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The EIMS fragmentation mechanisms of the sesquiterpenes corvol ethers A and B, epi -cubebol and isodauc-8-en-11-ol

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ABSTRACT

Farnesyl diphosphate (FPP) and all fifteen positional isomers of (13C1)FPP were enzymatically converted by the bacterial terpene cyclases corvol ether synthase from Kitasatospora setae, the epi-cubebol synthase from Streptosporangium roseum, and the isodauc-8-en-11-ol synthase from Streptomyces venezuelae. The enzyme products were analysed by GC–MS and GC–QTOF MS2 and the obtained data were used to delineate the EIMS fragmentation mechanisms of the two sesquiterpene ethers corvol ethers A and B, and the sesquiterpene alcohols epi-cubebol and isodauc-8-en-11-ol.

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Mass spectra of unlabelled 3 and all fifteen positional isomers of (13C1)-3.
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Figure 3: Mass spectra of unlabelled 3 and all fifteen positional isomers of (13C1)-3.

Mentions: The mass spectra of 3 and the fifteen corresponding isotopomers of (13C1)-3 are shown in Fig. 3. For the unlabelled compound a small, but clearly visible molecular ion is detected at m/z = 222. The base peak is recorded at m/z = 207 and other important fragment ions are observed at m/z = 179, 161, 119 and 43. As revealed by PMA207, the base peak ion in the mass spectrum of 3 is only formed by loss of the methyl group C15, while cleavages of any of the other methyl groups do not contribute to its formation (Scheme 4). This is easily understood by electron impact ionisation of 3 at the hydroxy function to 3a+·, followed by α-cleavage of C15. PMA179 indicates a formation of the fragment ion m/z = 179 by loss of the isopropyl group C11–C12–C13 of 3 (Scheme 4). Usually in sesquiterpene alcohols the electron impact ionisation proceeds with loss of an electron from one of the oxygen lone pairs, but in the special case of epi-cubebol that contains a 3-membered ring the ionisation step may also proceed with removal of an electron from the energetically high molecular orbitals of the cyclopropane moiety, resulting in 3b+·. Neutral loss of the isopropyl group is then possible by α-cleavage to B3+. PMA161 shows that the fragment ion m/z = 161 represents the same part of the carbon skeleton of 3 as m/z = 179 (Scheme 4), requiring the elimination of water that is plausible starting from 3b+· by a hydrogen rearrangement to C3+·, neutral loss of water by inductive cleavage to D3+· and α-cleavage to E3+. The reverse order of steps for the losses of the isopropyl group and water is also possible, but MS2 analyses of m/z = 204 and m/z = 179 show that this alternative mechanism is much less pronounced (Figures S7 and S8, Supporting Information File 1).


The EIMS fragmentation mechanisms of the sesquiterpenes corvol ethers A and B, epi -cubebol and isodauc-8-en-11-ol
Mass spectra of unlabelled 3 and all fifteen positional isomers of (13C1)-3.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4979868&req=5

Figure 3: Mass spectra of unlabelled 3 and all fifteen positional isomers of (13C1)-3.
Mentions: The mass spectra of 3 and the fifteen corresponding isotopomers of (13C1)-3 are shown in Fig. 3. For the unlabelled compound a small, but clearly visible molecular ion is detected at m/z = 222. The base peak is recorded at m/z = 207 and other important fragment ions are observed at m/z = 179, 161, 119 and 43. As revealed by PMA207, the base peak ion in the mass spectrum of 3 is only formed by loss of the methyl group C15, while cleavages of any of the other methyl groups do not contribute to its formation (Scheme 4). This is easily understood by electron impact ionisation of 3 at the hydroxy function to 3a+·, followed by α-cleavage of C15. PMA179 indicates a formation of the fragment ion m/z = 179 by loss of the isopropyl group C11–C12–C13 of 3 (Scheme 4). Usually in sesquiterpene alcohols the electron impact ionisation proceeds with loss of an electron from one of the oxygen lone pairs, but in the special case of epi-cubebol that contains a 3-membered ring the ionisation step may also proceed with removal of an electron from the energetically high molecular orbitals of the cyclopropane moiety, resulting in 3b+·. Neutral loss of the isopropyl group is then possible by α-cleavage to B3+. PMA161 shows that the fragment ion m/z = 161 represents the same part of the carbon skeleton of 3 as m/z = 179 (Scheme 4), requiring the elimination of water that is plausible starting from 3b+· by a hydrogen rearrangement to C3+·, neutral loss of water by inductive cleavage to D3+· and α-cleavage to E3+. The reverse order of steps for the losses of the isopropyl group and water is also possible, but MS2 analyses of m/z = 204 and m/z = 179 show that this alternative mechanism is much less pronounced (Figures S7 and S8, Supporting Information File 1).

View Article: PubMed Central - HTML - PubMed

ABSTRACT

Farnesyl diphosphate (FPP) and all fifteen positional isomers of (13C1)FPP were enzymatically converted by the bacterial terpene cyclases corvol ether synthase from Kitasatospora setae, the epi-cubebol synthase from Streptosporangium roseum, and the isodauc-8-en-11-ol synthase from Streptomyces venezuelae. The enzyme products were analysed by GC–MS and GC–QTOF MS2 and the obtained data were used to delineate the EIMS fragmentation mechanisms of the two sesquiterpene ethers corvol ethers A and B, and the sesquiterpene alcohols epi-cubebol and isodauc-8-en-11-ol.

No MeSH data available.