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We report the first experimental study of the intrinsic chemistry of a U-methylidyne species, focusing on reaction of [OUCH]+ with H2O, O2 and CH3CN in the gas phase. DFT was also used to determine reaction pathways, and establish the mechanism by which [OUCH]+ is formed through collision-induced dissociation of [UO2(CCH)]+.

The goals of the present study were (a) to create positively charged organo-uranyl complexes with general formula [UO2 (R)]+ (eg, R═CH3 and CH2 CH3 ) by decarboxylation of [UO2 (O2 C─R)]+ precursors and (b) to identify the pathways by which the complexes, if formed, dissociate by collisional activation or otherwise react when exposed to gas-phase H2 O. Collision-induced dissociation (CID) of both [UO2 (O2 C─CH3 )]+ and [UO2 (O2 C─CH2 CH3 )]+ causes H+ transfer and elimination of a ketene to leave [UO2 (OH)]+ . However, CID of the alkoxides [UO2 (OCH2 CH3 )]+ and [UO2 (OCH2 CH2 CH3 )]+ produced [UO2 (CH3 )]+ and [UO2 (CH2 CH3 )]+ , respectively. Isolation of [UO2 (CH3 )]+ and [UO2 (CH2 CH3 )]+ for reaction with H2 O caused formation of [UO2 (H2 O)]+ by elimination of ·CH3 and ·CH2 CH3 : Hydrolysis was not observed. CID of the acrylate and benzoate versions of the complexes, [UO2 (O2 C─CH═CH2 )]+ and [UO2 (O2 C─C6 H5 )]+ , caused decarboxylation to leave [UO2 (CH═CH2 )]+ and [UO2 (C6 H5 )]+ , respectively. These organometallic species do react with H2 O to produce [UO2 (OH)]+ , and loss of the respective radicals to leave [UO2 (H2 O)]+ was not detected. Density functional theory calculations suggest that formation of [UO2 (OH)]+ , rather than the hydrated UV O2+ , cation is energetically favored regardless of the precursor ion. However, for the [UO2 (CH3 )]+ and [UO2 (CH2 CH3 )]+ precursors, the transition state energy for proton transfer to generate [UO2 (OH)]+ and the associated neutral alkanes is higher than the path involving direct elimination of the organic neutral to form [UO2 (H2 O)]+ . The situation is reversed for the [UO2 (CH═CH2 )]+ and [UO2 (C6 H5 )]+ precursors: The transition state for proton transfer is lower than the energy required for creation of [UO2 (H2 O)]+ by elimination of CH═CH2 or C6 H5 radical.

While the strong axial U[double bond, length as m-dash]O bonds confer high stability and inertness to UO22+, it has been shown that the axial oxo ligands can be eliminated or replaced in the gas-phase using collision-induced dissociation (CID) reactions. We report here tandem mass spectrometry experiments initiated with a gas-phase complex that includes UO22+ coordinated by a 2,6-difluorobenzoate ligand. After decarboxylation to form a difluorophenide coordinated uranyl ion, [UO2(C6F2H3)]+, CID causes elimination of CO, and then CO and C2H2 in sequential dissociation steps, to leave a reactive uranium fluoride ion, [UF2(C2H)]+. Reaction of [UF2(C2H)]+ with CH3OH creates [UF2(OCH3)]+, [UF(OCH3)2]+ and [UF(OCH3)2(CH3OH)]+. Cleavage of C-O bonds within these species results in the elimination of methyl cation (CH3+). Subsequent CID steps convert [UF(OCH3)2]+ to [UO2(F)]+ and similarly, [U(OCH3)3]+ to [UO2(OCH3)]+. Our experiments show removal of both uranyl oxo ligands in "top-down" CID reactions and replacement in "bottom-up" ion-molecule and dissociation steps.

We recently reported a detailed investigation of the collision-induced dissociation (CID) of [UO

Electrospray ionization and multiple-stage tandem mass spectrometry were used to study the collision-induced dissociation of methanol-coordinated copper-acetate cations, and the ion-molecule reactions of specific product ions. Our experiments led to the discovery of unusual gas-phase ions with compositions such as [Cu(CO2)(CH3OH)]+ and [Cu(N2)(CH3OH)]+. The latter is generated by spontaneous exchange of CO2 for N2 in an ion-molecule reaction. Isotopic labeling studies and high mass-resolution measurements provide data to support the product ion composition assignments. Density functional theory calculations corroborate the experimental observations, both with respect to the preferential decarboxylation over methanol ligand elimination, and the spontaneous nature of the ion-molecule reactions.

RATIONALE In a previous study [Rapid Commun Mass Spectrom. 2004;18:3028-3034], collision-induced dissociation (CID) of [UVI O2 (ClO4 )]+ appeared to be influenced by the high levels of background H2 O in a quadrupole ion trap. The CID of the same species was re-examined here with the goal of determining whether additional, previously obscured dissociation pathways would be revealed under conditions in which the level of background H2 O was lower. METHODS Water- and methanol-coordinated [UVI O2 (ClO4 )]+ precursor ions were generated by electrospray ionization. Multiple-stage tandem mass spectrometry (MSn ) for CID and ion-molecule reaction (IMR) studies was performed using a linear ion trap mass spectrometer. RESULTS Under conditions of low background H2 O, CID of [UVI O2 (ClO4 )]+ generates [UVI O2 (Cl)]+ , presumably by elimination of two O2 molecules. Using low isolation/reaction times, we found that [UVI O2 (Cl)]+ will undergo an IMR with H2 O to generate [UVI O2 (OH)]+ . CONCLUSIONS With lower levels of background H2 O, CID experiments reveal that the intrinsic dissociation pathway for [UVI O2 (ClO4 )]+ leads to [UVI O2 (Cl)]+ , apparently by loss of two O2 molecules. We propose that the results reported in the earlier CID study reflected a two-step process: initial formation of [UVI O2 (Cl)]+ by CID, followed by a very rapid hydrolysis reaction to leave [UVI O2 (OH)]+ .

