Identification of bacterial lipo-amino acids: origin of regenerated fatty acid carboxylate from dissociation of lipo-glutamate anion

Amandine Hueber,Jean-Marie Galano,Geoffrey Langevin,Justine Bertrand-Michel,Yves Gimbert,Alexandre Guy,Jean-Claude Tabet,Thierry Durand,Nicolas Cenac

doi:10.1007/s00726-021-03109-1

Abstract

The identification of bacterial metabolites produced by the microbiota is a key point to understand its role in human health. Among them, lipo-amino acids (LpAA), which are able to cross the epithelial barrier and to act on the host, are poorly identified. Structural elucidation of few of them was performed by high-resolution tandem mass spectrometry based on electrospray combined with selective ion dissociations reach by collision-induced dissociation (CID). The negative ions were used for their advantages of yielding only few fragment ions sufficient to specify each part of LpAA with sensitivity. To find specific processes that help structural assignment, the negative ion dissociations have been scrutinized for an LpAA: the N-palmitoyl acyl group linked to glutamic acid (C16Glu). The singular behavior of [C16Glu-H]¯ towards CID showed tenth product ions, eight were described by expected fragment ions. In contrast, instead of the expected product ions due to CONH-CH bond cleavage, an abundant complementary dehydrated glutamic acid and fatty acid anion pair were observed. Specific to glutamic moiety, they were formed by a stepwise dissociation via molecular isomerization through ion–dipole formation prior to dissociation. This complex dissociated by partner splitting either directly or after inter-partner proton transfer. By this pathway, surprising regeneration of deprotonated fatty acid takes place. Such regeneration is comparable to that occurred from dissociation to peptides containing acid amino-acid. Modeling allow to confirm the proposed mechanisms explaining the unexpected behavior of this glutamate conjugate.

Highlights

IntroductionLipopeptides (Pérez-Berezo et al 2017) and lipo-amino acids (LpAA) (Vizcaino et al 2014) have been described for their capacity to regulate host homeostasis
Among intestinal microbiota metabolites, lipopeptides (Pérez-Berezo et al 2017) and lipo-amino acids (LpAA) (Vizcaino et al 2014) have been described for their capacity to regulate host homeostasis
LC-electrospray ionization (ESI)-HRMS/MS performed in negative ion mode for the LpAA analysis has some advantages in terms of sensibility and specificity

Summary

Introduction

Lipopeptides (Pérez-Berezo et al 2017) and lipo-amino acids (LpAA) (Vizcaino et al 2014) have been described for their capacity to regulate host homeostasis. Identification of LpAA and lipopeptides produced by Escherichia coli Nissle 1917, such as the N-lauroyl acyl group linked to asparagine (C12Asn), has been done in negative ESI mode yielding abundant [C12Asn-H] ̄ anions (Pérez-Berezo et al 2017). Analysis of these metabolites by hyphenated methods as liquid chromatography–tandem high-resolution mass spectrometry LC-HRMS/MS is especially versatile for complex mixture analysis. This method is characterized by a large selectivity, specificity and sensitivity when it is combined to desorption/ionization as electrospray (ESI). Using low-resolution instrumentation based on linear ion trap quadrupole, an encouraging study of ornithine and glutamine lipids (from Rhodobacter sphaeroides) proposed mechanisms (Zhang et al 2009) or a possible rationalization of ion dissociations under low-energy collision conditions in resonant excitation mode

Methods

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Discussion

Conclusion