IR ion spectroscopy in a combined approach with MS/MS and IM-MS to discriminate epimeric anthocyanin glycosides (cyanidin 3-O-glucoside and -galactoside)

Anthocyanins are widespread in plants and flowers, being responsible for their different colouring. Two representative members of this family have been selected, cyanidin 3-O-β-glucopyranoside and 3-O-β-galactopyranoside, and probed by mass spectrometry based methods, testing their performance in discriminating between the two epimers. The native anthocyanins, delivered into the gas phase by electrospray ionization, display a comparable drift time in ion mobility mass spectrometry (IM-MS) and a common fragment, corresponding to loss of the sugar moiety, in their collision induced dissociation (CID) pattern. However, the IR multiple photon dissociation (IRMPD) spectra in the fingerprint range show a feature particularly evident in the case of the glucoside. This signature is used to identify the presence of cyanidin 3-O-β-glucopyranoside in a natural extract of pomegranate. In an effort to increase any differentiation between the two epimers, aluminum complexes were prepared and sampled for elemental composition by FT-ICR-MS. CID experiments now display an extensive fragmentation pattern, showing few product ions peculiar to each species. More noteworthy is the IRMPD behavior in the OH stretching range showing significant differences in the spectra of the two epimers. DFT calculations allow to interpret the observed distinct bands due to a varied network of hydrogen bonding and relative conformer stability

Nanoemulsions of Satureja montana essential oil. Antimicrobial and antibiofilm activity against avian Escherichia coli strains

Satureja montana essential oil (SEO) presents a wide range of biological activities due to its high content of active phytochemicals. In order to improve the essential oil’s (EO) properties, oil in water nanoemulsions (NEs) composed of SEO and Tween-80 were prepared, characterized, and their antimicrobial and antibiofilm properties assayed against Escherichia coli strains isolated from healthy chicken. Since surfactant and oil composition can strongly influence NE features and their application field, a ternary phase diagram was constructed and evaluated to select a suitable sur-factant/oil/water ratio. Minimal inhibitory concentration and minimal bactericidal concentration of NEs, evaluated by the microdilution method, showed that the SEO NE formulation exhibited higher inhibitory effects against planktonic E. coli than SEO alone. The quantification of biofilm production in the presence of NEs, assessed by crystal violet staining and scanning electron microscopy, evi-denced that sub-MIC concentrations of SEO NEs enable an efficient reduction of biofilm production by the strong producer strains. The optimized nanoemulsion formulation could ensure food safety quality, and counteract the antibiotic resistance of poultry associated E. coli, if applied/aerosolized in poultry farms

Correlation between the antimicrobial activity and metabolic profiles of cell free supernatants and membrane vesicles produced by lactobacillus reuteri DSM 17938

The aim of the work is to assess the antimicrobial activities of Cell Free Supernatants (CFS) and Membrane Vesicles (MVs), produced by Lactobacillus reuteri DSM 17938, versus Gram-positive and Gram-negative bacteria and investigate their metabolic profiles. The Minimum Inhibitory Concentration was determined through the broth microdilution method and cell proliferation assay and the Minimum Bactericidal Concentration was determined by Colony Forming Units counts. The characteristics of the antimicrobial compounds were evaluated by pH adjustments, proteinase treatment, and size fractionation of the CFS. The cytotoxicity of CFS was tested on two human cell lines. A detailed snapshot of the L. reuteri metabolism was attained through an untargeted metabolic profiling by means of high resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) coupled with Electrospray Ionization Source (ESI). The results showed (i) a greater efficacy of CFS and its fractions towards Gram-negative compared to Gram-positive bacteria; (ii) an antimicrobial effect related to pH-dependent compounds but not to MVs; (iii) a molecular weight < 3 KDa as well as an a non-proteinaceous nature of the antimicrobial compounds; and (iv) more than 200 and 500 putative metabolites annotated in MVs and supernatants, covering several classes of metabolites, including amino acids, lipids, fatty and organic acids, polyalcohols, nucleotides, and vitamins. Some putative compounds were proposed not only as characteristic of specific fractions, but also possibly involved in antimicrobial activity

Long-Lived ipso-Silylated p-Toluenium Ions: Evidence from a Kinetic Isotope Effect

