A study of fragmentation of protonated amides of some acylated amino acids by tandem mass spectrometry: observation of an unusual nitrilium ion

A tandem mass spectrometric study of a series of secondary amides of acetylglycine and hippuric acid utilizing electrospray ionization (ESI) was conducted. Among the fragment ions observed was an unusual one, which we have determined to be a nitrilium ion having the structure or by loss of the full mass of glycine as a neutral fragment. A mechanism that we propose involves an initial protonation of the oxygen atom at the N‐terminus, followed by cyclization to a five‐membered imidazolium ring, and its subsequent collapse to the nitrilium ion. This mechanism is supported by extensive isotopic labels and considerable variation of substituents. A similar study of the amides of acyl β‐alanine and acyl γ‐aminobutyric acid revealed that the former furnishes the same nitrilium ion, but not the latter. Thus, a six‐membered intermediate is also possible and capable of losing the full mass of β‐alanine as a neutral fragment. When the size of the ring is forced to be seven‐membered, this pathway is blocked. When this study was expanded to include a variety of N‐acylproline amides, the nitrilium ion was observed in 100% abundance only when the acyl group was acetyl. Thus a proline effect (involvement of a strained bicyclic [3.3.0] structure) is being observed. Copyright © 2011 John Wiley & Sons, Ltd.     

Collision-induced dissociation of protonated tetrapeptides containing beta-alanine, gamma-aminobutyric acid, epsilon-aminocaproic acid or 4-aminomethylbenzoic acid residues

The influence of the presence and position of a single beta-alanine, gamma-aminobutyric acid, epsilon-aminocaproic acid or 4-aminomethylbenzoic acid residue on the tendency to form b(n)+ -and y(n)+ -type product ions was determined using a group of protonated tetrapeptides with general sequence XAAG, AXAG and AAXG (where X refers to the position of amino acid substitution). The hypothesis tested was that the 'alternative' amino acids would influence product ion signal intensities by inhibiting or suppressing either the nucleophilic attack or key proton transfer steps by forcing the adoption of large cyclic intermediates or blocking cyclization altogether. We found that specific b ions are diminished or eliminated completely when betaA, gammaAbu, Cap or 4AMBz residues are positioned such that they should interfere with the intramolecular nucleophilic attack step. In addition, differences in the relative proton affinities of the alternative amino acids influence the competition between complementary b(n) and y(n) ions. For both the AXAG and the XAAG series of peptides, collision-induced dissociation (CID) generated prominent b ions despite potential inhibition or suppression of intramolecular proton migration by the betaA, gammaAbu, Cap or 4AMBz residues. The prominent appearance of b ions from the AXAG and XAAG peptide is noteworthy, and suggests either that proton migration occurs through larger, 'whole' peptide cyclic intermediates or that fragmentation proceeds through a population of [M+H]+ isomers that are initially protonated at amide O atoms.     

Synthesis of 2-oxo and 6-hydroxylated benzimidazole analogs of DOPA as potential norepinephrine antagonists

The synthesis of benzimidazolone analogs of DOPA and α-methyl-DOPA and of the benzimidazole and benzimidazolone analogs of 6-hydroxy-DOPA is reported. These compounds showed a moderate amount of dopamine β-hydroxylase and tyrosine hydroxylase inhibitory activity but did not significantly alter tissue catacholamine levels.     

Single exposure fabrication and manipulation of 3D hydrogel cell microcarriers

We present a simple and high-throughput method for fabricating free-floating hydrogel cell microcarriers using single exposure UV patterning. We also demonstrate magnetic manipulation of the free-floating cell microcarriers using a magnetic nanoparticle-embedded structure for an active agitation and a solution exchange.     

A quantitation method for mass spectrometry imaging

A new quantitation method for mass spectrometry imaging (MSI) with matrix-assisted laser desorption/ionization (MALDI) has been developed. In this method, drug concentrations were determined by tissue homogenization of five 10 μm tissue sections adjacent to those analyzed by MSI. Drug levels in tissue extracts were measured by liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS). The integrated MSI response was correlated to the LC/MS/MS drug concentrations to determine the amount of drug detected per MSI ion count. The study reported here evaluates olanzapine in liver tissue. Tissue samples containing a range of concentrations were created from liver harvested from rats administered a single dose of olanzapine at 0, 1, 4, 8, 16, 30, or 100 mg/kg. The liver samples were then analyzed by MALDI-MSI and LC/MS/MS. The MALDI-MSI and LC/MS/MS correlation was determined for tissue concentrations of ~300 to 60,000 ng/g and yielded a linear relationship over two orders of magnitude (R(2) = 0.9792). From this correlation, a conversion factor of 6.3 ± 0.23 fg/ion count was used to quantitate MSI responses at the pixel level (100 μm). The details of the method, its importance in pharmaceutical analysis, and the considerations necessary when implementing it are presented.     

Desorption electrospray ionization with a portable mass spectrometer: in situ analysis of ambient surfaces

Desorption electrospray ionization (DESI) is implemented on a portable mass spectrometer and used to demonstrate in situ detection of active ingredients in pharmaceutical preparations, alkaloids in plant tissues, explosives, chemical warfare agent simulants and agricultural chemicals from a variety of surfaces; air monitoring applications using DESI are also introduced.     

New surfaces for desorption electrospray ionization mass spectrometry: porous silicon and ultra-thin layer chromatography plates

The performance of nanoporous silicon (pSi) and ultra-thin layer chromatography (UTLC) plates as surfaces for desorption electrospray ionization (DESI) was compared with that of polymethyl methacrylate (PMMA) and polytetrafluoroethylene (PTFE), both popular surfaces in previous DESI studies. The limits of detection (LODs) and other analytical characteristics for six different test compounds were determined using all four surfaces. The LODs for the compounds were in the fmol-pmol (pg-ng) range. The LODs with the pSi surface were further improved for each of the compounds when heat was applied to the surface during sample application which gave LODs as low as or lower than those achieved with PMMA and PTFE. The UTLC plates were successfully used as a rapid means of chromatographic separation prior to DESI-MS analysis. Another advantage achieved using the newer pSi and UTLC surfaces was increased speed of analysis, associated with drying of solution-phase samples. This took place immediately at the UTLC surface and it could be achieved rapidly by gently heating the pSi surface. The presence of salts in the sample did not cause suppression of the analyte signal with any of the surfaces.