In-Source Decay MALDI and High-Energy Collision-Induced Dissociation Mass Spectrometry of Alkali Metal-Adducted Underivatized Oligosaccharides.

In-source decay (ISD) and high-energy collision-induced dissociation (HE-CID) were explored to provide structural information on alkali metal-adducted linear and stacked oligosaccharides (oligosaccharides with increased flexibility due to linkage type). These oligosaccharides include isomeric tetrasaccharides, maltoheptaose, and several human milk oligosaccharides (HMOs). Matrix-assisted laser desorption ionization (MALDI) ion production efficiency, as well as the product ion intensities, and the number of product ions formed in ISD and HE-CID of these oligosaccharides were influenced by the matrix, the ionic radius of the metal ion used for adduction, and the affinity of metal ions for specific functional groups in the oligosaccharides.

Mechanistic Study of Enhanced Protonation by Chromium(III) in Electrospray Ionization: A Superacid Bound to a Peptide.

Addition of trivalent chromium, Cr(III), to solutions undergoing electrospray ionization (ESI) enhances protonation and leads to formation of [M + 2H] for peptides that normally produce [M + H]. This effect is explored using electronic structure calculations at the density functional theory (DFT) level to predict the energetics of various species that are potentially important to the mechanism. Gas- and solution-phase reaction free energies for glycine and its anion reacting with [Cr(III)(HO)] and for dehydration of these species have been predicted, where glycine is used as a simple model for a peptide.

Experimental and Computational Study of the Gas-Phase Acidities of Acidic Di- and Tripeptides.

Gas-phase acidities (GA or Δ G) of acidic di- and tripeptides are determined for the first time. The peptides studied are composed of inert alanine (A) residues and one X residue of either aspartic acid (D) or glutamic acid (E): AX, XA, AAX, AXA, and XAA. Experimental GAs were measured by the thermokinetic method of deprotonation ion/molecule reactions in a Fourier transform ion cyclotron resonance mass spectrometer.

The use of chromium(III) complexes to enhance peptide protonation by electrospray ionization mass spectrometry.

The addition of trivalent chromium, Cr(III), reagents to peptide solutions can increase the intensity of doubly protonated peptides, [M + 2H] , through electrospray ionization (ESI). Three model heptapeptides were studied: neutral (AAAAAAA), acidic (AAEEEAA), and basic (AAAKAAA). The neutral and acidic peptides form almost no 2+ ions in the absence of Cr(III).

Electron transfer dissociation mass spectrometry of acidic phosphorylated peptides cationized with trivalent praseodymium.

The lanthanide ion praseodymium, Pr(III), was employed to study metallated ion formation and electron transfer dissociation (ETD) of 27 biological and model highly acidic phosphopeptides. All phosphopeptides investigated form metallated ions by electrospray ionization (ESI) that can be studied by ETD to yield abundant sequence information. The ions formed are [M + Pr - H] , [M + Pr] , and [M + Pr + H] .

Electron Transfer Dissociation and Collision-Induced Dissociation of Underivatized Metallated Oligosaccharides.

Electron transfer dissociation (ETD) and collision-induced dissociation (CID) were used to investigate underivatized, metal-cationized oligosaccharides formed via electrospray ionization (ESI). Reducing and non-reducing sugars were studied including the tetrasaccharides maltotetraose, 3α,4β,3α-galactotetraose, stachyose, nystose, and a heptasaccharide, maltoheptaose. Univalent alkali, divalent alkaline earth, divalent and trivalent transition metal ions, and a boron group trivalent metal ion were adducted to the non-permethylated oligosaccharides.

Optimization of electrospray ionization conditions to enhance formation of doubly protonated peptide ions with and without addition of chromium(III).

Rationale: Production of multiply protonated ions by electrospray ionization (ESI) is important to the analysis of peptides by mass spectrometry. For small neutral and acidic peptides, addition of chromium(III) greatly increases the intensity of doubly protonated ions. The current study examines instrumental and solution parameters that maximize peptide ion charge by ESI.

Effects of acidic peptide size and sequence on trivalent praseodymium adduction and electron transfer dissociation mass spectrometry.

Using the lanthanide ion praseodymium, Pr(III), metallated ion formation and electron transfer dissociation (ETD) were studied for 25 biological and model acidic peptides. For chain lengths of seven or more residues, even highly acidic peptides that can be difficult to protonate by electrospray ionization will metallate and undergo abundant ETD fragmentation. Peptides composed of predominantly acidic residues form only the deprotonated ion, [M + Pr - H] ; this ion yields near complete ETD sequence coverage for larger peptides.

Citric Acid Capped Iron Oxide Nanoparticles as an Effective MALDI Matrix for Polymers.

