Laurent Serani

Structural investigation of cyclic peptidolipids from Bacillus subtilis by high-energy tandem mass spectrometry.

The natural products belonging to the surfactin family are cycloheptapeptides bearing a long beta-hydroxy-fatty acyl chain at the N-terminal position. The structure of these compounds, often isolated as complex mixtures, can be elucidated by high-energy tandem mass spectrometry (MS/MS). The protonated molecules generated by cesium ion bombardment (LSIMS) undergo charge-proximate fragmentations leading to the b- and y-type ion series useful for the sequence determination. The sodium-cationised molecules show a radically different behaviour towards high-energy collisional activation. Besides the well-known charge-remote dissociation products of the alkyl side chain, complete series of d- and w-type fragments allow easy distinction between leucine and isoleucine. The complementary MS/MS data obtained from the protonated and cationised molecules prove to be of great interest for the structural characterisation of this type of compounds.

Muricadienin, muridienins and chatenaytrienins, the early precursors of annonaceous acetogenins

Abstract Chatenaytrienins-1,-2 and -3, muridienins-3 and -4 and muricadienin were characterized by tandem mass spectrometry (MS/MS) in a mixture of natural precursors of annonaceous acetogenins from Annona muricata . Chatenaytrienin-1, -2, -3 and -4 were then isolated from A. nutans and fully characterized by spectroscopic methods (NMR, MS) and by chemical and enzymatic oxidative processes. Isolation of these trienes confirmed the postulated biosynthetic pathway leading to the acetogenins.

Isolation and structure elucidation of sabadelin, an acetogenin from roots of Annona muricata

Abstract A new acetogenin, sabadelin, has been isolated from the roots of Annona muricata and was characterised using tandem mass spectrometry (MS/MS). The structure determination of this compound was strengthened by epoxidation and transformation leading to a known mono-tetrahydrofuran acetogenin. Sabadelin is probably an intermediate in the biosynthetic pathway of mono-THF acetogenins, and it is proposed as a biogenetic precursor of cis -panatellin.

Sesquiterpenes and Alkaloids from Scorodocarpus borneensis

A new sesquiterpene, scodopin, and a mixture of three tryptamine-type alkaloids, scorodocarpines A-C, were isolated from the fruits of Scorodocarpus borneensis, together with a known hemisynthetic sesquiterpene, cadalene-beta-carboxylic acid, which was isolated from the bark. The structures of the new compounds were elucidated by interpretation of spectral data, especially tandem mass spectrometry for the alkaloid mixture.

Cohibins A and B, acetogenins from roots of Annona muricata☆

Abstract Cohibins A and B, Biogenetic intermediates of acetogenins, were characterized using tandem mass spectrometry (MS/MS) from the roots of Annona muricata.

Isolation and Structure Elucidation of Three New Diglyceride Compounds from Ajuga Iva Leaves

Abstract Three novel compounds were isolated from the leaves of Ajuga iva (Labiatae). Their structures were established, on the basis of spectroscopic data, as symmetric and asymmetric 1,3-diacyl derivatives of glycerol with 3-hydroxy-tetradecanoic and 3-hydroxy-hexadecanoic acids.

Differential chemical labeling of the AlcR DNA-binding domain from Aspergillas nidulans versus its complex with a 16-mer DNA target: Identification of an essential tryptophan involved in the recognition and the interaction with the nucleic acid

DNA binding of the ethanol regulon transcription factor AlcR from Aspergillus nidulans was shown to involve a consensus basic region as in the other zinc cluster proteins. However, additional interactions between some residues and DNA were suspected, among which were a hypothetic hydrophobic interaction between Trp45 and the T residue of the consensus TGCGG sequence. In the present study, the differential chemical labeling of both the free protein and the protein/DNA complex showed significantly different behaviors of the three tryptophan residues comprised in the AlcR sequence toward the Koshland reagent. The spectacular decreased reaction rate for Trp45 within the complex confirmed the location of this residue at the protein/DNA interface. A similar result obtained with Trp53, an amino acid present at the C‐terminal side of AlcR, also indicated its involvement in the DNA recognition. In contrast, the formation of the complex accompanied by an allosteric rearrangement allowed the Trp36 to be much more exposed to the solvent than in the free protein. These data provide additional evidence that the unique specificity of AlcR among the zinc binuclear cluster family results in new types of interactions between AlcR and its cognate targets. From a methodological point of view, the approach of differential chemical labeling combined with mass spectrometric analyses proved to be an interesting tool for the recognition of hydrophobic interactions between the tryptophan residues of a protein and its macromolecular target.

Energy-resolved electrospray mass spectrometry of diexo-norbornane and norbornene derivatives containing a fused, phenyl-substituted, 1,3-oxazine ring: competitive retro-Diels-Alder fragmentation processes

The fragmentation behaviour of an organic ion depends on both the amount and the distribution of internal energy resulting from the ionisation process. Thus, investigations of the fragmentation pathways of complex ions can gain some advantage from energy-resolved mass spectrometry (ERMS) techniques. These methods are able to establish a relationship between the internal energy level of a precursor ion of interest and the nature and extent of the generated fragment ions. In the present study, the competitive retro-Diels–Alder fragmentation processes from norbornane and norbornene ions linked to 1,3-oxazine moieties were examined by ERMS, using an electrospray ionisation source and in-source collisional activation.

Structural elucidation of eighteen cerebrosides from Holothuria coronopertusa in a complex mixture by high-energy collision-induced dissociation of (M + Li) + ions

Eighteen cerebrosides were detected in admixture from the sea cucumber Holothuria coronopertusa. Their structures have been established on the basis of liquid-secondary ion mass spectrometry (LSIMS)experiments. The collision-induced dissociation (CID)of the lithiated molecules ([M + Li]+)led to diagnostic fragment ions which were further identified by tandem mass spectrometry (MS/MS). In the present study, the combination of LSIMS as ionization mode and high-energy MS/MS analysis appears to be a very powerful method for the rapid analysis of unresolved mixtures of complex molecules.

State-selected dissociation of dehydrovincamine alkaloid stereo-isomers

We have obtained results on alkaloid stereoisomers that show strongly stereoselective fragmentation processes. An original technique for such molecules, i.e. photoionisation by synchrotron radiation in the VUV range coupled to the photoelectron–photoion coincidence technique, is used to prepare state-selected parent ions with a very wide internal energy range. Mass spectra obtained after photoionisation, and metastable spectra obtained after electron impact ionisation, exhibit stereospecific fragmentations leading either to water loss and subsequent fragmentations in dehydrovincamine, or to carbomethoxy and ethyl loss in epi-dehydrovincamine. The results are in agreement with previous measurements obtained on the protonated samples, or on the similar vincamine molecule. Appearance energies for major fragmentations were recorded as well as a state-selected breakdown diagram for the epi-dehydrovincamine sample. They show that thresholds for the fragmentation processes are almost identical in the two isomers, indicating very similar thermodynamics. Very slow fragmentation decay for the first dissociation channels leads to the coexistence of parent and fragment ions in a very wide internal energy range of the breakdown diagram. The results show that the fragmentation of the system is most probably driven by strong steric factors. Further modelling with statistical theories should give the precise transition states and lead to a better understanding of this stereospecificity.