The following gas-phase uranyl/12-crown-4 (12C4) complexes were synthesized by electrospray ionization: [UO2(12C4)2]2+ and [UO2(12C4)2(OH)]+. Collision-induced dissociation (CID) of the dication resulted in [UO2(12C4-H)]+ (12C4-H is a 12C4 that has lost one H), which spontaneously adds water to yield [UO2(12C4-H)(H2O)]+. The latter has the same composition as complex [UO2(12C4)(OH)]+ produced by CID of [UO2(12C4)2(OH)]+ but exhibits different reactivity with water. The postulated structures as isomeric [UO2(12C4-H)(H2O)]+ and [UO2(12C4)(OH)]+ were confirmed by comparison of infrared multiphoton dissociation (IRMPD) spectra with computed spectra. The structure of [UO2(12C4-H)]+ corresponds to cleavage of a C–O bond in the 12C4 ring, with formation of a discrete U–Oeq bond and equatorial coordination by three intact ether moieties. Comparison of IRMPD and computed IR spectra furthermore enabled assignment of the structures of the other complexes. Theoretical studies of the chemical bonding features of the complexes provide an understanding of their stabilities and reactivities. The results reveal bonding and structures of the uranyl/12C4 complexes and demonstrate the synthesis and identification of two different isomers of gas-phase uranyl coordination complexes.

Developing a comprehensive understanding of the reactivity of uranium species remains an important goal in areas ranging from the development of nuclear fuel processing methods to studies of the migration and fate of the element in the environment. Electrospray ionization (ESI) can provide relatively easy access to gas-phase complexes containing uranium in high oxidation states for subsequent studies of intrinsic structure and reactivity. We report here the formation of a superoxo- complex, [UVIO2(O2)(O2C-CF3)2]−, which is created by ESI using “gentle” conditions (low sheath gas flow rate and low desolvation temperature). CID of [UVIO2(O2)(O2C-CF3)2]− causes elimination of O2, presumably with concomitant reduction of UVIO22+ to UVO2+. Remarkably, subsequent CID of [UVO2(O2C-CF3)2]− creates a species at m/z 330, which is attributed to formation of [UVO(F)4]−. A similar species is generated by multiple-stage CID in a linear ion trap, and collision-cell CID in a Fourier-transform ion-cyclotron resonance (FT-ICR) mass spectrometer, when initiated with the tris-trifluoroacetato complex [UVIO2(O2C-CF3)3]−. High accuracy mass measurement using the FT-ICR instrument confirms the composition assignment for the species at m/z 330.

We present a thorough characterization of fragmentations observed in threshold collision-induced dissociation (TCID) experiments of protonated triglycine (H+GGG) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Kinetic energy-dependent cross-sections for 10 ionic products are observed and analyzed to provide 0 K barriers for six primary products: [b2]+, [y1 + 2H]+, [b3]+, CO loss, [y2 + 2H]+, and [a1]+; three secondary products: [a2]+, [a3]+, and [y2 + 2H – CO]+; and two tertiary products: high energy [y1 + 2H]+ and [a2 – CO]+ after accounting for multiple ion-molecule collisions, internal energy of reactant ions, unimolecular decay rates, competition between channels, and sequential dissociations. Relaxed potential energy surface scans performed at the B3LYP-D3/6-311+G(d,p) level of theory are used to identify transition states (TSs) and intermediates of the six primary and one secondary products. Geometry optimizations and single point energy calculations were performed at several levels of theory. These theoretical energies are compared with experimental energies and are found to give reasonably good agreement, in particular for the M06-2X level of theory. This good agreement between experiment and theory validates the reaction mechanisms explored computationally here and elsewhere and allows identification of the product structures formed at threshold energies. The present work presents the first measurement of absolute experimental threshold energies of important sequence ions and non-sequence ions: [y1 + 2H]+, [b3]+, CO loss, [a1]+, and [a3]+, and refines those for [b2]+ and [y2 + 2H]+ previously measured.

© The Royal Society of Chemistry 2016. Understanding of the nature and extent of chemical bonding in uranyl coordination complexes is crucial for the design of new ligands for nuclear waste separation, uranium extraction from seawater, and other applications. We report here the synthesis, infrared spectroscopic characterization, and quantum chemical studies of a molecular uranyl-di-15-crown-5 complex. The structure and bonding of this unique complex featuring a distinctive 6-fold coplanar coordination staggered sandwich structure and an unusual non-perpendicular orientation of the uranyl moiety are evaluated using density functional theory and chemical bonding analyses. The results provide fundamental understanding of the coordination interaction of uranyl with oxygen-donor ligands.

The speciation and reactivity of uranium are topics of sustained interest because of their importance to the development of nuclear fuel processing methods, and a more complete understanding of the factors that govern the mobility and fate of the element in the environment. Tandem mass spectrometry can be used to examine the intrinsic reactivity (i.e., free from influence of solvent and other condensed phase effects) of a wide range of metal ion complexes in a species-specific fashion. Here, electrospray ionization, collision-induced dissociation, and gas-phase ion-molecule reactions were used to create and characterize ions derived from precursors composed of uranyl cation (UVIO22+) coordinated by formate or acetate ligands. Anionic complexes containing UVIO22+ and formate ligands fragment by decarboxylation and elimination of CH2=O, ultimately to produce an oxo-hydride species [UVIO2(O)(H)]-. Cationic species ultimately dissociate to make [UVIO2(OH)]+. Anionic complexes containing acetate ligands exhibit an initial loss of acetyloxyl radical, CH3CO2•, with associated reduction of uranyl to UVO2+. Subsequent CID steps cause elimination of CO2 and CH4, ultimately to produce [UVO2(O)]-. Loss of CH4 occurs by an intra-complex H+ transfer process that leaves UVO2+ coordinated by acetate and acetate enolate ligands. A subsequent dissociation step causes elimination of CH2=C=O to leave [UVO2(O)]-. Elimination of CH4 is also observed as a result of hydrolysis caused by ion-molecule reaction with H2O. The reactions of other anionic species with gas-phase H2O create hydroxyl products, presumably through the elimination of H2. Graphical Abstract ᅟ.