A new matrix-assisted laser desorption ionization (MALDI) mass spectrometry matrix is proposed for molecular mass determination of polymers. This matrix contains an iron oxide nanoparticle (NP) core with citric acid (CA) molecules covalently bound to the surface. With the assistance of additives, the particulate nature of NPs allows the matrix to mix uniformly with polar or nonpolar polymer layers and promotes ionization, which may simplify matrix selection and sample preparation procedures.

The Effects of Trivalent Lanthanide Cationization on the Electron Transfer Dissociation of Acidic Fibrinopeptide B and its Analogs.

Electrospray ionization (ESI) on mixtures of acidic fibrinopeptide B and two peptide analogs with trivalent lanthanide salts generates [M + Met + H](4+), [M + Met](3+), and [M + Met -H](2+), where M = peptide and Met = metal (except radioactive promethium). These ions undergo extensive and highly efficient electron transfer dissociation (ETD) to form metallated and non-metallated c- and z-ions. All metal adducted product ions contain at least two acidic sites, which suggest attachment of the lanthanide cation at the side chains of one or more acidic residues.

Spectroscopic and biological activity studies of the chromium-binding peptide EEEEGDD.

While trivalent chromium has been shown at high doses to have pharmacological effects improving insulin resistance in rodent models of insulin resistance, the mechanism of action of chromium at a molecular level is not known. The chromium-binding and transport agent low-molecular-weight chromium-binding substance (LMWCr) has been proposed to be the biologically active form of chromium. LMWCr has recently been shown to be comprised of a heptapeptide of the sequence EEEEDGG.

Negative Ion In-Source Decay Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry for Sequencing Acidic Peptides.

Matrix-assisted laser desorption/ionization (MALDI) in-source decay was studied in the negative ion mode on deprotonated peptides to determine its usefulness for obtaining extensive sequence information for acidic peptides. Eight biological acidic peptides, ranging in size from 11 to 33 residues, were studied by negative ion mode ISD (nISD). The matrices 2,5-dihydroxybenzoic acid, 2-aminobenzoic acid, 2-aminobenzamide, 1,5-diaminonaphthalene, 5-amino-1-naphthol, 3-aminoquinoline, and 9-aminoacridine were used with each peptide.

Gas-Phase Acidities of Phosphorylated Amino Acids.

Gas-phase acidities and heats of formation have been predicted at the G3(MP2)/SCRF-COSMO level of theory for 10 phosphorylated amino acids and their corresponding amides, including phospho-serine (pSer), -threonine (pThr), and -tyrosine (pTyr), providing the first reliable set of these values. The gas-phase acidities (GAs) of the three named phosphorylated amino acids and their amides have been determined using proton transfer reactions in a Fourier transform ion cyclotron mass spectrometer. Excellent agreement was found between the experimental and predicted GAs.

An Experimental and Computational Study of the Gas-Phase Acidities of the Common Amino Acid Amides.

Using proton-transfer reactions in a Fourier transform ion cyclotron resonance mass spectrometer and correlated molecular orbital theory at the G3(MP2) level, gas-phase acidities (GAs) and the associated structures for amides corresponding to the common amino acids have been determined for the first time. These values are important because amino acid amides are models for residues in peptides and proteins. For compounds whose most acidic site is the C-terminal amide nitrogen, two ions populations were observed experimentally with GAs that differ by 4-7 kcal/mol.

The use of chromium(III) to supercharge peptides by protonation at low basicity sites.

The addition of chromium(III) nitrate to solutions of peptides with seven or more residues greatly increases the formation of doubly protonated peptides, [M + 2H](2+), by electrospray ionization. The test compound heptaalanine has only one highly basic site (the N-terminal amino group) and undergoes almost exclusive single protonation using standard solvents. When Cr(III) is added to the solution, abundant [M + 2H](2+) forms, which involves protonation of the peptide backbone or the C-terminus.

An experimental and computational investigation into the gas-phase acidities of tyrosine and phenylalanine: three structures for deprotonated tyrosine.

Using mass spectrometry and correlated molecular orbital theory, three deprotonated structures were revealed for the amino acid tyrosine. The structures were distinguished experimentally by ion/molecule reactions involving proton transfer and trimethylsilyl azide. Gas-phase acidities from proton transfer reactions and from G3(MP2) calculations generally agree well.

MALDI MS in-source decay of glycans using a glutathione-capped iron oxide nanoparticle matrix.

