Generation and characterization of ionic and neutral P(OH)2+/* in the gas phase by tandem mass spectrometry and computational chemistry.

The bicoordinated dihydroxyphosphenium ion P(OH)2+ (1+) was generated specifically by charge-exchange dissociative ionization of triethylphosphite and its connectivity was confirmed by collision induced dissociation and neutralization-reionization mass spectra. The major dissociation of 1+ forming PO+ ions at m/z 47 involved another isomer, O=P-OH2+ (2+), for which the optimized geometry showed a long P-OH2 bond. Dissociative 70-eV electron ionization of diethyl phosphite produced mostly 1+ together with a less stable isomer, HP(O)OH+ (3+). Ion 2+ is possibly co-formed with 1+ upon dissociative 70-eV electron ionization of methylphosphonic acid. Neutralization-reionization of 1+ confirmed that P(OH)2* (1) was a stable species. Dissociations of neutral 1, as identified by variable-time measurements, involved rate-determining isomerization to 2 followed by fast loss of water. A competitive loss of H occurs from long-lived excited states of 1 produced by vertical electron transfer. The A and B states undergo rate-determining internal conversion to vibrationally highly excited ground state that loses an H atom via two competing mechanisms. The first of these is the direct cleavage of one of the O-H bonds in 1. The other is an isomerization to 3 followed by cleavage of the P-H bond to form O=P-OH as a stable product. The relative, dissociation, and transition state energies for the ions and neutrals were studied by ab initio and density functional theory calculations up to the QCISD(T)/6-311+G(3df,2p) and CCSD(T)/aug-cc-pVTZ levels of theory. RRKM calculations were performed to investigate unimolecular dissociation kinetics of 1. Excited state geometries and energies were investigated by a combination of configuration interaction singles and time-dependent density functional theory calculations.

Design and synthesis of substrate and internal standard conjugates for profiling enzyme activity in the Sanfilippo syndrome by affinity chromatography/electrospray ionization mass spectrometry.

We describe the design and synthesis of substrate and internal standard conjugates for application in profiling enzyme activity of the enzymes alpha-D-2-deoxy-2-N-sulfonamido-glucosamine sulfamidase, alpha-D-2-deoxy-2-N-acetyl-glucosamine hydrolase, acetyl-coenzymeA:alpha-D-2-deoxy-2-amino-glucosamine transferase, and alpha-D-2-deoxy-2-N-acetyl-glucosamine-6-sulfate sulfatase. Deficiency of any one of these enzymes results in a single clinical phenotype known as Sanfilippo syndrome. Such substrates have been proven effective in the confirmation of enzyme deficiency by a combination of affinity chromatography (AC) and electrospray ionization mass spectrometry (ESIMS), which forms the foundation for a new analytical technology (ACESIMS) of general interest and application to clinical and biomedical research.

Direct profiling of multiple enzyme activities in human cell lysates by affinity chromatography/electrospray ionization mass spectrometry: application to clinical enzymology.

We describe a new method for enzyme analysis using affinity capture followed by electrospray ionization mass spectrometry (ACESIMS) for the quantitative determination of the initial velocities of four heparin-modifying enzymes. These enzymes, when defective in affected children, lead to the lysosomal storage disease known as Sanfilippo syndrome. The method relies on substrates and internal standards conjugated to the molecular handle biotin via a heavy isotope-encodable, mass-adjustable linker. Reaction velocities of the Sanfilippo enzymes in a crude lysate prepared from as little as 2500 human skin fibroblasts can be determined. In addition, the ACESIMS method is widely applicable to the simultaneous analysis of multiple enzymes in a complex biological sample by a single analytical technique and will thus serve as a useful tool in basic and clinical biomedical research.

Sequence information, distinction and quantitation of C-terminal leucine and isoleucine in ternary complexes of tripeptides with Cu(II) and 2,2'-bipyridine.