Past studies of fragmentation reactions of doubly-charged uranyl (UO22+) complexes have been impeded by very rapid water addition reactions that cause H2O adducts to dominate product ion spectra. The fragmentation of uranyl-acetone (aco) complexes ([UO2(aco)n]2+, n = 1–5), generated by electrospray ionization, is revisited here using: (a) collisional activation in a linear ion trap (LIT) mass spectrometer in which the level of background H2O is significantly lower, and (b) infrared multiple-photon photodissociation (IRMPD, 10.6 μm) in the LIT and a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. Lower levels of adventitious H2O in the LIT provided access to fragmentation of [UO2(aco)n]2+, n = 1–5. For n = 3–5, direct elimination of aco ligands is the favored fragmentation pathway. For n = 1 and 2, charge reduction reactions are dominant. For [UO2(aco)2]2+, the most abundant product ion is [UO2(aco)]+, while UO2+ is observed following collision-induced dissociation (CID) of [UO2(aco)]2+. Minor peaks corresponding to ligated [UO2OH]+ are also observed. The IRMPD experiments in the FT-ICR yielded highly accurate mass measurements that confirm composition assignments, and shed light on dissociation reactions in a gas-phase environment that is entirely free of adventitious H2O. For [UO2(aco)n]2+, n = 3–5, the primary photodissociation channel is direct aco elimination, along with charge-reduction pathways that involve intra-complex proton transfer and formation of species that contain enolate ligands. Similar pathways are observed for IRMPD measurements in the LIT.

In this study, peptides were derivatized by reaction with salicylaldehyde to create N-terminal imines (Schiff bases). Collision-induced dissociation of the imine-modified peptides produces a complete series of b and a ions (which reveal sequence). However, an unusual pathway is also observed, one that leads to elimination of the residue mass of the N-terminal amino acid despite the chemical modification to create the imine. This pathway was investigated further using infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory with alanine-glycine-glycine (AGG) as the test peptide. The IRMPD spectrum for the product generated by loss of 71 from modified AGG (Sal-AGG) matches one predicted for protonated Sal-GG, as well as the IRMPD spectrum of glycine-glycine derivatized independently to produce a Schiff base. We conclude that the residue mass of the N-terminal amino acid is likely eliminated through a pathway that involves nucleophilic attack by an amide N atom and possible formation of an imidazole-4-one intermediate.

Developing a comprehensive understanding of the reactivity of uranium-containing species remains an important goal in areas ranging from the development of nuclear fuel processing methods to studies of the migration and fate of the element in the environment. Electrospray ionization (ESI) is an effective way to generate gas-phase complexes containing uranium for subsequent studies of intrinsic structure and reactivity. Recent experiments by our group have demonstrated that the relatively low levels of residual H

The synthesis, reactivity, structures, and bonding in gas-phase binary and complex oxide anion molecules of protactinium and uranium have been studied by experiment and theory. The oxalate ions, An

RATIONALE Hydroxynicotinic acids (2-, 4-, 5- and 6-hydroxy) are widely used in the manufacture of industrial products, and hydroxypyridines are important model systems for study of the tautomerization of N-heterocyclic compounds. Here we determined the gas-phase structure of deprotonated 6-hydroxynicotinic acid (6OHNic). METHODS Anions were generated by electrospray ionization, and isolated and stored in a Fourier transform ion cyclotron resonance mass spectrometer. Infrared (action) spectra were collected by monitoring photodissociation yield versus photon energy. Experimental spectra were then compared with those predicted by density functional theory (DFT) and second-order Moller-Plesset (MP2) perturbation theory calculations. RESULTS For neutral 6OHNic, DFT and MP2 calculations strongly suggest that the 6-pyridone tautomer is favored when solvent effects are included. The lowest energy isomer of deprotonated 6OHNic, in the aqueous or gas phase, is predicted to be the 6-pyridone structure deprotonated by the carboxylic acid group. CONCLUSIONS The deprotonated, 6-pyridone structure is confirmed by comparison of the infrared multiple-photon photodissociation (IRMPD) spectrum in the region of 1100–1900 cm–1 with those predicted using DFT and MP2 calculations. Copyright © 2014 John Wiley & Sons, Ltd.

RATIONALE: Hydroxamates are essential growth factors for some microbes, acting primarily as siderophores that solubilize iron for transport into a cell. Here we determined the intrinsic structure of 1:1 complexes between Boc-protected hydroxylamine and group I ([M(L)](+)) and group II ([M(L-H)](+)) cations, where M and L are the cation and ligand, respectively, which are convenient models for the functional unit of hydroxamate siderphores. METHODS: The relevant complex ions were generated by electrospray ionization (ESI) and isolated and stored in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Infrared spectra of the isolated complexes were collected by monitoring (infrared) photodissociation yield as a function of photon energy. Experimental spectra were then compared to those predicted by density functional theory (DFT) calculations. RESULTS: The infrared multiple photon dissociation (IRMPD) spectra collected are in good agreement with those predicted to be lowest-energy by DFT. The spectra for the group I complexes contain six resolved absorptions that can be attributed to amide I and II type and hydroxylamine N-OH vibrations. Similar absorptions are observed for the group II cation complexes, with shifts of the amide I and amide II vibrations due to the change in structure with deprotonation of the hydroxylamine group. CONCLUSIONS: IRMPD spectroscopy unequivocally shows that the intrinsic binding mode for the group I cations involves the O atoms of the amide carbonyl and hydroxylamine groups of Boc-hydroxylamine. A similar binding mode is preferred for the group II cations, except that in this case the metal ion is coordinated by the O atom of the deprotonated hydroxylamine group. Copyright (c) 2013 John Wiley & Sons, Ltd.

Reported here is a comparison of electron transfer dissociation (ETD) and collision-induced dissociation (CID) of solvent-coordinated dipositive uranyl and plutonyl ions generated by electrospray ionization. Fundamental differences between the ETD and CID processes are apparent, as are differences between the intrinsic chemistries of uranyl and plutonyl. Reduction of both charge and oxidation state, which is inherent in ETD activation of [An(VI) O(2) (CH(3) COCH(3) )(4) ](2+) , [An(VI) O(2) (CH(3) CN)(4) ](2) , [U(VI) O(2) (CH(3) COCH(3) )(5) ](2+) and [U(VI) O(2) (CH(3) CN)(5) ](2+) (An = U or Pu), is accompanied by ligand loss. Resulting low-coordinate uranyl(V) complexes add O(2) , whereas plutonyl(V) complexes do not. In contrast, CID of the same complexes generates predominantly doubly-charged products through loss of coordinating ligands. Singly-charged CID products of [U(VI) O(2) (CH(3) COCH(3) )(4,5) ](2+) , [U(VI) O(2) (CH(3) CN)(4,5) ](2+) and [Pu(VI) O(2) (CH(3) CN)(4) ](2+) retain the hexavalent metal oxidation state with the addition of hydroxide or acetone enolate anion ligands. However, CID of [Pu(VI) O(2) (CH(3) COCH(3) )(4) ](2+) generates monopositive plutonyl(V) complexes, reflecting relatively more facile reduction of Pu(VI) to Pu(V).

