Post-column in-source derivatisation in LC-MS: a tool for natural products characterisation and metabolomics

INTRODUCTION

An efficient characterisation of metabolites is a crucial task in many aspects of basic research, such as the de-replication of crude extracts in natural products chemistry or the tentative identification of compounds in untargeted metabolomics.

OBJECTIVE

The goal of this work is the evaluation of the reaction with phenylhydrazine for the derivatisation post-column in situ of carbonyl-containing compounds in liquid chromatography-mass spectrometry (LC-MS).

MATERIALS AND METHODS

LC-MS was performed using electrospray, Atmospheric Pressure Chemical Ionisation (APCI) or Atmospheric Pressure Photoionization (APPI) as ionisation techniques. The post-column addition of phenylhydrazine was done through a syringe pump via a T-junction before entrance to the ion source.

RESULTS

A variety of natural products having carbonyl groups, such as cycloartanes, steroids, cardenolides and other terpenoids, were analysed by this method. In the case of compounds with non-hindered aldehyde or keto groups, the main signals of the mass spectra were those corresponding to the phenylhydrazones. However, the spectra of compounds with hindered carbonyl groups displayed mainly those signals corresponding to the product of the nucleophilic addition adduct of phenylhydrazine to the carbonyl, which is the first step of the derivatisation process. Finally, those compounds with conjugated ketones did not react with phenylhydrazine. This methodology was applied in the analysis of crude natural extracts.

CONCLUSION

The results show that in situ derivatisation of carbonyl compounds in the ionisation source was achieved, yielding the typical derivatives of carbonyl compounds with phenylhydrazine.

Liquid chromatography coupled to different atmospheric pressure ionization sources-quadrupole-time-of-flight mass spectrometry and post-column addition of metal salt solutions as a powerful tool for the metabolic profiling of Fusarium oxysporum

Fusarium oxysporum L11 is a non-pathogenic soil-borne fungal strain that yielded an extract that showed antifungal activity against phytopathogens. In this study, reversed-phase high-performance liquid chromatography (RP-HPLC) coupled to different atmospheric pressure ionization sources-quadrupole-time-of-flight mass spectrometry (API-QTOF-MS) was applied for the comprehensive profiling of the metabolites from the extract. The employed sources were electrospray (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI). Post-column addition of metal solutions of Ca, Cu and Zn(II) was also tested using ESI. A total of 137 compounds were identified or tentatively identified by matching their accurate mass signals, suggested molecular formulae and MS/MS analysis with previously reported data. Some compounds were isolated and identified by NMR. The extract was rich in cyclic peptides like cyclosporins, diketopiperazines and sansalvamides, most of which were new, and are reported here for the first time. The use of post-column addition of metals resulted in a useful strategy for the discrimination of compound classes since specific adducts were observed for the different compound families. This technique also allowed the screening for compounds with metal binding properties. Thus, the applied methodology is a useful choice for the metabolic profiling of extracts and also for the selection of metabolites with potential biological activities related to interactions with metal ions.

A New Antifeedant Withanolide from Jaborosa lanigera

A new trechonolide-type withanolide, jaborosolactol, possessing a hemiketal ring system and a 5-membered lactol, has been isolated from the aerial parts of Jaborosa lanigera. The structure was elucidated based on spectroscopic and spectrometric methods (1D and 2D NMR, MS). The feeding deterrent activity of the compound against the Mediterranean fruit fly Ceratitis capitata was examined.

Multicomponent synthesis of 4,4-dimethyl sterol analogues and their effect on eukaryotic cells

Most sterols, such as cholesterol and ergosterol, become functional only after the removal of the two methyl groups at C-4 from their biosynthetic precursors. Nevertheless, some findings suggest that 4,4-dimethyl sterols might be involved in specific physiological processes. In this paper we present the synthesis of a collection of analogues of 4,4-dimethyl sterols with a diamide side chain and a preliminary analysis of their in vitro activity on selected biological systems. The key step for the synthesis involves an Ugi condensation, a versatile multicomponent reaction. Some of the new compounds showed antifungal and cytotoxic activity.

Differentiation of cyclosporin A from isocyclosporin A by liquid chromatography/electrospray ionization mass spectrometry with post-column addition of divalent metal salt

RATIONALE

Cyclosporin A (CsA) rearranges to its isomer isocyclosporin A (isoCsA) upon acid hydrolysis and also during ionization in the ion source of the mass spectrometer. It has been reported that both compounds could not be differentiated by tandem mass spectrometry (MS/MS) using atmospheric pressure ionization (API) sources and ambiguously differentiated by using other sources. In order to analyze these compounds which are common fungal metabolites, it is relevant to develop a simple method for their differentiation.

METHODS

CsA and isoCsA were analyzed by liquid chromatography/mass spectrometry (LC/MS) with post-column addition of metal ion solutions in a quadrupole time-of-flight instrument equipped with an electrospray ionization (ESI) source.

RESULTS

Mass spectra of CsA obtained upon post-column addition of solutions of Ca(II), Cu(II) and Zn(II) showed complexes between cyclosporin and the metal, including [2CsA + Me](2+) and [CsA-H + Me](+). These complexes were not observed in the spectra of isoCsA. The same results were observed at different metal concentrations.

