Tandem mass spectrometry in the structural analysis of an oviposition‐deterring pheromone

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Abstract
The potential of tandem mass Spectrometry (MS/MS) as a stand‐alone technique in the structural analyses of an oviposition‐deterring pheromone (ODP, 1) is reviewed. Two facets of the salt‐like glycolipid structure of 1 were of major interest in this context: the substitution pattern of the lipid backbone (15‐glucosyioxy‐8‐hydroxypalmitate) and, more specifically, the configurational identity of the sugar portion (glucopyra nose). Throughout this study, trideuterioacetyl derivatives of ODP (1 → pentakis(trideuterioacetyl)‐ODP la) and the reference substrates were used. Probing of the sugar moiety by fast atom bombardment (FAB) and both low‐ and high‐energy collision‐induced dissociation (CID) of B2‐type sugar ions surprisingly failed as a single exception within a larger number of glycosidic substrates subjected to this approach. However, electrospray ionization (ESI) of la with the formation of the sugar ions in the gas phase by ‘first‐stage’ CID before mass selection circumvented this difficulty and provided an unambiguous and sensitive probe for sugar stereochemistry. When studying the ODP molecule as a whole, FAB‐generated M‐like ions such as [M − H]−, [M + Na]+ and [M − H + 2Na]+ were subjected to high‐energy CID using a four‐sector tandem mass spectrometer. Analyses of simple model substrates such as the 12‐trideuterioacetoxystearate anion facilitated the interpretation of the distinct charge‐remote fragmentation (CRF) behaviour of la. Whereas all M‐like species provided complete records of the lipid portion of la and its oxygenation pattern, only the sodiated cations allowed reliable location of the individual substituents. In these latter species complementary series of ‘sequence ions’ were observed that incorporated either the taurine or the sugar terminus and thus reflected CRF for both alternatives of terminal charge fixation.