A new matrix-assisted laser desorption ionization (MALDI) mass spectrometry matrix is proposed for molecular mass and structural determination of glycans. This matrix contains an iron oxide nanoparticle (NP) core with gluthathione (GSH) molecules covalently bound to the surface. As demonstrated for the monosaccharide glucose and several larger glycans, the mass spectra exhibit good analyte ion intensities and signal-to-noise ratios, as well as an exceptionally clean background in the low mass-to-charge (m/z) region.

Paramagnetic F NMR and Electrospray Ionization Mass Spectrometric Studies of Substituted Pyridine Complexes of Chromium(III): Models for Potential Use of F NMR to Probe Cr(III)-Nucleotide Interaction.

The synthesis and characterization of chromium basic carboxylate complexes, [Cr(OCR)L], containing trifluoroacetate, 3-fluoropyridine, 3-trifluoromethylpyridine, and 4-trifluoromethylpyridine are described. The substituted pyridine ligands are used as models of DNA bases to determine whether F NMR would be a potentially useful probe of the binding of Cr to DNA. The F NMR resonances of the coordinated ligands, while broadened by delocalization of unpaired electron density from the S=3/2 chromic centers, are readily discernable, and the contact shifts are of sufficient magnitude that the signals from coordinated and free ligands can easily be differentiated.

Gas-phase deprotonation of the peptide backbone for tripeptides and their methyl esters with hydrogen and methyl side chains.

The gas-phase acidities (GAs) of six tripeptides (GlyGlyGly, GlyAlaGly, AlaGlyAla, AlaAlaAla, AibAibAib, and SarSarSar) and their methyl esters were obtained by proton transfer reactions in a Fourier transform ion cyclotron resonance mass spectrometer and G3(MP2) molecular orbital theory calculations. All six peptides have GAs in the range 321.0-323.

A comparison of the effects of amide and acid groups at the C-terminus on the collision-induced dissociation of deprotonated peptides.

The dissociative behavior of peptide amides and free acids was explored using low-energy collision-induced dissociation and high level computational theory. Both positive and negative ion modes were utilized, but the most profound differences were observed for the deprotonated species. Deprotonated peptide amides produce a characteristic c(m-2)(-) product ion (where m is the number of residues in the peptide) that is either absent or in low abundance in the analogous peptide acid spectrum.

Fundamental thermochemical properties of amino acids: gas-phase and aqueous acidities and gas-phase heats of formation.

The gas-phase acidities of the 20 L-amino acids have been predicted at the composite G3(MP2) level. A broad range of structures of the neutral and anion were studied to determine the lowest energy conformer. Excellent agreement is found with the available experimental gas-phase deprotonation enthalpies, and the calculated values are within experimental error.

Effects of transition metal ion coordination on the collision-induced dissociation of polyalanines.

Transition metal-polyalanine complexes were analyzed in a high-capacity quadrupole ion trap after electrospray ionization. Polyalanines have no polar amino acid side chains to coordinate metal ions, thus allowing the effects metal ion interaction with the peptide backbone to be explored. Positive mode mass spectra produced from peptides mixed with salts of the first row transition metals Cr(III), Fe(II), Fe(III), Co(II), Ni(II), Cu(I), and Cu(II) yield singly and doubly charged metallated ions.

Characterization of the organic component of low-molecular-weight chromium-binding substance and its binding of chromium.

Chromium was proposed to be an essential element over 50 y ago and was shown to have therapeutic potential in treating the symptoms of type 2 diabetes; however, its mechanism of action at a molecular level is unknown. One chromium-binding biomolecule, low-molecular weight chromium-binding substance (LMWCr or chromodulin), has been found to be biologically active in in vitro assays and proposed as a potential candidate for the in vivo biologically active form of chromium. Characterization of the organic component of LMWCr has proven difficult.

A comparison of positive and negative ion collision-induced dissociation for model heptapeptides with one basic residue.

The effects of the identity and position of basic residues on peptide dissociation were explored in the positive and negative modes. Low-energy collision-induced dissociation (CID) was performed on singly protonated and deprotonated heptapeptides of the type: XAAAAAA, AAAXAAA, AAAAAXA and AAAAAAX, where X is arginine (R), lysine (K) or histidine (H) residues and A is alanine. For [M + H](+), the CID spectra are dominated by cleavages adjacent to the basic residues and the majority of the product ions contain the basic residues.

Mass spectrometric and spectroscopic studies of the nutritional supplement chromium(III) nicotinate.

Despite chromium nicotinate's popular use as a chromium nutritional supplement, the structure and composition of chromium nicotinate have only been poorly described. As solid chromium nicotinate is intractable, being insoluble or unstable in common solvents, studies on the solid have been limited, and studies of the solution from which the "compound" precipitates have additionally provided little additional data. The results of mass spectrometric and spectroscopic investigations designed to further elucidate the structure and composition of chromium nicotinate are described.