Tripeptides form ternary complexes with Cu(2+) and 2,2'-bipyridine (bpy) that self-assemble upon mixing the components in aqueous methanol solution. Electrospray ionization (ESI) of the complex solutions provides abundant singly charged [Cu(peptide -- H)bpy](+) and doubly charged [Cu(peptide)bpy](2+) ions. Collision-induced dissociation (CID) at low ion kinetic energies of several tripeptides, AGG, GGA, LGG, GGL, GGI, FGG, GGF, LGF, GLF, GFL, GYA and GAY, showed fragments that were indicative of the amino acid sequence in the peptide. In addition, CID of single and doubly charged complexes of isomeric tripeptides GGL and GGI provided unambiguous distinction of the isomeric leucine and isoleucine residues. Leucine peptides eliminated C(3)H(7) radicals from the amino acid side-chain whereas isoleucine eliminated C(2)H(5) radicals. CID of gas-phase doubly charged peptide complexes in a quadrupole ion trap produced a series of singly charged sequence fragments that following isolation and further CID furnished distinct fragments that allowed quantitation of leucine and isoleucine-containing peptides in mixtures.

Quantification of cellular acid sphingomyelinase and galactocerebroside beta-galactosidase activities by electrospray ionization mass spectrometry.

BACKGROUND: Diagnosis of Niemann-Pick (A and B) and Krabbe diseases is achieved by measurement of the lysosomal enzymes acid sphingomyelinase (ASM) and galactocerebroside beta-galactosidase (GCG), respectively. Conventional assays use radiolabeled or fluorescent substrates and do not allow simultaneous determination of two or more enzymes in the sample. METHODS: We developed a sensitive and specific method to assay ASM and GCG in skin fibroblast homogenates using biotinylated substrate conjugates. The products were purified by bioaffinity capture on streptavidin-agarose beads and, following release, were analyzed by electrospray ionization mass spectrometry. Quantification was achieved using stable-isotope-labeled internal standards that were chemically identical to the products of the enzymatic reactions. RESULTS: The method demonstrated excellent linearity of ASM and GCG enzymatic product formation with the amount of cellular protein and incubation time. The range of ASM activities in fibroblast lysates from six healthy patients was 39-70 nmol. mg(-1). h(-1) compared with 3.7-5.1 nmol. mg(-1). h(-1) in cell lysates from two patients affected with Niemann-Pick A disease. The GCG activities toward the corresponding substrate conjugate were 4.0-6.8 nmol. mg(-1). h(-1) in cell lysates from healthy patients compared with 0.1-0.2 nmol. mg(-1). h(-1) in cell lysates from two patients affected with Krabbe disease. The amounts of substrate conjugates needed per analysis were 15 nmol (14 microg) for both ASM and GCG. CONCLUSIONS: Electrospray mass spectrometry combined with the use of biotinylated substrate conjugates and bioaffinity purification represents a new approach for the diagnosis of lysosomal storage diseases as demonstrated for Niemann-Pick A and Krabbe diseases. No radioactive substrates are used, and the method uses a single instrumental platform to determine both ASM and GCG in one cell sample.

Proton affinity of peroxyacetyl nitrate. A computational study of topical proton affinities.

The structure and energetics of the peroxyacetyl nitrate conformers syn- and anti-PAN and several cations formed by PAN protonation were investigated by a combination of density functional theory and ab initio calculations. syn-PAN is the more stable conformer that is predicted to predominate in gas-phase equilibria. The acetyl carbonyl oxygen was found to be the most basic site in PAN, the oxygen atoms of the peroxide and NO(2) groups being less basic. The 298 K proton affinity of syn-PAN was calculated as 759-763 kJ mol(-1) by effective QCISD(T)/6-311 + G(3df,2p) and 771-773 kJ mol(-1) by B3-MP2/6-311 + G(3df,2p). The calculated values are 25-39 kJ mol(-1) lower than the previous estimate by Srinivasan et al. (Rapid Commun. Mass Spectrom. 1998; 12: 328) that was based on competitive dissociations of proton-bound dimers (the kinetic method). The calculated threshold dissociation energies predicted the formation of CH(3)CO(+) + syn - HOONO(2) and CH(3)COOOH + NO(2)(+) to be the most favorable fragmentations of protonated PAN that required 83 and 89 kJ mol(-1) at the respective thermochemical thresholds at 298 K. The previously observed dissociation to CH(3)COOH + NO(3)(+) was calculated by effective QCISD(T)/6-311 + G(3df,2p) to require 320 kJ mol(-1). The disagreement between the experimental data and calculated energetics is discussed.

