Analysis of glycolipids by fast atom bombardment mass spectrometry

Rhamnolipids from the rhizosphere bacterium Pseudomonas sp. GRP(3) that reduces damping-off disease in Chilli and tomato nurseries

A detailed screening of bacterial isolates from the Central Himalayan region for plant growth promotion and antimycelial activity against Pythium and Phytophthora strains afforded seven isolates, of which three were particularly effective against the incidence of damping-off in field trials on chilli and tomato. In this investigation an initial spectroscopic survey of the methanolic extracts of the seven bacterial isolates showed complex mixtures except for Pseudomonas sp. GRP3, one of the most promising isolates on the basis of field studies. Strain GRP3 was selected for structural characterization of its secondary metabolites, particularly glycolipids. The extracellular secondary metabolites were enriched by Amberlite XAD-16 adsorber resin followed by separation and structural analysis using TLC, LC-MS, MS-MS, and NMR spectroscopy. Acquired data show the presence of a number of mono- and dirhamnolipids and include rhamnose (Rha)-C8-C10, Rha-C10-C8, Rha-C10-C10, Rha-C10-C12:1, Rha-C10-C12, Rha-Rha-C8-C10, Rha-Rha-C10-C10, Rha-Rha-C10-C10:1, Rha-Rha-C10-C12, Rha-Rha-C10-C12:1, Rha-Rha-C12-C12:1, and Rha-Rha-C12-C12 in strain GRP3. Since rhamnolipids are involved in the lysis of the plasma membrane of zoospores of fungi, application of such rhamnolipid-producing rhizobacteria could facilitate control of damping-off plant pathogens.

Mass spectrometry monitoring of rhamnolipids from a growing culture of Pseudomonas aeruginosa strain 57RP

Two rapid and simple methods for the characterisation and quantification of rhamnolipids produced by a growing culture of the Pseudomonas aeruginosa strain 57RP were developed. Two rhamnolipids were purified and their response factors determined. The various rhamnolipids produced were then measured using liquid chromatography/mass spectrometry. The culture supernatants were injected directly, without prior purification, in a HPLC equipped with a C(18) reverse-phase column. The complete profile of rhamnolipid congeners produced during a 2 week cultivation period was monitored. In order to shorten the analysis time, another method was developed which did not require chromatographic separation of the rhamnolipids prior to their detection. Quantification of rhamnolipids using the direct infusion method gave results very similar to those obtained with HPLC separation. These two methods were very well correlated with the standard colorimetric orcinol method. The rhamnolipid profiles obtained show that the various rhamnolipid congeners are secreted simultaneously, and that their relative proportion remained unchanged throughout the cultivation period.

Mechanism of cross-ring cleavage reactions in dirhamnosyl lipids: effect of the alkali ion

Liquid secondary ion mass spectrometry and high-energy collision-induced dissociation were used to analyze a dirhamnosyl lipid mixture. The negative fast-atom bombardment spectrum reveals a mixture of four homologous dirhamnosyl lipids with the following general structure: Rha-Rha-Cn-Cm (where Cn and Cm denote 3-hydroxy fatty acid moieties). The mass region 450-600 u in the collision-induced dissociation spectra of the negative [M - H]- ions shows product ions that can be rationalized by terminal loss of a 3-hydroxyalkanoic acid residue; these ions can be used for the characterization of the fatty acid substituents. A unique effect of alkali-metal ions on the course of fragmentation of dirhamnosyl lipid attachment ions was observed. The strong chelation of sodium is revealed from the stability of the [M - H + 2Na]+ ion that does not lose a sodium ion with the eliminated neutrals, contrary to what is observed for the dilithium adduct. Cross-ring cleavages occur during high-energy collision-induced dissociation of both positively and negatively charged precursor ions. The results suggest a concerted decomposition pathway involving the six-membered rings of the monosaccharide residues. The formation of cross-ring cleavage products, which retain the C10-C10 moiety during high-energy collision-induced dissociation of all the precursor ions that contain sodium or lithium, strongly supports a retro [2 + 2 + 2] mechanism.

Liquid chromatography/mass spectrometry analysis of mixtures of rhamnolipids produced by Pseudomonas aeruginosa strain 57RP grown on mannitol or naphthalene

Liquid chromatography/mass spectrometry using electrospray ionisation was used to analyse rhamnolipids produced by a Pseudomonas aeruginosa strain with mannitol or naphthalene as carbon source. Identification and quantification of 28 different rhamnolipid congeners was accomplished using a reverse-phase C(18) column and a 30 min chromatographic run. Isomeric rhamnolipids that were not chromatographically resolved could be identified by interpretation of their mass spectra and their relative proportions estimated. The most abundant rhamnolipid produced on mannitol contained two rhamnoses and two 3-hydroxydecanoic acid groups. The most abundant rhamnolipid produced from naphthalene contained two rhamnoses and one 3-hydroxydecanoic acid group.

High-performance thin-layer chromatography/fast atom bombardment (tandem) mass spectrometry of Pseudomonas rhamnolipids

Several rhamnolipid preparations from Pseudomonas strains were studied by thin-layer chromatography/fast atom bombardment (TLC/FAB) mass spectrometry and TLC/FAB tandem mass spectrometry (MS/MS). The preparations were separated with normal-phase (Silica 60) and reversed-phase (RP-8) chromatography. Silica 60 plates appeared to be very useful in the separation of rhamnolipids according to the number of monosaccharide residues present. Spectra which show characteristic fragment ions could be obtained from components of mixtures with a total sample size of less than 200 ng. Chromatography on RP-8 plates gave a good separation of the rhamnolipids based on the length of the fatty acid alkyl chain. MS/MS of the sodium cationized molecules gave information about the sequence of the building blocks. Particularly, heterogeneity in beta-hydroxy fatty acid composition was determined for the principal as well as minor components present in natural rhamnolipid mixtures.

Structure identification of natural rhamnolipid mixtures by fast atom bombardment tandem mass spectrometry

Two rhamnobiose-lipid preparations have been studied by fast atom bombardment (FAB) tandem mass spectrometry. The principal rhamnobiose-lipids contain the beta-hydroxydecanoyl-beta-hydroxydecanoate Rha-Rha-C10-C10 and the beta-hydroxytetradecanoyl-beta-hydroxytetradecanoate Rha-Rha-C14-C14. Both preparations contain minor components which are heterogenous in beta-hydroxy fatty acid composition. FAB ionization of rhamnobiose-lipids in the presence of Na+ shows the formation of both [M + Na]+, [M +2Na-H]+, [M + 3Na-2H]+ and [M - H]- ions. Tandem mass spectrometry of the [M + 2Na-H]+ and [M - H]- ions give information about the sequence of the building blocks. Particularly, heterogeneity in beta-hydroxy fatty acid composition is determined for the principal components and all the minor components present in the preparations.