Abstract
Seco-sativene sesquiterpenoids are an important member of phytotoxins and plant growth regulators isolated from a narrow spectrum of fungi. In this report, eight seco-sativene sesquiterpenoids (1–8) were first analyzed using the UPLC-Q-TOF-MS/MS technique in positive mode, from which their mass fragmentation pathways were suggested. McLafferty rearrangement, 1,3-rearrangement, and neutral losses were considered to be the main fragmentation patterns for the [M+1]+ ions of 1–8. According to the structural features (of different substitutes at C-1, C-2, and C-13) in compounds 1–8, five subtypes (A–E) of seco-sativene were suggested, from which subtypes A, B/D, and E possessed the diagnostic daughter ions at m/z 175, 189, and 203, respectively, whereas subtype C had the characteristic daughter ion at m/z 187 in the UPLC-Q-TOF-MS/MS profiles. Based on the fragmentation patterns of 1–8, several known compounds (1–8) and two new analogues (9 and 10) were detected in the extract of plant pathogen fungus Bipolaris sorokiniana based on UPLC-Q-TOF-MS/MS analysis, of which 1, 2, 9, and 10 were then isolated and elucidated by NMR spectra. The UPLC-Q-TOF-MS/MS spectra of these two new compounds (9 and 10) were consistent with the fragmentation mechanisms of 1–8. Compound 1 displayed moderate antioxidant activities with IC50 of 0.90 and 1.97 mM for DPPH and ABTS+ scavenging capacity, respectively. The results demonstrated that seco-sativene sesquiterpenoids with the same subtypes possessed the same diagnostic daughter ions in the UPLC-Q-TOF-MS/MS profiles, which could contribute to structural characterization of seco-sativene sesquiterpenoids. Our results also further supported that UPLC-Q-TOF-MS/MS is a powerful and sensitive tool for dereplication and detection of new analogues from crude extracts of different biological origins.
Highlights
Seco-sativenes are a member of sesquiterpenoids possessing a unique bicyclo[3.2.1]octane ring system and different substitutions including glycosylation, methylation, and acylation; different heterocyclic rings such as lactone, furan, and pyran ring; and diverse oxygenation sites on the core skeleton, increasing the chemical diversity
The protonated parent ion m/z 267, [M+H]+ of cochliobolin A (1) was not observed in the UPLC-Q-TOF-MS/MS spectra, and it might be that this ion could lose one molecule of H2O to produce a daughter ion at m/z 249 [M+1-H2O]+
The fragmentation routes according to ESI-Q-TOFMS/MS analysis were depicted in Scheme 1, in which typical neutral losses, McLafferty rearrangement, and 1,3-rearrangement were the main fragmentation patterns for the parent ion m/z 267 [M+1]+ (Liang et al, 2018)
Summary
Seco-sativenes are a member of sesquiterpenoids possessing a unique bicyclo[3.2.1]octane ring system and different substitutions including glycosylation, methylation, and acylation; different heterocyclic rings such as lactone, furan, and pyran ring; and diverse oxygenation sites (hydroxylation) on the core skeleton, increasing the chemical diversity. Qader et al (2017) isolated three seco-sativene sesquiterpenoids from B. sorokiniana, in which helminthosporal acid and helminthosporol displayed a strong phytotoxic effect on lettuce seed germination and toxicity against brine shrimps, and helminthosporal acid showed antifungal activity. Seco-sativene analogues displayed strong phytotoxic effects on cereals and gramineous plants, (Ludwig et al, 1956; Ludwig, 1957; Mayo et al, 1961, 1962b, 1963; Spencer, 1965; Katsumi et al, 1967; Taniguchi and White, 1967; White and Taniguchi, 1972; Pena-Rodriguez et al, 1988; PenaRodriguez and Chilton, 1989; Qader et al, 2017; Phan et al, 2019) whereas others possessed plant-growth-promoting biological activities to rice, lettuce, cucumber, and wheat seedlings (Briggs, 1966; Hashimoto et al, 1967; Nukina et al, 1975; Pena-Rodriguez and Chilton, 1989; Miyazaki et al, 2017, 2018; Qader et al, 2017). Efficient approaches for mining novel structures of seco-sativene sesquiterpenoids are urgent
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