Distinction and quantitation of leucine-isoleucine isomers and lysine-glutamine isobars by electrospray ionization tandem mass spectrometry (MS(n), n = 2, 3) of copper(II)-diimine complexes.

Electrospray ionization of mixtures of isomeric and isobaric amino acids was investigated with the goal of distinguishing and quantifying the components. Isomeric amino acids leucine and isoleucine were readily distinguished and quantified in 90 : 10 to 10 : 90 binary mixtures using two-stage (MS(2)) and three-stage (MS(3)) tandem mass spectrometric dissociations of ternary Cu(2+)-2, 2'-bipyridyl (bpy) complexes, [Cu(AA - H)bpy](+). The complexes self-assembled in solution upon mixing the components and provided a convenient means of efficient derivatization that increased the efficiency of amino acid ionization by electrospray and shifted the mass of the analytes to a region which was free of solvent interferences. Low-energy dissociations of [Cu(AA - H)bpy](+) complexes in a quadrupole ion trap were achieved at >90% conversions and >80% trapping efficiencies for the MS(2) and MS(3) precursor and fragment ions. Isobaric amino acids glutamine and lysine were also distinguished through MS(2) and MS(3) of their ternary complexes with Cu(2+) and bpy. ESI of [Cu(Gln - H)bpy](+) was enhanced in the presence of [Cu(Lys - H)bpy](+), which resulted in non-linear response at low Lys concentrations.

Protonation sites in methyl nitrate and the formation of transient CH4NO3 radicals. A neutralization-reionization mass spectrometric and computational study

Protonation sites in methyl nitrate (1) were evaluated computationally at the Gaussian 2(MP2) level of ab initio theory. The methoxy oxygen was the most basic site that had a calculated proton affinity of PA = 728-738 kJ mol-1 depending on the optimization method used to calculate the equilibrium geometry of the CH3O(H)-NO2+ ion (2+). Protonation at the terminal oxygen atoms in methyl nitrate was less exothermic; the calculated proton affinities were 725, 722, and 712 kJ mol-1 for the formation of the syn-syn, anti-syn, and syn-anti ion rotamers 3a+, 3b+, and 3c+, respectively. Ion 2+ was prepared by an ion-molecule reaction of NO2+ with methanol and used to generate the transient CH3O(H)-NO2. radical (2) by femtosecond collisional electron transfer. Exothermic protonation of 1 produced a mixture of 3a(+)-3c+ with 2+ that was used to generate transient radicals 3a-3c. Radical 2 was found to be unbound and dissociated without barrier to methanol and NO2. Radicals 3a-3c were calculated to be weakly bound. When formed by vertical neutralization, 3a-3c dissociated completely on the 4.2 microseconds time scale of the experiment. The main dissociations of 3a-3c were formations of CH3O. + HONO and CH3ONO + OH.. The gas-phase chemistry of radicals 3a-3c and their dissociation products, as studied by neutralization-reionization mass spectrometry, was dominated by Franck-Condon effects on collisional neutralization and reionization. The adiabatic ionization energies of 3a-3c were calculated as 7.54, 7.57, and 7.66 eV, respectively.

Quantitative electrospray ionization mass spectrometric studies of ternary complexes of amino acids with Cu(2+) and phenanthroline.

