A monopole mass filter with a hyperbolic V-shaped electrode

In this article we compare the classical monopole mass filter of von Zahn and the monopole mass filter with a hyperbolic V-shaped electrode. The experimental results and those of computer simulation for both mass spectrometers are presented. We show that the replacement of a conventional 90 degrees V-shaped electrode by an electrode with a hyperbolic profile substantially improves the peak shape of any given mass, and increases the mass resolution by a factor of 3-4 and the abundance sensitivity by a factor of 100. The potential of high analytical performance combined with electroforming techniques for electrode manufacture indicate future practical uses of such instruments. Copyright 1999 John Wiley & Sons, Ltd.     

Determination of calcium binding sites in gas-phase small peptides by tandem mass spectrometry

Low-energy (LE) and high-energy (HE) collisionally activated decompositions (CAD) of calcium/peptide complexes of the form [M-H+Ca]⁺ and [M+Ca]²⁺ reflect the site of calcium binding in various gas-phase peptides that are models of the calcium binding site III of rabbit skeletal troponin C. The Ca²⁺ binding sites involve an aspartic acid, glutamic acid, and asparagine, which are in the metal-binding loops of calcium-binding proteins. Both fast atom bombardment (FAB) and electrospray ionization (ESI) were used to generate the metal/peptide complexes. When submitted to LE CAD, ESI-produced Ca²⁺/peptide complexes undergo fragmentations that are controlled by Ca²⁺ binding and provide information on the Ca²⁺ binding site. The LE CAD spectra are simple, indicating that Ca²⁺ binding involves specific oxygen ligands including acidic side chains and that only a few low-energy fragmentation channels exist. The HE CAD spectra of FAB-produced Ca²⁺/peptide complexes are more complex, owing to the introduction of high internal energy into the precursor ion. Interactions of the other alkaline-earth metal ions Mg²⁺ and Ba²⁺ with these peptides reveal that the ligand preferences of these metal ions are slightly different than those of Ca²⁺.     

Investigation of calcium-induced, noncovalent association of calmodulin with melittin by electrospray ionization mass spectrometry

Noncovalent association of Ca²⁺-loaded calmodulin with a target peptide melittin was studied by electrospray ionization mass spectrometry (ESI-MS). ESI-MS does not reveal any binding of the apocalmodulin to the melittin. Partial loading of calmodulin with calcium leads to weak association with melittin. Upon binding of two calcium ions to the protein, changes in the conformation of calmodulin occur; these changes are sufficient to promote binding of melittin. Saturation of the protein with Ca²⁺ (a distribution of up to seven calcium ions is detected) induces a large increase of the binding to melittin. The stoichiometry of peptide binding to calmodulin is 1:1.     

Do charge-remote fragmentations occur under matrix-assisted laser desorption ionization post-source decompositions and matrix-assisted laser desorption ionization collisionally activated decompositions?

The precursor ions of tetraphenylporphyrins that are substituted with fatty acids can be introduced into the gas phase by matrix-assisted laser desorption ionization (MALDI) and undergo post-source and collisionally activated decompositions (CAD) in a time-of-flight mass spectrometer. The goal of the research is to obtain a better understanding of post-source decompositions (PSD); specifically, we asked the question of whether ions undergoing PSD have sufficient energy to give charge-remote fragmentations along an alkyl chain. We chose the porphyrin macrocycle because we expected it to act as an inert "support," allowing the molecule to be desorbed by MALDI and to be amenable to charge-remote fragmentation. MALDI-PSD and MALDI-CAD spectra are similar to high-energy CAD spectra and considerably more informative than low-energy CAD spectra, showing that charge-remote fragmentations of the fatty acid moieties do occur upon MALDI-PSD and MALDI-CAD.     

High- and low-energy collisionally activated decompositions of octaethylporphyrin and its metal complexes

High-energy (HE) and low-energy (LE) collisionally activated decompositions of octaethylporphyrin (OEP) and its metal complexes (ZnOEP and CuOEP) depend on whether the precursor is produced by electrospray ionization as protonated molecules or by fast atom bombardment as radical cations or protonated molecules. LE activation leads to such simple product-ion spectra that a complete picture of fragmentation emerges only after nine stages of tandem mass spectrometry (MS⁹). HE activation, on the other hand, gives product-ion spectra that afford an integrated view of all the decomposition channels in a single MS/MS experiment. These results are the basis of a recommendation that OEP is an appropriate model compound for investigating energy effects in the collisional activation of organic and bioorganic molecule ions.     

