss of amino acid residues in kahalalide F. It is observed that there is sequential loss of amino acids in the linear peptide chain but in the cyclic part, the ring opens at the amide bond rather than at the lactone linkage and the loss of amino acid residues is not sequential. The CID experiment of the alkali metal cationized molecular ions shows that the sodium & potassium ions coordinate to the amide nitrogen/oxygen in the linear peptide chain of the molecule and not lactone oxygen of the lactone. In the case of kahalalide D, CID of the protonated peptide opens the depsipeptide ring to form a linear peptide with acylium ion and fragment ions signals indicate losses of amino acids in sequential order. In this study, tandem mass spectrometry provides detailed information required to fully characterize the new peptides.
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