Electrospray ionization (ESI) mass spectra of ternary complexes of Cu(2+) and 1,10-phenanthroline with the 20 essential amino acids (AA) were investigated quantitatively. Non-basic amino acids formed singly charged complexes of the [Cu(AA - H)phen](+) type. Lysine (Lys) and arginine (Arg) formed doubly charged complexes of the [Cu(HAA - H)phen](2+) type. Detection limits were determined for the complexes of phenylalanine (Phe), glutamic acid (Glu) and Arg, which were at low micromolar or submicromolar concentrations under routine conditions. Detection limits of low nanomolar concentrations are possible for amino acids with hydrophobic side-chains (Phe, Tyr, Trp, Leu, Ile) as determined for Phe. The efficiencies for the formation by ESI of gaseous [Cu(AA - H)phen](+) ions were determined and correlated with the acid-base properties of the amino acids, ternary complex stability constants and amino acid hydrophobicities expressed as the Bull-Breese indices (DeltaF). A weak correlation was found between DeltaF and the ESI efficiencies for the formation of gaseous [Cu(AA - H)phen](+) [Cu(HAA - H]phen](2+) and [AA + H](+) ions that showed that amino acids with hydrophobic side-chains were ionized more efficiently. In the ESI of binary and ternary amino acid mixtures, the formation of gas-phase Cu-phen complexes of amino acids with hydrophobic side-chains was enhanced in the presence of complexes of amino acids with polar or basic side-chains. An interesting enhancement of the ESI formation of [Cu(Glu - H)phen](+) was observed in mixtures. The effect is explained by ion-cluster formation at the droplet interface that results in enhanced desorption of the glutamic acid complex.

Quantitative analysis of complex protein mixtures using isotope-coded affinity tags.

We describe an approach for the accurate quantification and concurrent sequence identification of the individual proteins within complex mixtures. The method is based on a class of new chemical reagents termed isotope-coded affinity tags (ICATs) and tandem mass spectrometry. Using this strategy, we compared protein expression in the yeast Saccharomyces cerevisiae, using either ethanol or galactose as a carbon source. The measured differences in protein expression correlated with known yeast metabolic function under glucose-repressed conditions. The method is redundant if multiple cysteinyl residues are present, and the relative quantification is highly accurate because it is based on stable isotope dilution techniques. The ICAT approach should provide a widely applicable means to compare quantitatively global protein expression in cells and tissues.

Mass spectrometric analysis of catechol-histidine adducts from insect cuticle.

Adducts of catechols and histidine, which are produced by reactions of 1,2-quinones and p-quinone methides with histidyl residues in proteins incorporated into the insect exoskeleton, were characterized using electrospray ionization mass spectrometry (ESMS), tandem electrospray mass spectrometry (ESMS-MS, collision-induced dissociation), and ion trap mass spectrometry (ITMS). Compounds examined included adducts obtained from acid hydrolysates of Manduca sexta (tobacco hornworm) pupal cuticle exuviae and products obtained from model reactions under defined conditions. The ESMS and ITMS spectra of 6-(N-3')-histidyldopamine [6-(N-3')-His-DA, pi isomer] isolated from M. sexta cuticle were dominated by a [M + H]+ ion at m/z 308, rather than the expected m/z 307. High-resolution fast atom bombardment MS yielded an empirical formula of C14H18N3O5, which was consistent with this compound being 6-(N-1')-histidyl-2-(3, 4-dihydroxyphenyl)ethanol [6-(N-1')-His-DOPET] instead of a DA adduct. Similar results were obtained when histidyl-catechol compounds linked at C-7 of the catechol were examined; the (N-1') isomer was confirmed as a DA adduct, and the (N-3') isomer identified as an (N-1')-DOPET derivative. Direct MS analysis of unfractionated cuticle hydrolysate revealed intense parent and product ions characteristic of 6- and 7-linked adducts of histidine and DOPET. Mass spectrometric analysis of model adducts synthesized by electrochemical oxidative coupling of N-acetyldopamine (NADA) quinone and N-acetylhistidine (NAcH) identified the point of attachment in the two isomers. A prominent product ion corresponding to loss of CO2 from [M + H]+ of 2-NAcH-NADA confirmed this as being the (N-3') isomer. Loss of (H2O + CO) from 6-NAcH-NADA suggested that this adduct was the (N-1') isomer. The results support the hypothesis that insect cuticle sclerotization involves the formation of C-N cross-links between histidine residues in cuticular proteins, and both ring and side-chain carbons of three catechols: NADA, N-beta-alanyldopamine, and DOPET.