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 CH3-C≡N⊕-Ph or Ph-C≡N⊕-Ph 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.

We present infrared photodissociation spectra of two protonated peptides that are cooled in a ~10 K quadrupole ion trap and "tagged" with weakly bound H(2) molecules. Spectra are recorded over the range of 600-4300 cm(-1) using a table-top laser source, and are shown to result from one-photon absorption events. This arrangement is demonstrated to recover sharp (Δν ~6 cm(-1)) transitions throughout the fingerprint region, despite the very high density of vibrational states in this energy range. The fundamentals associated with all of the signature N-H and C=O stretching bands are completely resolved. To address the site-specificity of the C=O stretches near 1800 cm(-1), we incorporated one (13)C into the tripeptide. The labeling affects only one line in the complex spectrum, indicating that each C=O oscillator contributes a single distinct band, effectively "reporting" its local chemical environment. For both peptides, analysis of the resulting band patterns indicates that only one isomeric form is generated upon cooling the ions initially at room temperature into the H(2) tagging regime.

The infrared spectrum of the bis-uranyl nitrate complex [(UO(2))(2)(NO(3))(5)](-) was measured in the gas phase using multiple photon dissociation (IRMPD). Intense absorptions corresponding to the nitrate symmetric and asymmetric vibrations, and the uranyl asymmetric vibration were observed. The nitrate nu(3) vibrations indicate the presence of nitrate in a bridging configuration bound to both uranyl cations, and probably two distinct pendant nitrates in the complex. The coordination environment of the nitrate ligands and the uranyl cations were compared to those in the mono-uranyl complex. Overall, the uranyl cation is more loosely coordinated in the bis-uranyl complex [(UO(2))(2)(NO(3))(5)](-) compared to the mono-complex [UO(2)(NO(3))(3)](-), as indicated by a higher O-U-O asymmetric stretching (nu(3)) frequency. However, the pendant nitrate ligands are more strongly bound in the bis-complex than they are in the mono-uranyl complex, as indicated by the nu(3) frequencies of the pendant nitrate, which are split into nitrosyl and O-N-O vibrations as a result of bidentate coordination. These phenomena are consistent with lower electron density donation per uranyl by the nitrate bridging two uranyl centers compared to that of a pendant nitrate in the mono-uranyl complex. The lowest energy structure predicted by density functional theory (B3LYP functional) calculations was one in which the two uranyl molecules bridged by a single nitrate coordinated in a bis-bidentate fashion. Each uranyl molecule was coordinated by two pendant nitrate ligands. The corresponding vibrational spectrum was in excellent agreement with the IRMPD measurement, confirming the structural assignment. (C) 2011 Elsevier B.V. All rights reserved.

Multiple-stage tandem mass spectrometry and collision-induced dissociation were used to investigate loss of H(2)O or CH(3)OH from protonated versions of GGGX (where X = G, A, and V), GGGGG, and the methyl esters of these peptides. In addition, wavelength-selective infrared multiple photon dissociation was used to characterize the [M + H - H(2)O](+) product derived from protonated GGGG and the major MS(3) fragment, [M + H - H(2)O - 29](+) of this peak. Consistent with the earlier work [ Ballard , K. D. ; Gaskell , S. J. J. Am. Soc. Mass Spectrom. 1993 , 4 , 477 - 481 ; Reid , G. E. ; Simpson , R. J. ; O'Hair , R. A. J. Int. J. Mass Spectrom. 1999 , 190/191 , 209 -230 ], CID experiments show that [M + H - H(2)O](+) is the dominant peak generated from both protonated GGGG and protonated GGGG-OMe. This strongly suggests that the loss of the H(2)O molecule occurs from a position other than the C-terminal free acid and that the product does not correspond to formation of the b(4) ion. Subsequent CID of [M + H - H(2)O](+) supports this proposal by resulting in a major product that is 29 mass units less than the precursor ion. This is consistent with loss of HN horizontal lineCH(2) rather than loss of carbon monoxide (28 mass units), which is characteristic of oxazolone-type b(n) ions. Comparison between experimental and theoretical infrared spectra for a group of possible structures confirms that the [M + H - H(2)O](+) peak is not a substituted oxazolone but instead suggests formation of an ion that features a five-membered ring along the peptide backbone, close to the amino terminus. Additionally, transition structure calculations and comparison of theoretical and experimental spectra of the [M + H - H(2)O - 29](+) peak also support this proposal.

Tris-carboxylate complexes of uranyl [UO2](2+) with acetate and benzoate were generated using electrospray ionization mass spectrometry, and then isolated in a Fourier transform ion cyclotron resonance mass spectrometer. Wavelength-selective infrared multiple photon dissociation (IRMPD) of the tris-acetato uranyl anion resulted in a redox elimination of an acetate radical, which was used to generate an IR spectrum that consisted of six prominent absorption bands. These were interpreted with the aid of density functional theory calculations in terms of symmetric and antisymmetric -CO2 stretches of the monodentate and bidentate acetate, CH3 bending and umbrella vibrations, and a uranyl O-U-O asymmetric stretch. The comparison of the calculated and measured IR spectra indicated that the predominant conformer of the tris-acetate complex contained two acetate ligands bound in a bidentate fashion, while the third acetate was monodentate. In similar fashion, the tris-benzoate uranyl anion was formed and photodissociated by loss of a benzoate radical, enabling measurement of the infrared spectrum that was in close agreement with that calculated for a structure containing one monodentate and two bidentate benzoate ligands. (J Am Soc Mass Spectrom 2010, 21, 719-727) (C) 2010. Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry

Tandem mass spectrometry and wavelength-selective infrared photodissociation were used to generate an infrared spectrum of gas-phase triethylphosphate cationized by attachment of K+. Prominent absorptions were observed in the region of 900 to 1300 cm(-1) that are characteristic of phosphate P=O and P-O-R stretches. The relative positions and intensities of the IR absorptions were reproduced well by density functional theory (DFT) calculations performed using the B3LYP functional and the 6-31+G(d), 6-311+G(d,p) and 6-311++G(3df,2pd) basis sets. Because of good correspondence between experiment and theory for the cation, DFT was then used to generate a theoretical spectrum for neutral triethylphosphate, which in turn accurately reproduces the IR spectrum of the neat liquid when solvent effects are included in the calculations. Copyright (C) 2009 John Wiley & Sons, Ltd.

