A systematic pathway of fragmentation for several protonated cyclohexadepsipeptides is demonstrated by positive-ion fast-atom bombardment linked-scan mass spectrometry. It involves classical sequential acylium and immonium ions, but also, unusual fragment ions characteristic of this family of compounds.Since fast-atom bombardment (FAB) and liquid secondary ion mass spectrometry (LSIMS) ionization techniques have been introduced, structural analyses of linear and cyclic peptides have improved. 1 . 2 3 3 Thus, numerous examples of primary structure elucidation have been r e p~r t e d .~. ' In a recent review, Traldi gives important views of tandem mass spectrometry for cyclo(depsi)peptides.6 More recently, an exhaustive review from Eckart summarizes all the strategies developed for sequencing cy~lopeptides.~ In the case of cyclodepsipeptides, Lange et al. proposed two methodologies to determine the primary structure of destruxins', 9, lo which are natural toxins from Metarhizium anisopliae. The first is direct sequence determination by means of sequential carboxylate anions observed in conventional negative-ion FAB mass spectra.' The second approach used positive-ion FAB tandem mass spectrometry with low9 and high" energy collision-induced dissociation (CID). Under the latter conditions, a rapid and unambiguous sequence elucidation of destruxin-E (DTX-E) was performed by two-and three-stage tandem mass spectrometry (MS' and MS3), sequential product-ion spectra and by reaction-intermediate scanning (MS3). These strategies concerned the two most active destruxins, i.e. DTX-E and DTX-A, which possess varied biological activity and more particularly insecticidal value." Due to the high insecticidal potential, a general strategy for the synthesis of such compounds was successfully developed. 12. l3 Thus, various synthetic analogues were prepared.I4,In the present work, we attempt to show that their rapid identification becomes possible by means of characteristic and diagnostic transitions, even using a conventional (EB) double-focusing mass spectrometer.We have analysed a series of six analogues including natural and synthetic compounds (Fig. 1) by positiveion fast-atom bombardment linked-scan mass spectrometry. DTX-A, which was previously analysed on a tandem mass spectrometer: is now examined for comparison with DTX-B and the four synthetic analogues.
EXPERIMENTAL ProductsDestruxins from Meturhizium anisopliae (DTX-A and DTX-B) were purified at the Institut de Chimie des Substances Naturelles, CNRS-France.I6 Synthetic analogues (Lac-6 DTX-E, Hpy-6 DTX-E, Hpp-6 DTX-E and Hpy(TMS)-6 DTX-E) were prepared at the URA-CNRS 0468-France, according to the previously published methodology.". 13, '' Glycerol (G) and mnitrobenzyl alcohol (NBA) were purchased from Prolabo (France). Xenon and helium were provided by Air Liquide (France).
Mass spectrometryMass spectra were obtained on a conventional E B geometry mass spectrometer JEOL model AX-500 equipped with a DEC data system (JEOL-Europe SA, France). Ionization was realized by...
