Dissociation of deprotonated microcystin variants by collision‐induced dissociation following electrospray ionization

Abstract

Microcystins (MC) are a family of hepatotoxic cyclic heptapeptides produced by a number of different cyanobacterial species. Considering the recent advances in the characterization of deprotonated peptides by mass spectrometry, the fragmentation behavior of four structurally related microcystin compounds was investigated using collision-induced dissociation (CID) experiments on an orbitrap mass spectrometer. It is demonstrated in this study that significant structural information can be obtained from the CID spectra of deprotonated microcystins. A predominant ring-opening reaction at the isoMeAsp residue, as well as two major complementary fragmentation pathways, was observed, reducing the complexity of the product ion spectra in comparison with spectra observed from protonated species. This proposed fragmentation behavior was applied to characterize [Leu(1)]MC-LR from a cyanobacterial cell extract. In conclusion, CID spectra of microcystins in the negative ion mode provide rich structurally informative mass spectra which greatly enhance confidence in structural assignments, in particular when combined with complementary positive ion CID spectra.

An integrated strategy for rapid and accurate determination of free and cell-bound microcystins and related peptides in natural blooms by liquid chromatography–electrospray-high resolution mass spectrometry and matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometry using both positive and negative ionization modes

“…Negative ionization mode by MALDI and ESI presented low absolute intensities but also low background and therefore provided suitable S/N ratios. According to other authors [19][20][21]44], as shown Table S1 of supplementary data, the analysis of MCs containing no Arg residue such as MC-LA, -LF, -LW and -LY presented high sensitivity in negative ionization mode.…”

“…The main fragment ions of MC-LR have been included in Table 2. These fragment ions have been already described in the literature [19,21,52] but in the present manuscript all the fragment ions have been evaluated for confirmatory purposes. As shown in Fig.…”

“…S3b) were obtained. Dörr et al identified this fragment ion as a product of a charge-transfer reaction at the leaving cyclic acid unit [19].…”

“…Typically, the positive ionization mode is the most usually ionization mode employed. Additionally, electrospray (ESI) recorded in negative ion mode has recently been applied to the analysis of MCs [19][20][21]. In order to have faster methods for toxin analysis: ultra-high performance liquid chromatography (UHPLC) [22][23][24], on-line solid phase extraction (SPE) [25][26][27][28] and direct analysis without prior extraction [29,30] and chromatographic separation [31,32] have been implemented in several works.…”

Liquid chromatography/negative electrospray ionization ion trap MS2 mass spectrometry application for the determination of microcystins occurrence in Southern Portugal water reservoirs

“…Single [MþH] þ and double charged [Mþ2H] 2þ ions are frequently observed in the positive mode, especially for variants containing basic arginine residues, such as MC-RR (Dörr et al, 2010). However, recently, Dörr et al (2011) presented an extensive study with the mass spectra characterization of several microcystins after negative electrospray ionization and Gambaro et al (2012) presented a negative ionization electrospray quantitative method, using liquid chromatography coupled to a triple quadrupole, applied to aerosol samples from Venice Lagoon, reaching detection limits of 1 ng/L. This work presents the development and application of an analytical LC/ESI/MS 2 ion trap method, using the negative ionization mode, which can be used as a sensitive and accurate tool for the characterization of environmental and health risks due to cyanotoxins.…”