Infrared multiple photon dissociation (IRMPD) spectroscopy was used to study formation of b2+ from nicotinyl-glycine-glycine-methyl ester (NicGGOMe). IRMPD shows that NicGGOMe is protonated at the pyridine ring of the nicotinyl group, and more importantly, that b2+ from NicGGOMe is not protonated at the oxazolone ring, as would be expected if the species were generated on the conventional bn+/yn+ oxazolone pathway, but at the pyridine ring instead. IRMPD data support a hypothesis that formation of b2+ from NicGGOMe involves mobilization and transfer of an amide position proton during the fragmentation reaction.

Cerium oxyhydroxide cluster anions were produced by irradiating ceric oxide particles by using 355 nm laser pulses that were synchronized with pulses of nitrogen gas admitted to the irradiation chamber. The gas pulse stabilized the nascent clusters that are largely anhydrous [Ce(x)O(y)] ions and neutrals. These initially formed species react with water, principally forming oxohydroxy species that are described by the general formula [Ce(x)O(y)(OH)(z)](-) for which all the Ce atoms are in the IV oxidation state. In general, the extent of hydroxylation varies from a value of three OH per Ce atom when x = 1 to a value slightly greater than 1 for x >or= 8. The Ce(3) and Ce(6) species deviate significantly from this trend: the x = 3 cluster accommodates more hydroxyl moieties compared to neighboring congeners at x = 2 and 4. Conversely, the x = 6 cluster is significantly less hydroxylated than its x = 5 and 7 neighbors. Density functional theory (DFT) modeling of the cluster structures shows that the hydrated clusters are hydrolyzed, and contain one-to-multiple hydroxide moieties, but not datively bound water. DFT also predicts an energetic preference for formation of highly symmetric structures as the size of the clusters increases. The calculated structures indicate that the ability of the Ce(3) oxyhydroxide to accommodate more extensive hydroxylation is due to a more open, hexagonal structure in which the Ce atoms can participate in multiple hydrolysis reactions. Conversely the Ce(6) oxyhydroxide has an octahedral structure that is not conducive to hydrolysis. In addition to the fully oxidized (Ce(IV)) oxyhydroxides, reduced oxyhydroxides (containing a Ce(III) center) are also formed. These become more prominent as the size of the clusters increases, suggesting that the larger ceria clusters have an increased ability to accommodate a reduced Ce(III) moiety. In addition, the spectra offer evidence for the formation of superoxide derivatives that may arise from reaction of the reduced oxyhydroxides with dioxygen. The overall intensity of the clusters tends to monotonically decrease as the cluster size increases; however, this trend is interrupted at Ce(13), which is significantly more stable compared to neighboring congeners, suggesting formation of a dehydrated Keggin-type structure.

The syntheses, characterization, and single-crystal X-ray crystal structures are reported for four complexes of iron and cobalt with the pentadentate ligands, 2,6-diacetylpyridinebis(thiosemicarbazone) (H2L1) and 2,6-diacetylpyridinebis(phenylthiosemicarbazone) (H2L2), including a cobalt dimer displaying a deviation from planarity which is unprecedented for this class of ligands and allows the ligand to occupy five positions of a pseudo-octahedral coordination sphere. This dimer reacts with KCN to produce a mononuclear complex of relevance to the active site of cobalt nitrile hydratase.

Gas-phase complexes of the formula [UO_2(lig)]^+ (lig = acetone (aco) or dimethylsulfoxide (dmso)) were generated by electrospray ionization (ESI) and studied by tandem ion-trap mass spectrometry to determine the general effect of ligand charge donation on the reactivity of UO_2^+ with respect to water and dioxygen. The original hypothesis that addition of O_2 is enhanced by strong σ-donor ligands bound to UO_2^+ is supported by results from competitive collision-induced dissociation (CID) experiments, which show near exclusive loss of H_2O from [UO_2(dmso)(H_2O)(O_2)]^+, whereas both H_2O and O_2 are eliminated from the corresponding [UO_2(aco)(H_2O)(O_2)]^+ species. Ligand-addition reaction rates were investigated by monitoring precursor and product ion intensities as a function of ion storage time in the ion-trap mass spectrometer: these experiments suggest that the association of dioxygen to the UO_2^+ complex is enhanced when the more basic dmso ligand was coordinated to the metal complex. Conversely, addition of H_2O is favored for the analogous complex ion that contains an aco ligand. Experimental rate measurements are supported by density function theory calculations of relative energies, which show stronger bonds between UO_2^+ and O_2 when dmso is the coordinating ligand, whereas bonds to H_2O are stronger for the aco complex.

The structures and predicted infrared (IR) spectra of M + ClO 3 - and M + ClO 3 - · H 2 O , where M = Li, Na, K, Rb and Cs, have been investigated using density functional theory calculations. The structures identified in this study for Li + ClO 3 - , Na + ClO 3 - and K + ClO 3 - ion pairs are in good general agreement with earlier experimental and theoretical investigations, notably bidentate binding of the cation by chlorate in Li + ClO 3 - , two isomers of Na + ClO 3 - and preferred tridentate binding of chlorate in K + ClO 3 - . Our study extends the body of work to include Rb+ and Cs+ ion pairs, which both adopt the tridentate coordination geometry favored by K + ClO 3 - . For M + ClO 3 - · H 2 O our calculations yielded multiple structures, including one not previously reported or suggested that features the H2O ligand “bridging” the metal ion and one oxygen atom of the chlorate anion. The “bridging” structure represents the global minimum for several of the hydrated ion pairs, and the importance of this conformation to the correct interpretation of matrix isolation infrared spectra of the hydrated ions pairs is discussed.