Electrospray ionization mass spectrometry and hydrogen/deuterium exchange for probing the interaction of calmodulin with calcium

The extent of H/D exchange of the protein calmodulin in solution was monitored by mass spectrometry following electrospray ionization (ESI) of the protein. In the absence of Ca2+, approximately 115 protons are exchanged for deuteriums after 60 min. As the calmodulin is titrated with Ca2+, the extent of exchange decreases significantly (i.e., by 24 protons), indicating Ca(2+)-induced folding of the protein to a tighter, less solvent-accessible form. The extent of H/D exchange ceases to decrease when the amount of added Ca2+ is sufficient to convert greater than 80% of the calmodulin to a form bound by four calcium ions. Lysozyme, a protein of similar molecular weight, does not show a significant decrease in the extent of H/D exchange as it binds to Ca2+, indicating that the changes in H/D exchange for calmodulin reflect tertiary structural change that occur upon binding with Ca2+.     

Differentiation of positional isomers of nitro meso-tetraphenylporphyrins by tandem mass spectrometry

We studied by tandem mass spectrometry two isomers of nitro meso-tetraphenylporphyrin, one with a nitro group in the para position of a phenyl ring and the other with the same group in a beta-pyrrolic position, and their copper complexes. Collisional activation of the molecular ions of both free-base porphyrins and of their copper complexes produces an array of product ions that permit ready differentiation of the two positional isomers. The diagnostic ions, when the nitro group is in a beta-pyrrolic position, may be produced through intramolecular and double cyclization processes, triggered by the interaction of the nitro substituent with the neighboring meso-phenyl ring. These diagnostic ions do not form when the nitro group is in the para position. The gas-phase processes have precedents in solution chemistry.     

Unexpected fragmentation of β-substituted meso-tetraphenylporphyrins induced by high-energy collisional activation

The protonated molecules and radical cations of meso-tetraphenylporphyrins with β-pyrrolic substituents, when formed by fast atom bombardment (FAB) and subjected to high-energy collisions, give rise to unexpected fragment ions. The reaction involves hydrogen migration from the ortho position of the phenyl ring to the atom of the substituent, with formation of an intramolecular, six-membered ring. The process is analogous to condensed-phase cyclizations described for the same type of compounds. The fragmentation requires the presence of a double bond in the substituent group attached to the pyrrolic ring. A rearrangement process involving anchimeric assistance by the phenyl group (analogous to an ortho effect) is proposed for the formation of these ions.     

Complexes of iron(II) with cysteine-containing peptides in the gas phase

Gas-phase interactions of peptides that contain cysteine with iron(II) atoms were examined by using fast-atom bombardment and tandem mass spectrometry. Specific and strong interactions of iron and sulfur from the thiol group of the cysteine side chain occur in the gas phase and are the basis for highly specific fragmentation to give abundant [a _n ?⁺ ions. For peptides that contain two cysteines, an internal ion, which results from the interaction of Fe and both thiol groups, is formed upon collisional activation. The mechanism for the formation of [a _n ?2H+Fe]⁺ fragment ions requires the metal to be coordinated at sulfur in close proximity to the site of reaction. Iron-bis(pentapeptide) complexes, which form under the same conditions, decompose predominantly to lose a pentapeptide molecule and, to a lesser extent, to give [a _a ?2H+Fe]⁺ ions.     

Justification of the Kirchhoff hypotheses and error estimation for two-dimensional models of anisotropic and inhomogeneous plates, including laminated plates

Asymptotic analysis of the problem describing deformation ofa thin cylindrical plate with clamped lateral side is performed.The problem is considered under the most general statement withthe plate being laminated and consisting of an arbitrary numberof nonhomogeneous and anisotropic (21 elastic moduli) layers.Explicit integral representations of the differential operatorswhich form the two-dimensional model of the plate are derived.In the case when the elastic moduli of each of the layers areconstant, these integral representations turn into algebraicones. The asymptotic procedure is justified with the help ofa weighted inequality of Korn's type. The error estimates obtainedgive a rigorous mathematical proof of both of Kirchhoff's hypotheses(kinematic and static) and shed light on the well-known intrinsicinconsistency of two of the hypotheses.