Autoxidative and Photooxidative Reactivity of Highly Branched Isoprenoid (HBI) Alkenes

Electron ionization mass spectrometric fragmentation and detection of autoxidation products of 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5-ene in Arctic sediments

RATIONALE

Some highly branched isoprenoid (HBI) alkenes are commonly used as proxies for palaeoceanographic reconstructions. However, there is a need to identify compounds that are sufficiently stable and abundant to be used as tracers of HBI oxidation in sediments. 2,6,10,14-Tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5(Z/E)-en-4-ols resulting from 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5-ene appear to be useful for this purpose.

METHODS

Comparison of electron ionization (EI) mass spectra and retention times with those of standards allowed formal identification of autoxidation products of 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5-ene. EI-MS fragmentations of TMS ethers of the main oxidation products (2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5(Z/E)-en-4-ols) were deduced by gas chromatography/electron ionization mass spectrometry (GC/EI-MS), low-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS) and accurate mass measurements. These compounds were then quantified in Arctic sediment samples in MS/MS multiple reaction monitoring (MRM) mode using transitions based on the main fragmentation pathways elucidated.

RESULTS

2,6,10,14-Tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5(Z/E)-en-4-ols were identified after autoxidation of the HBI alkene 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5-ene. Low-energy CID-MS/MS analyses and accurate mass measurements allowed the EI-MS fragmentation pathways of their trimethylsilyl (TMS) derivatives to be elucidated. Some specific fragment ions and chromatographic retention times were also useful for further characterization. As an application of some of the described fragmentations, TMS derivatives of these metabolites were characterized and quantified in MRM mode in Arctic sediments.

CONCLUSIONS

Due to their production in high proportions during autoxidation of their parent HBI diene, their apparent stability in sediments, and their specific EIMS fragmentations, (Z and E)-2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5-en-4-ol TMS derivatives appeared to be useful tracers of HBI autoxidation in sediments.

Electron ionization mass spectrometry fragmentation pathways of trimethylsilyl derivatives of isomeric allylic alcohols derived from HBI alkene oxidation

RATIONALE

C25 tri-unsaturated highly branched isoprenoid (HBI) alkenes are produced by a number of marine diatoms around the world yet are very easily oxidized during senescence to yield several isomeric allylic 9-hydroperoxides. Elucidation of the electron ionization mass spectrometry (EIMS) fragmentation pathways of the trimethylsilyl (TMS) derivatives of the alcohols (obtained by reduction of the corresponding 9-hydroperoxides) is essential for their characterization and quantification in natural samples.

METHODS

EIMS fragmentation pathways of TMS derivatives of isomeric allylic alcohols resulting from NaBH4 reduction of photo- and autoxidation products of HBI alkenes were investigated. These pathways were deduced by: (i) low-energy collision-induced dissociation gas chromatography/tandem mass spectrometry (CID-GC/MS/MS) and (ii) deuterium labelling.

RESULTS

CID-MS/MS analyses and deuterium labelling allowed us to elucidate EIMS fragmentations of TMS derivatives of several isomeric allylic alcohols resulting from NaBH4 reduction of HBI alkene oxidation products and to propose some specific fragment ions for differentiating individual isomers. As an application of some of the described fragmentations, these oxidized compounds in phytoplanktonic cells collected from the Antarctic were characterized and quantified in multiple reaction monitoring (MRM) mode.

CONCLUSIONS

EIMS fragmentations of TMS derivatives of several isomeric allylic alcohols resulting from NaBH4 reduction of HBI alkene oxidation products are shown to be strongly dependent on the position and configuration of double bonds, allowing simple, yet robust differentiation of individual isomers in natural samples.