UO(2) (+)-solvent complexes having the general formula [UO(2)(ROH)](+) (R=H, CH(3), C(2)H(5), and n-C(3)H(7)) are formed using electrospray ionization and stored in a Fourier transform ion cyclotron resonance mass spectrometer, where they are isolated by mass-to-charge ratio, and then photofragmented using a free-electron laser scanning through the 10 mum region of the infrared spectrum. Asymmetric O=U=O stretching frequencies (nu(3)) are measured over a very small range [from approximately 953 cm(-1) for H(2)O to approximately 944 cm(-1) for n-propanol (n-PrOH)] for all four complexes, indicating that the nature of the alkyl group does not greatly affect the metal centre. The nu(3) values generally decrease with increasing nucleophilicity of the solvent, except for the methanol (MeOH)-containing complex, which has a measured nu(3) value equal to that of the n-PrOH-containing complex. The nu(3) frequency values for these U(V) complexes are about 20 cm(-1) lower than those measured for isoelectronic U(VI) ion-pair species containing analogous alkoxides. nu(3) values for the U(V) complexes are comparable to those for the anionic [UO(2)(NO(3))(3)](-) complex, and 40-70 cm(-1) lower than previously reported values for ligated uranyl(VI) dication complexes. The lower frequency is attributed to weakening of the O=U=O bonds by repulsion related to reduction of the U metal centre, which increases electron density in the antibonding pi* orbitals of the uranyl moiety. Computational modelling of the nu(3) frequencies using the B3LYP and PBE functionals is in good agreement with the IRMPD measurements, in that the calculated values fall in a very small range and are within a few cm(-1) of measurements. The values generated using the LDA functional are slightly higher and substantially overestimate the trends. Subtleties in the trend in nu(3) frequencies for the H(2)O-MeOH-EtOH-n-PrOH series are not reproduced by the calculations, specifically for the MeOH complex, which has a lower than expected value.

A new cyanoscorpionate ligand, hydrotris(3- t-butyl-4-cyanopyrazolyl)borate (Tp ( t-Bu,4CN )) is reported. Both Tp ( t-Bu,4CN ) and hydrotris(4-cyano-3-phenylpyrazolyl)borate (Tp (Ph,4CN)) form one-dimensional coordination polymers with Cu(I). The polymeric chains align to form channels which, in the case of Tp ( t-Bu,4CN ), can encapsulate solvent molecules, as evidenced by the characterization of one such polymer with encapsulated acetonitrile molecules.

Singly and doubly charged cluster ions of ammonium tetrafluoroborate (NH4BF4) with general formula [(NH4BF4)nNH4]+ and [(NH4BF4)m(NH4)2]2+, respectively, were generated by electrospray ionization (ESI) and their fragmentation examined using collision-induced dissociation (CID) and ion-trap tandem mass spectrometry. CID of [(NH4BF4)nNH4]+ caused the loss of one or more neutral NH4BF4 units. The n = 2 cluster, [(NH4BF4)2NH4]+, was unique in that it also exhibited a dissociation pathway in which HBF4 was eliminated to create [(NH4BF4)(NH3)NH4]+. Dissociation of [(NH4BF4)m(NH4)2]2+ occurred through two general pathways: (a) 'fission' to produce singly charged cluster ions and (b) elimination of one or more neutral NH4BF4 units to leave doubly charged product ions. CID profiles, and measurements of changing precursor and product ion signal intensity as a function of applied collision voltage, were collected for [(NH4BF4)nNH4]+ and compared with those for analogous [(NaBF4)nNa]+ and [(KBF4)nK]+ ions to determine the influence of the cation on the relative stability of cluster ions. In general, the [(NH4BF4)nNH4]+ clusters were found to be easier to dissociate than both the sodium and potassium clusters of comparable size, with [(KBF4)nK]+ ions the most difficult to dissociate.

Reaction of the new cyanoscorpionate ligand, hydrotris(4-cyano-3-phenyl)pyrazolylborate (Tp(Ph),(4CN)) with Co(II), Mn(II), and Fe(II) unexpectedly results in the isolation only of crystals containing sandwich complexes in which the ligands have been isomerized to produce the heterocyanoscorpionate hydrobis(4-cyano-3-phenylpyrazolyl)(4-cyano-5-phenylpyrazolyl)borate (Tp(Ph),(4CN*)). The three complexes have been characterized crystallographically and are isostructural, with each ligand acting in a tridentate manner toward the metal. The isomerization of the ligand appears to be more facile than that of the analogous non-cyano ligand, Tp(Ph), with which crystals of the unisomerized sandwich compound have been isolated for Mn(II) and Fe(II).

A new pyrazole ligand, 3-tert-butyl-4-cyano pyrazole (Hpzt-Bu,4CN), has been synthesized. The crystal structure of this pyrazole, along with the syntheses and crystal structures of Co, Cu, and Mn complexes of this ligand, are reported. The uncoordinated pyrazole shows the formation of a cyclic hydrogen-bound dimer. The Co complex is octahedral, with four coordinated pyrazoles and two coordinated waters. The Mn complex is octahedral, with two coordinated pyrazoles, two coordinated triflates and two coordinated waters. A hydrogen bonding network involving the triflates and waters results in a linear double chain of Mn complexes. The Cu complex has two coordinated pyrazoles and two coordinated chlorides in a slightly distorted square-planar geometry, with a long bond to the cyano N atom of a neighboring Cu complex, forming a pseudodimer.

Formation of [bn+17+cat]+ is a prominent collision-induced dissociation (CID) pathway for Li+- and Na+-cationized peptides. Dissociation of protonated and Ag+-cationized peptides instead favors formation of the rival bn+/[bn-1+cat]+ species. In this study the influence of a 4-aminomethylbenzoic acid (4AMBz) residue on the relative intensities of [b(3)-1+cat]+ and [b(3)+17+cat]+ fragment ions was investigated using several model tetrapeptides including those with the general formula A(4AMBz)AX and A(4AMBz)GX (where X=G, A, V). For Li+- and Na+-cationized versions of the peptides there was a significant increase in the intensity of [b(3)-1+cat]+ for the peptides that contain the 4AMBz residue, and in some cases the complete elimination of the [b(3)+17+cat]+ pathway. The influence of the 4AMBz residue may be attributed to the fact that [b(3)-1+cat]+ would be a highly conjugated species containing an aromatic ring substituent. Comparison of CID profiles generated from Na+-cationized AAGV and A(4AMBz)GV suggests an apparent decrease in the critical energy for generation of [b(3)-1+Na]+ relative to that of [b(3)+17+Na]+ when the aromatic amino acid occupies a position such that it leads to the formation of the highly conjugated oxazolinone, thus leading to an increase in formation rate for the former compared to the latter.

