Effect of N-terminal glutamic acid and glutamine on fragmentation of peptide ions

A prominent dissociation path for electrospray generated tryptic peptide ions is the dissociation of the peptide bond linking the second and third residues from the ammo-terminus. The formation of the resulting b₂ and y _n−2 fragments has been rationalized by specific facile mechanisms. An examination of spectral libraries shows that this path predominates in diprotonated peptides composed of 12 or fewer residues, with the notable exception of peptides containing glutamine or glutamic acid at the N-terminus. To elucidate the mechanism by which these amino acids affect peptide fragmentation, we synthesized peptides of varying size and composition and examined their MS/MS spectra as a function of collision voltage in a triple quadrupole mass spectrometer. Loss of water from N-terminal glutamic acid and glutamine is observed at a lower voltage than any other fragmentation, leading to cyclization of the terminal residue. This cyclization results in the conversion of the terminal amine group to an imide, which has a lower proton affinity. As a result, the second proton is not localized at the N-terminus but is readily transferred to other sites, leading to fragmentation near the center of the peptide. Further confirmation was obtained by examining peptides with N-terminal pyroglutamic acid and N-acetyl peptides. Peptides with N-terminal proline maintain the trend of forming b₂ and y _n−2 because their ring contains an imine rather than imide and has sufficient proton affinity to retain the proton at the N-terminus.     

The dioxanone approach to (2S,3R)-2-C-methylerythritol 4-phosphate and 2,4-cyclodiphosphate, and various MEP analogues

Efficient syntheses of the non-mevalonate pathway intermediates 2-C-methylerythritol 4-phosphate (MEP) and 2-C-methylerythritol 2,4-cyclodiphosphate (ME-2,4-cycloPP), as well as the parent tetrol 2-C-methylerythritol, in enantiopure form from (2S,4R)-cis-2-phenyl-4-tert-butyldimethylsilyloxy-1,3-dioxan-5-one are reported. The 2S configuration of the C-methyl group was installed by highly axial-face selective addition of CH3MgBr (20:1) to the chiral dioxanone carbonyl group. Primary selective mono-phosphorylation and 2,4-bis-phosphorylation, followed by desilation and hydrogenolysis to the free mono- and diphosphates, and, in the latter case, cyclization to form the eight-membered phosphoryl anhydride, afforded MEP and ME-2,4-cycloPP in good yields. The C2 epimeric analogues, 2-C-methylthreitol and its 4-phosphate, were accessed by LiAlH4 reduction of the cis,cis epoxide of (2S,4R)-4-tert-butyldimethylsilyloxymethyl-5-methylene-2-phenyl-1,3-dioxane, primary-selective phosphorylation, and cleavage of the silyl, benzylidene, and benzyl protecting groups. Regioselective cleavage of the acetal ring of 1,3-benzylidene 2-C-methylerythritol silyl ether by ozonolysis afforded a 1,2,3-triol 3-monobenzoate intermediate that was converted to the novel amino sugar, 1-amino-1-deoxy-2-C-methylerythritol.     

A formal total synthesis of crocacin C

An efficient total synthesis of a potent antifungal and moderate cytotoxic agent crocacin C is described. The synthesis involves the generation of four contiguous stereogenic centres via desymmetrization of a meso bicyclic dihydrofuran using asymmetric hydroboration.     

Solvent and catalyst-free synthesis of imidazo[1,2-a]pyridines by grindstone chemistry

The present work describes the solvent and catalyst-free synthesis of imidazo[1,2-a]pyridines in excellent to nearly quantitative yields from 2-aminopyridines and a wide variety of ω-bromomethylketones using a grindstone procedure at 25°C to 30°C for 3 to 5 minutes. The absolute structure of the compound, 2-(3-bromophenyl)-7-methylimidazo[1,2-a]pyridine is determined by X-ray crystallography. This green strategy has several noteworthy advantages such as wide spread substrate scope, short reaction times, water work up and the products do not require any chromatographic purification. Moreover, the method does not require any specialized equipment and is highly economical, environmentally benign and easy to carry out in any laboratory. Hence, the developed method meets the concept of “benign by design” and is greener alternative to the reported procedures for the synthesis of imidazo[1,2-a]pyridines.     

