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

• Published in: European journal of mass spectrometry (Chichester, England) England), 2017-06, Vol.23 (3), p.92-97
• Main Authors: Tilvi, Supriya, Devi, Prabha, Majik, Mahesh S
• Format: Article
• Language: English
• Subjects: Animals; Depsipeptides - analysis; Depsipeptides - chemistry; Mollusca - chemistry; Peptide Mapping - methods; Reproducibility of Results; Sensitivity and Specificity; Sequence Analysis, Protein - methods; Spectrometry, Mass, Electrospray Ionization - methods; Tandem Mass Spectrometry - methods
• ISSN: 1469-0667; 1751-6838
• DOI: 10.1177/1469066717699218

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.