In a previous study we found that a dominant fragmentation pathway observed for collision-induced dissociation (CID) of b derived from peptides with sequence AXAG, where X is γ-aminobutyric acid (γAbu) or e-aminocaproic acid (Cap), involved the loss of 89 mass units (u). A neutral loss of 89 u corresponded to the free acid mass of an alanine (A) residue. This specific pathway was studied in greater detail here using a series of A(γAbu)AG peptides with strategic positioning of 15N, 13C and 2H isotope labels. Based on the extensive labeling, several possible routes to the net elimination of 89 u are proposed. One is based on initial elimination of either aziridinone or imine and CO, followed by opening of an oxazolinone, tautomerization and elimination of H2O. Another involves formation of an aziridinone by cleavage of the N-terminal amide bond, and transfer of O and H atoms to this fragment via an H-bonded ion-molecule complex to complete the loss of 89 u. Both types of pathway include the transfer/migration of H atoms from the α-carbon position of γAbu or A residues. Copyright © 2007 John Wiley & Sons, Ltd.

We report here that electrospray ionization (ESI) of uranyl nitrate dissolved in a mixture of H 2 O and acetone causes the formation of doubly charged, gas-phase complexes containing UO 2 2 + "solvated" by neutral ligands.Using mild conditions, the dominant species observed in the ESI mass spectrum contained the uranyl ion coordinated by five acetone ligands, consistent with proposed most-stable structures in the solution phase. However, chemical mass shift data, ion peak shapes, and a plot of fractional ion abundance versus ion desolvation temperature suggest that in the gas phase, and under the ion-trapping and ejection conditions imposed, complexes with five equatorial acetone ligands are less stable than those with four. Multiple-stage tandem mass spectrometry showed that uranyl-acetone complexes dissociate via the elimination of acetone ligands and through pathways that involve reactive collisions with adventitious H 2 O in the ion trap. At no point was complete removal of ligands to generate the UO 2 2 + ion achieved. ESI was also used to generate complex ions of similar composition and ligand number but different charge state for an investigation of the influence of complex charge on the tendency to add ligands by gas-phase association reactions. We found that the addition of a fifth acetone molecule to complexes initially containing four equatorial ligands is more facile for the doubly charged species. The singly charged complex shows a significant back-reaction to eliminate the fifth ligand, suggesting an intrinsic difference in the preferred coordination number for the U(VI) and U(V) complexes in the gas phase.

We demonstrate, by way of multi-stage tandem mass spectrometry and extensive deuterium labeling, that 2-propanol is converted to acetone, and 2-propoxide to acetaldehyde, when monopositive 2-propanol-coordinated uranyl-ligand cations are subjected to collision-induced dissociation in the gas-phase environment of an ion trap mass spectrometer. A species with formula [(UO2OCH(CH3)2)(HOCH(CH3)2)]+, derived from dissociation of the gas-phase precursor [(UO2NO3)(HOCH(CH3)2)3]+ eliminates two H atoms and CH3 in consecutive stages to generate a monopositive complex composed of the U(V) species U O 2 + coordinated by acetone and acetaldehyde, i.e. [ U O 2 + (O C(CH3)2)(O C(H)CH3)]. Dissociation of this latter ion resulted in elimination of the two coordinating carbonyl ligands in two consecutive dissociation stages to leave U O 2 + . Analogous reactions were not observed for uranyl complexes containing 1-propanol or 2-methyl-2-propanol, or for cationic complexes with divalent metals such as Ni2+, Co2+, Pb2+ and Ca2+. One explanation for these reactions is bond insertion by the metal center in the bis-ligated uranyl complex, which would be expected to have an LUMO consisting of unoccupied 6d-orbitals that would confer transition metal-like behavior on the complex.

We describe herein the design and in vitro biochemical evaluation of a novel class of mechanism-based inhibitors of human leukocyte elastase (HLE) that inactivate the enzyme via an unprecedented enzyme-induced sulfonamide fragmentation cascade. The inhibitors incorporate in their structure an appropriately functionalized saccharin scaffold. Furthermore, the inactivation of the enzyme by these inhibitors was found to be time-dependent and to involve the active site. Biochemical, HPLC, and mass spectrometric studies show that the interaction of these inhibitors with HLE results in the formation of a stable acyl complex and is accompanied by the release of (L) phenylalanine methyl ester. The data are consistent with initial formation of a Michaelis–Menten complex and subsequent formation of a tetrahedral intermediate with the active site serine (Ser195). Collapse of the tetrahedral intermediate with tandem fragmentation results in the formation of a highly reactive conjugated sulfonyl imine which can either react with water to form a stable acyl enzyme and/or undergo a Michael addition reaction with an active site nucleophilic residue (His57). It is also demonstrated herein that this class of compounds can be used in the design of inhibitors of serine proteases having either a neutral or basic primary substrate specificity. Thus, the results suggest that these inhibitors constitute a potential general class of mechanism-based inhibitors of (chymo)trypsin-like serine proteases.

The reactivity of mono-alcohol/Ag+ complexes (methanol, ethanol, n-propanol, n-butanol, and tert-butyl alcohol) when stored, without collisional activation, in an ion trap mass spectrometer for periods ranging from 1 to 1000 ms was investigated. During the isolation/reaction time, association reactions between the complex ions and adventitious water and alcohol present within a He bath gas occurred to varying degrees. While the free Ag+ ion was unreactive, complexes composed of Ag+ coordinated by a single alcohol molecule demonstrated reactivity consistent with the following reactions: (1) formation of an adduct by the addition of a single water molecule, (2) formation of a bis-alcohol complex by the addition of a second alcohol molecule, and (3) formation of the bis-alcohol complex via the exchange of a water molecule for alcohol when investigated under similar experimental conditions. The trend in reactivity followed the order n-butanol > n-propanol > ethanol > methanol. To quantify observed trends in ...