In vivo and in vitro metabolite profiling of nirmatrelvir using LC–Q-ToF–MS/MS along with the in silico approaches for prediction of metabolites and their toxicity

Nirmatrelvir (NRV), a 3C-like protease or M^pro inhibitor of SARS-CoV-2, is used for the treatment of COVID-19 in adult and paediatric patients. The present study was accomplished to investigate the comprehensive metabolic fate of NRV using in vitro and in vivo models. The in vitro models used for the study were microsomes (human liver microsomes, rat liver microsomes, mouse liver microsomes) and S9 fractions (human liver S9 fractions and rat liver S9 fractions) with the appropriate cofactors, whereas Sprague–Dawley rats were used as the in vivo models. Nirmatrelvir was administered orally to Sprague–Dawley rats, which was followed by the collection of urine, faeces and blood at pre-determined time intervals. Protein precipitation was used as the sample preparation method for all the samples. The samples were then analysed by liquid chromatography–quadrupole time-of-flight tandem mass spectrometry (LC-Q-ToF-MS/MS) using an Acquity BEH C18 column with 0.1% formic acid and acetonitrile as the mobile phase. Four metabolites were found to be novel, which were formed via amide hydrolysis, oxidation and hydroxylation. Furthermore, an in silico analysis was performed using Meteor Nexus software to predict the probable metabolic changes of NRV. The toxicity and mutagenicity of NRV and its metabolites were also determined using DEREK Nexus and SARAH Nexus.     

Electrospray tandem quadrupole fragmentation of quinolone drugs and related ions. On the reversibility of water loss from protonated molecules

Selected reaction monitoring (SRM) of quinolone drugs showed different sensitivities in aqueous solution vs. biological extract. The authors suggested formation of two singly protonated molecules with different behavior, one undergoing loss of H(2)O and the other loss of CO(2), so that SRM transitions might depend on the ratios of these forms generated by the electrospray. These surprising results prompted us to re-examine several quinolone drugs and some simpler compounds to further elucidate the mechanisms. We find that the relative contributions of loss of H(2)O vs. loss of CO(2) in tandem mass spectrometric (MS/MS) experiments depend not only on molecular structure and collision energy, but also, in certain cases, on the cone voltage. We further find that many product ions formed by loss of H(2)O can reattach a water molecule in the collision cell, whereas ions formed by loss of CO(2) do not. Since reattachment of H(2)O can occur after water loss in the cone region and prior to selection of the precursor ion, this effect leads to the dependence of MS/MS spectra on the cone voltage used in creating the precursor ion, which explains the formerly observed effect on SRM ratios. Our results support the earlier conclusion that varying amounts of two ions of the same m/z value are responsible for problems in the analysis of these drugs, but the origin is in dehydration/rehydration reactions. Thus, SRM transitions for certain complex compounds may be comparable only when monitored under equivalent ion-forming conditions, including the voltage used in the production of the protonated molecules in the electrospray ionization (ESI) source.     

Stereoselective total synthesis of (+)-mueggelone, a novel inhibitor of fish development

An efficient stereoselective total synthesis of (+)-mueggelone, a novel inhibitor of fish development, is described. Highlights of the strategy include the utilization of Sharpless asymmetric epoxidation, Yamaguchi lactonization and an olefin cross-metathesis as the key steps.     

A study of the fundamental and first overtone bands of NO in NO-rare gas mixtures at pressures up to 10,000 PSI

The absorption intensities of the 1-0 and 2-0 vibration-rotation bands of NO are determined from the absorption coefficients of NOHe and NOAr gaseous mixtures at high pressures at room temperature. The values obtained are: A_1-0 = 121 ± 6 cm^?2 Agt^?1 and A_2-0 = 2.17 ± 0.11 cm^?2 Agt^?1. A theory developed by Tipping is applied to evaluate the dipole moment coefficients unambiguously, including their signs, from the absolute intensity values and the difference between the mean frequency factor and the band origin. The following expansion for the dipole moment function in the ground state of NO is determined: M(x) = ?0.166 + 2.54x ? 1.99x² (in Debye). The absorption profiles of both the 1-0 and 2-0 bands in NOAr mixtures show marked changes as gas pressure increases; some of the factors influencing the shapes of the bands are also discussed. The plots of the integrated intensity vs rare gas density are found to be straight lines with positive slopes. This linear increase of the band intensity with density is interpreted as mainly due to the apparent induced absorption.