Quick elucidation of cyclodepsipeptide sequence from sacoglossan Elysia grandifolia using electrospray ionisation-tandem mass spectrometry

Butanol fraction of sacoglossan Elysia grandifolia was investigated for identifying peptides using electrospray ionisation-tandem mass spectrometry (ESI-MS/MS). Without prior isolation, the structural determination is achieved on the basis of mass fragmentation pattern and comparison with the previously established data. The ESI-MS of the fraction in the positive ion mode gave clusters of singly and doubly charged molecular ion peaks. The ESI-MS spectrum showed peaks for the presence of the peptides kahalalides F, G, R and S reported earlier. In addition, it also showed molecular ion peaks at m/z 1557.8 [M+H]⁺ and doubly charged ions at m/z 779.4 [M+2H]²⁺, 790.4 [M+Na]²⁺ and 796.4 [M+K]²⁺. The MS/MS at m/z 779.4 [M+2H]⁺² at collision energy 40 V obtained series of b and y fragment ions. The MS/MS spectrum showed identical fragment ion y₆ at m/z 643 which revealed that cyclic part is identical with kahalalide F, R and S. Careful examination of the fragment ions b₁ to b₇ with their corresponding y fragment ions y₁₂ to y₆, respectively and by comparison of MS/MS pattern of kahalalide S, established that proline can be replaced by tyrosine amino acid residue. The mass difference between b₄ (m/z 511) and b₅ (m/z 674) is equal to 163 which is equivalent to mass residue of tyrosine. Their y fragment ions also quickly helped in fixing the puzzle. This resulted in the identification of the peptide sequence cyclo-[Val-(5-MeHex-Val-Thr-Val-Val-Tyr-Lys-Ile)Thr-Ile-Val-Phe-Dhb)] for the new cyclodepsipeptide, kahalalide Z₃. Thus, ESI-MS/MS has set a trend in quick identification of new marine molecules.