Hydroxamates are essential growth factors for some microbes, acting primarily as siderophores that solubilize iron for transport into a cell. Here we determined the intrinsic structure of 1:1 complexes between Boc-protected hydroxylamine and group I ([M(L)](+)) and group II ([M(L-H)](+)) cations, where M and L are the cation and ligand, respectively, which are convenient models for the functional unit of hydroxamate siderphores.The relevant complex ions were generated by electrospray ionization (ESI) and isolated and stored in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Infrared spectra of the isolated complexes were collected by monitoring (infrared) photodissociation yield as a function of photon energy. Experimental spectra were then compared to those predicted by density functional theory (DFT) calculations.The infrared multiple photon dissociation (IRMPD) spectra collected are in good agreement with those predicted to be lowest-energy by DFT. The spectra for the group I complexes contain six resolved absorptions that can be attributed to amide I and II type and hydroxylamine N-OH vibrations. Similar absorptions are observed for the group II cation complexes, with shifts of the amide I and amide II vibrations due to the change in structure with deprotonation of the hydroxylamine group.IRMPD spectroscopy unequivocally shows that the intrinsic binding mode for the group I cations involves the O atoms of the amide carbonyl and hydroxylamine groups of Boc-hydroxylamine. A similar binding mode is preferred for the group II cations, except that in this case the metal ion is coordinated by the O atom of the deprotonated hydroxylamine group.

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 CH3-C≡N⊕-Ph or Ph-C≡N⊕-Ph 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.

Singly and doubly charged cluster ions of ammonium tetrafluoroborate (NH4BF4) with general formula [(NH4BF4)nNH4]+ and [(NH4BF4)m(NH4)2]2+, respectively, were generated by electrospray ionization (ESI) and their fragmentation examined using collision-induced dissociation (CID) and ion-trap tandem mass spectrometry. CID of [(NH4BF4)nNH4]+ caused the loss of one or more neutral NH4BF4 units. The n = 2 cluster, [(NH4BF4)2NH4]+, was unique in that it also exhibited a dissociation pathway in which HBF4 was eliminated to create [(NH4BF4)(NH3)NH4]+. Dissociation of [(NH4BF4)m(NH4)2]2+ occurred through two general pathways: (a) 'fission' to produce singly charged cluster ions and (b) elimination of one or more neutral NH4BF4 units to leave doubly charged product ions. CID profiles, and measurements of changing precursor and product ion signal intensity as a function of applied collision voltage, were collected for [(NH4BF4)nNH4]+ and compared with those for analogous [(NaBF4)nNa]+ and [(KBF4)nK]+ ions to determine the influence of the cation on the relative stability of cluster ions. In general, the [(NH4BF4)nNH4]+ clusters were found to be easier to dissociate than both the sodium and potassium clusters of comparable size, with [(KBF4)nK]+ ions the most difficult to dissociate.

Desferrioxamine (DEF) is a trihydroxamate siderophore typical of those produced by bacteria and fungi for the purpose of scavenging Fe(3+) from environments where the element is in short supply. Since this class of molecules has excellent chelating properties, reaction with metal contaminants such as actinide species can also occur. The complexes that are formed can be mobile in the environment. Because the natural environment is extremely diverse, strategies are needed for the identification of metal complexes in aqueous matrices having a high degree of chemical heterogeneity, and electrospray ionization mass spectrometry (ESI-MS) has been highly effective for the characterization of siderophore-metal complexes. In this study, ESI-MS of solutions containing DEF and either UO(2)(2+), Fe(3+) or Ca(2+) resulted in generation of abundant singly charged ions corresponding to [UO(2)(DEF - H)](+), [Fe(DEF - 2H)](+) and [Ca(DEF - H)](+). In addition, less abundant doubly charged ions were produced. Mass spectrometry/mass spectrometry (MS/MS) studies of collision-induced dissociation (CID) reactions of protonated DEF and metal-DEF complexes were contrasted and rationalized in terms of ligand structure. In all cases, the most abundant fragmentation reactions involved cleavage of the hydroxamate moieties, consistent with the idea that they are most actively involved with metal complexation. Singly charged complexes tended to be dominated by cleavage of a single hydroxamate, while competitive fragmentation between two hydroxamate moieties increased when the doubly charged complexes were considered. Rupture of amide bonds was also observed, but these were in general less significant than the hydroxamate fragmentations. Several lower abundance fragmentations were unique to the metal examined: abundant loss of H(2)O occurred only for the singly charged UO(2)(2+) complex. Further, NH(3) was eliminated only from the singly charged Fe(3+) complex; this and fragmentation of C-C and C-N bonds derived from neither the hydroxamate nor the amide groups suggested that Fe(3+) insertion reactions were competing with ligand complexation. In no experiments were coordinating solvent molecules observed, attached either to the intact complexes or to the fragment ions, which indicated that both intact DEF and its fragments were occupying all of the coordination sites around the metal centers. This conclusion was based on previous experiments that showed that undercoordinated UO(2)(2+) and Fe(3+) readily added H(2)O and methanol in the ESI quadrupole ion trap mass spectrometer that was used in this study.

Wavelength-selective infrared multiple-photon photodissociation (WS-IRMPD) was used to study isotopically-labeled ions generated by McLafferty rearrangement of nicotinyl-glycine-tert-butyl ester and betaine-glycine-tert-butyl ester. The tert-butyl esters were incubated in a mixture of D(2)O and CH(3)OD to induce solution-phase hydrogen-deuterium exchange and then converted to gas-phase ions using electrospray ionization. McLafferty rearrangement was used to generate the free-acid forms of the respective model peptides through transfer of an H atom and elimination of butene. The specific aim was to use vibrational spectra generated by WS-IRMPD to determine whether the H atom remains at the acid group, or migrates to one or more of the other exchangeable sites. Comparison of the IRMPD results in the region from 1200-1900 cm(-1) to theoretical spectra for different isotopically-labeled isomers clearly shows that the H atom is situated at the C-terminal acid group and migration to amide positions is negligible on the time scale of the experiment. The results of this study suggest that use of the McLafferty rearrangement for peptide esters could be an effective approach for generation of H-atom isotope tracers, in situ, for subsequent investigation of intramolecular proton migration during peptide fragmentation studies.