Gas-phase protonation of pyridine. A variable-time neutralization-reionization and Ab initio study of pyridinium radicals.

Gas-phase protonation of pyridine with CH3NH3+, NH4+, t-C4H9+, H3O+ and CH5+ under thermal conditions was studied by variable-time neutralization-reionization mass spectrometry and ab initio calculations. N-Protonation was found to occur exclusively for CH3NH3+ through H3O+ and predominantly for CH5+. The calculated MP2/6-311G(2d,p) energies gave the proton affinities of N, C-2, C-3 and C-4 in pyridine as 924, 658, 686 and 637 kJ mol-1, respectively, which were in good agreement with previous experimental and theoretical results. Vertical neutralization of the N-protonated isomer (1H+) was accompanied by moderate Franck-Condon effects that deposited 20-21 kJ mol-1 in the 1H-pyridinium radicals (1H) formed. 1H was calculated by UMP2/6-311G(2d,p) and B3LYP/6-311G(2d,p) to be a bound species in its ground electronic state. A substantial fraction of stable 1H was detected in the spectra, which depended on the precursor ion internal energy. Deuterium labeling showed a specific loss of the N-bound hydrogen or deuterium in the radicals. The specificity increased with increasing internal energy in the radicals and decreasing contribution of ion dissociations following reionization. Variable-time measurements established specific loss of the N-bound deuterium also in dissociating low-energy 1D. Loss of hydrogen from 1H+ cations following reionization was highly endothermic and was accompanied by rearrangements that partially scrambled the ring hydrogens.

Protonation sites in gaseous pyrrole and imidazole: a neutralization-reionization and ab initio study.

Mild gas-phase acids C4H9+ and NH4+ protonate pyrrole at C-2 and C-3 but not at the nitrogen atom, as determined by deuterium labeling and neutralization-reionization mass spectrometry. Proton affinities in pyrrole are calculated by MP2/6-311G(2d,p) as 866, 845 and 786 kJ mol-1 for protonation at C-2, C-3 and N, respectively. Vertical neutralization of protonated pyrrole generates bound radicals that in part dissociate by loss of hydrogen atoms. Unimolecular loss of hydrogen atom from C-2- and C-3-protonated pyrrole cations is preceded by proton migration in the ring. Protonation of gaseous imidazole is predicted to occur exclusively at the N-3 imine nitrogen to yield a stable aromatic cation. Proton affinities in imidazole are calculated as 941, 804, 791, 791 and 724 for the N-3, C-4, C-2, C-5 and N-1 positions, respectively. Radicals derived from protonated imidazole are only weakly bound. Vertical neutralization of N-3-protonated imidazole is accompanied by large Franck-Condon effects which deposit on average 183 kJ mol-1 vibrational energy in the radicals formed. The radicals dissociate unimolecularly by loss of hydrogen atom, which involves both direct N-H bond cleavage and isomerization to the more stable C-2 H-isomer. Potential energy barriers to isomerizations and dissociations in protonated pyrrole and imidazole isomers and their radicals were investigated by ab initio calculations.

Energy effects in collisional neutralization with organic molecules.