CONCLUSIONS

Differentiation via metal complexation in positive ion mode LC/ESI-MS was performed to simultaneously distinguish CsA and its isomer isoCsA.

Synthesis and antifungal activity of C-21 steroids with an aromatic D ring

Six analogues of salpichrolides with a simplified side chain (6-11) were synthesized using a new methodology to obtain steroids with an aromatic D-ring. The key step was the elimination of HBr in a vicinal dibromo D-homosteroid by treatment with 1,4-diazabicyclo[2.2.2]octane (DABCO). All new compounds were completely characterized by 2D NMR techniques and tested on two fungal pathogenic species, Fusarium virguliforme and Fusarium solani.

Multicomponent synthesis of acylated short peptoids with antifungal activity against plant pathogens

In this article, we describe the synthesis of a small library of short peptoids composed of four glycine residues and acylated with a fatty acid that showed a remarkable in vitro activity against two fungal plant pathogens. Their straightforward synthesis implied two consecutive Ugi reactions and can be efficiently extended to the construction of highly diverse libraries.

Synthesis and preliminary biological screening of sterol analogues as new antifungal agents against plant pathogens

In this paper we report the synthesis of a new family of sterol analogues that have two amidic bonds on the side chain. These azasterols were obtained by a straightforward procedure including an Ugi condensation that allows the facile attachment of a polyfunctionalized side chain into the steroidal framework. Some of the new compounds showed an interesting inhibitory effect on the growth of two pathogenic fungi involved in plant diseases.

Withanolides from Jaborosa laciniata

Six new trechonolide type withanolides (compounds 1- 6), together with trechonolide A, jaborotetrol, and 12- O-methyl jaborosotetrol, were isolated from the aerial parts of Jaborosa laciniata. The structures were elucidated on the basis of spectroscopic methods (1D and 2D NMR, MS).

Spiranoid Withanolides from Jaborosa odonelliana and Jaborosa runcinata

Two new spiranoid withanolides, (20R,22R,23R)-12β ,17β ,22-trihydroxy-1-oxo-12,23-cycloergosta- 2,24-dien-26,23-olide (3) and (23R)-5α,6β ,12β ,21-tetrahydroxy-1,22-dioxo-12,23-cycloergosta- 2,17,24-trien-26,23-olide (4) were isolated from plants of Jaborosa odonelliana and Jaborosa runcinata, respectively. Compounds 3 and 4 were characterized by a combination of spectroscopic methods (1D and 2D NMR, MS) and molecular modelling.

Spiranoid withanolides from Jaborosa odonelliana

Six new spiranoid withanolides, (20R,22R,23S)-5alpha-chloro-6beta,12beta,17beta,22-tetrahydroxy-1-oxo-12,23-cycloergosta-2,24-dien-26,23-olide (2), (20R,22R,23S)-5beta,6beta-epoxy-12beta,17beta,22-trihydroxy-1-oxo-12,23-cycloergosta-2,24-dien-26,23-olide (3), (20R,22R,23S)-5beta,6beta-epoxy-4beta,12beta,17beta,22-tetrahydroxy-1-oxo-12,23-cycloergosta-2,24-dien-26,23-olide (4), (20R,22R,23S)-5alpha,6beta,12beta,17beta,22-pentahydroxy-1-oxo-12,23-cycloergosta-2,24-dien-26,23-olide (5), (20R,22R,23S)-6beta,12beta,17beta,22-tetrahydroxy-5alpha-methoxy-1-oxo-12,23-cycloergosta-2,24-dien-26,23-olide (6), and (20R,22R,23S)-6beta,12beta,17beta,22-tetrahydroxy-2alpha,5alpha-epidioxy-1-oxo-12,23-cycloergosta-3,24-dien-26,23-olide (7), were isolated from the leaves of Jaborosa odonelliana. Compounds 2-7 were characterized by a combination of spectroscopic methods (1D and 2D NMR, MS) and molecular modeling.

Induction of quinone reductase by withanolides

Thirty-seven naturally occurring withanolides (1-37), previously isolated in our laboratories, were evaluated for their potential to induce quinone reductase with cultured murine hepatoma cells (Hepa 1c1c7). Spiranoid (29, 32) and 18-functionalized withanolides (2-5, 7-9, 24) were found to be potent inducers of the enzyme, while 5alpha-substituted derivatives exhibited weak activity. Preliminary studies were performed with compound 29 to evaluate enzyme-inducing capacity in multiple organ sites of BALB/c mice. Significant induction was observed in liver and colon, but not in lung, stomach, or mammary gland.

7-Hydroxywithanolides from datura ferox

Three new withanolides were isolated from leaves of Datura ferox. Their structures were established as (17R,20S,22R)-5alpha,6alpha, 7beta,12alpha-tetrahydroxy-1-oxowitha-2,24-dienolide (1), (17R,20S, 22R,24S,25S)-24,25-epoxy-5alpha,6alpha,7beta, 12alpha-tetrahydroxy-1-oxowitha-2-enolide (2), and (17R,20S, 22R)-5alpha,6alpha-epoxy-7alpha-hydroxy-1,12-dioxowitha-2, 24-dienolide (3), using spectroscopic methods (NMR, MS) and molecular modeling.