A sizeable hydrogen isotope effect arises from the competing abstractions of Me3Si+ and X (X = H, D) from the σ complex, which is formed by addition of gaseous Me3Si+ to toluene. This species may adequately describe the Et3Si+ -toluene pair recently reported in the solid state

Aromatic Alkylation by Gaseous Me3C+ Ions. Kinetic Role of Deprotonation of Intermediate Arenium Ions

The alkylation of m-xylene by Me3C+ ions has been studied in the gas phase with the aim of assessing the kinetic role of proton transfer from intermediate arenium ions. Hydrogen kinetic isotope effects (KIEs) emerging from the reaction of m-xylene-4-d and of m-xylene/m-xylene-d10 mixtures have been exploited as mechanistic probes. The Me3C substitution at the 4-position displays a base-strength-dependent KIE related to rate-determining deprotonation of arenium intermediates. The Me3C+ reaction at the 5-position is instead characterized by a base-independent KIE due to 1,2-hydrogen shift to form a highly stable isomeric arenium ion. When the latter species originates from m-xylene-4-d, its neutralization involves an intramolecular competition of proton vs deuteron abstraction, manifesting a net primary KIE. Factors affecting its magnitude and base-strength dependence are discussed

Gaseous Arenium Ions at Atmospheric Pressure: Elementary Reactions and Internal Solvation Effects

The title species, of paramount importance as key intermediates of electrophilic aromatic substitution reactions, are amenable to mechanistic studies by mass spectrometric and radiolytic techniques. They have proven to be versatile benchmark models for the study of elementary processes such as proton migration, which may occur in both intra- and intermolecular fashions. This Account primarily focuses on proton migration reactions involving arenium ions. Additionally, the effect of a second unsaturated ring bound by an aliphatic chain of variable length on the stability and reactivity of the arenium ion is examined and quantitatively evaluated

Gas Phase Heteroaromatic Substitution. Silylation of Simple Five-Membered Heteroaromatic Rings by Trimethylsilyl Cations

Trimethylsilyl cation, obtained in the gas phase from the y-radiolysis of CH4/(CH3)4Si mixtures, has been allowed to react with pyrrole, N-methylpyrrole, furan, and thiophene, both neat and in competition with toluene. Experiments have been carried out at pressures around 620-720 Torr and in the presence of variable concentrations of a gaseous base (NEt3 0-10 Torr). The mechanism of the silylation process and of the subsequent isomerization of the relevant ionic intermediates is discussed and the intrinsic substrate and positional selectivity of the (CH3)3Si+ ions evaluated. The poor substrate discrimination of (CH3)3Si+ as well as its tendency to attack preferentially the substrate positions with the highest net negative charge, i. e. the beta C of pyrrole (70%) and N-methylpyrrole (75%), the oxygen of furan, and the alfa C of thiophene (loo%), characterize the reaction as dominated by extensive electrostatic interactions within the encounter pair. The pronounced site discrimination of (CH3)3Si+ toward pyrroles ranks it as a “hard” electrophile, but less “hard” than expected on the grounds of its calculated LUMO energy. A plausible explanation is found in the much larger 3p LUMO of (CH3),Si+, if compared to the 2p LUMOs of alkylating carbocations

Prevailing charge transfer in the reaction of protonated and neutral nitric oxide: A theoretical and experimental study

The thermal gas-phase reaction of [NOH]+ with nitric oxide (NO) has been studied using FT-ICR mass spectrometry complemented by high level quantum chemical calculations. Among different competitive paths that may be envisioned, using D- and 15N labelling in the reagent species has allowed to unequivocally observe an exclusive electron transfer (ET) reactivity. This outcome is well accounted by the energy profile for the envisioned plausible pathways calculated at UCCSD(T)/aug-ccpVTZ//UB3LYP/def2-TZVP level of theory. The seemingly barrierless ET process (as predicted by classical Marcus theory) is exoergonic by 20.8 kcal/mol. The two reaction partners may alternatively yield an adduct, where a large extent of charge transfer has taken place. This [HNO⋅⋅⋅NO]+ adduct may proceed by undergoing transfer of hydrogen, entailing largely hydride character. However, in agreement with a calculated barrier of ca. 12 kcal/mol, no experimental evidence is obtained for the occurrence of this alternative route