Neutralization-reionization of CH3OH+., CH3NH2+. and (CH3)2NH+. ions was studied under conditions of exothermic and endothermic electron transfer and distributions of internal energy in the reionized ions were determined. The internal energy deposited on neutralization at kiloelectronvolt collision energies is governed by Franck-Condon effects for the systems under study, whereas the endothermic or exothermic energy balance in electron transfer from molecular targets has only a small effect on the high-energy fraction of the molecules formed. Electron transfer from low-lying molecular orbitals of the organic donor is suggested to occur during random orientations of the ion-molecule collision pair. The internal energy of the precursor ions to be neutralized has a large effect on the relative abundances of survivor ions in the spectra. Vertical recombination energies of CH3OH+. and CH3NH2+. are found by Gaussian-2 level calculations to differ substantially from vertical ionization energies of the corresponding neutral molecules. Franck-Condon effects are analyzed for the vibrational modes that are affected most by vertical electron transfer.

Carboxylate and amine terminus directed fragmentations in gaseous dipeptide complexes with copper (II) and diimine ligands formed by electrospray.

Singly and doubly charged peptide complexes with copper(II) and 2,2'-bipyridyl (bpy) are formed in the gas phase by electrospraying water-methanol solutions containing the components. Collisionally activated dissociations at low kinetic energies of singly charged complexes of the [CuII(peptide-H)(bpy)].+ type provide information about the amino acid sequence for L-Phe-Leu, L-Leu-Phe, L-Phe-Pro, L-Pro-Phe, L-Phe-Met, L-Met-Phe, L-Ser-Phe, L-Asp-Phe, and L-His-Phe. Dissociations of doubly charged complexes of the [CuII(peptide)(bpy).2+ type also allow identification of the N- and C-terminal amino acid residues. Leucine and isoleucine residues are readily distinguished in L-Ala-Leu and L-Ala-Ile through dissociations of their Cu complexes. Ion dissociation mechanisms, as elucidated by deuterium labeling, are discussed.

Hydrogen bonding in transient bifunctional hypervalent radicals by neutralization-reionization mass spectrometry.

Neutralization-reionization mass spectrometry is used to generate hypervalent 9-N-4 (ammonium) and 9-O-3 (oxonium) radicals derived from protonated α,ω-bis-(dimethylamino)alkanes and α,ω-dimethoxyalkanes, which exist as cyclic hydrogen-bonded structures in the gas phase. Collisional neutralization with dimethyl disulfide, trimethylamine, and xenon of the hydrogen-bonded onium cations followed by reionization with oxygen results in complete dissociation. Bond cleavages at the hypervalent nitrogen atoms are found to follow the order CH2-N>CH3-N>N-H, which differs from that in the monofunctional hydrogen-n-heptyldimethylammonium radical, which gives CH2-N>N-H>CH3-N. No overall stabilization through hydrogen bonding of the bifunctional hypervalent ammonium and oxonium radicals is observed. Subtle effects of ring size are found that tend to stabilize large ring structures and are attributed to intramolecular hydrogen bonding.

Novel tandem quadrupole-acceleration-deceleration mass spectrometer for neutralization-reionization studies.

A new tandem mass spectrometer of the quadrupole-acceleration lens-deceleration. lens-quadrupole (QADQ) configuration is described. The instrument is designed for neutralization-reionization studies and consists of a 2000-u quadrupole mass analyzer as MS-I, an acceleration electrostatic lens, a series of three differentially pumped collision cells, and an electrostatic deceleration lens, energy filter, and another 2000-u quadrupole mass analyzer as MS-II. The ion optical system achieves high total ion transmission for 5-9-keV ions. Unit mass resolution in neutralization-reionization mass spectra of aromatic compounds is demonstrated. Mass, kinetic energy, and linked scans at various levels of mass resolution and sensitivity are described.

Angle-resolved neutralization-reionization mass spectrometry.

Neutralization -reionization mass spectra of 2-propenal, isomeric butenes, and isomeric n-hexenes have been found to depend significantly on the z-axis scattering angle of the neutralization event. As shown by Cooks for ion dissociations, increasing scattering angles generally favor products of higher activation-energy reactions. For isomeric butenes and n-hexenes, these reactions provide more definitive information for isomeric characterization.