Electrospray mass spectrometry and tandem mass spectrometry of the natural mixture of cyclic peptides from linseed.

The cyclic peptides from linseed are composed exclusively of the hydrophobic amino acids: Phe, Leu, Ile, Val, Met, Pro, and Trp. Because these compounds does not contain functional groups which undergo easily protonation or deprotonation. their ionization in solvents used usually for peptide analysis is not efficient. A rapid and sensitive procedure for detection and structure elucidation of the cyclic peptides based on ionization with Na+ and NH4+ ions. A cationisation of methionine containing peptides with methyl iodide has been also described. The extract of seeds of Linum utitatissimum was analyzed directly by ESI-MS and neutral loss ESI-MS/MS technique. The analysis confirms the presence of cyclolinopeptides reported previously: CLA (c(Pro-Pro-Phe-Phe-Leu-Ile-Ile-Leu-Val), and CLB (c(Pro-Pro-Phe-Phe-Val-Ile-Met-Ile-Leu)). Cyclolinopeptides CLC, CLD, CLE, and CLG, which contain methionine oxide, were detected in relatively small quantities. These peptides results likely from the oxidation of their not reported precursors: CLD' (c (Pro- Phe-Phe-Trp-Ile-Met-Leu-Leu)), CLE'(c (Pro-Leu-Phe-Ile-Met-Leu-Val-Phe)), CLF (c (Pro-Phe-Phe-Trp- Val-Met-Leu-Met), and CLG (c (Pro-Phe-Phe-Trp-Ile-Met-Leu-Met), present at higher concentrations in the extract protected from atmospheric oxygen. The sequences of the unreported cyclic peptides were proposed on the basis of CID experiments and homology with peptides described by Morita,1,2 and supported by the fragmentation of synthetic analogues of CLA of a known structure.