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1.
Phys Chem Chem Phys ; 22(27): 15716-15722, 2020 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-32618970

RESUMO

Microspheres containing radioactive 166holmium-acetylacetonate are employed in emerging radionuclide therapies for the treatment of malignancies. At the molecular level, details on the coordination geometries of the Ho complexes are however elusive. Infrared ion spectroscopy (IRIS) was used to characterize several 165Ho-acetylacetonate complexes derived from non-radioactive microspheres. The coordination geometry of four distinct ionic complexes were fully assigned by comparison of their measured IR spectra with spectra calculated at the density functional theory (DFT) level. The coordination of each acetylacetonate ligand is dependent on the presence of other ligands, revealing an asymmetric chelation motif in some of the complexes. A fifth, previously unknown constituent of the microspheres was identified as a coordination complex containing an acetic acid ligand. These results pave the way for IRIS-based identification of microsphere constituents upon neutron activation of the metal center.

2.
Phys Chem Chem Phys ; 22(23): 13084-13091, 2020 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-32490449

RESUMO

We report herein the first detailed study of the mechanism of redox reactions occurring during the gas-phase dissociative electron transfer of prototypical ternary [CuII(dien)M]˙2+ complexes (M, peptide). The two final products are (i) the oxidized non-zwitterionic π-centered [M]˙+ species with both the charge and spin densities delocalized over the indole ring of the tryptophan residue and with a C-terminal COOH group intact, and (ii) the complementary ion [CuI(dien)]+. Infrared multiple photon dissociation (IRMPD) action spectroscopy and low-energy collision-induced dissociation (CID) experiments, in conjunction with density functional theory (DFT) calculations, revealed the structural details of the mass-isolated precursor and product cations. Our experimental and theoretical results indicate that the doubly positively charged precursor [CuII(dien)M]˙2+ features electrostatic coordination through the anionic carboxylate end of the zwitterionic M moiety. An additional interaction exists between the indole ring of the tryptophan residue and one of the primary amino groups of the dien ligand; the DFT calculations provided the structures of the precursor ion, intermediates, and products, and enabled us to keep track of the locations of the charge and unpaired electron. The dissociative one-electron transfer reaction is initiated by a gradual transition of the M tripeptide from the zwitterionic form in [CuII(dien)M]˙2+ to the non-zwitterionic M intermediate, through a cascade of conformational changes and proton transfers. In the next step, the highest energy intermediate is formed; here, the copper center is 5-coordinate with coordination from both the carboxylic acid group and the indole ring. A subsequent switch back to 4-coordination to an intermediate IM1, where attachment to GGW occurs through the indole ring only, creates the structure that ultimately undergoes dissociation.

3.
Nat Commun ; 11(1): 2664, 2020 05 29.
Artigo em Inglês | MEDLINE | ID: mdl-32471982

RESUMO

Controlling the chemical glycosylation reaction remains the major challenge in the synthesis of oligosaccharides. Though 1,2-trans glycosidic linkages can be installed using neighboring group participation, the construction of 1,2-cis linkages is difficult and has no general solution. Long-range participation (LRP) by distal acyl groups may steer the stereoselectivity, but contradictory results have been reported on the role and strength of this stereoelectronic effect. It has been exceedingly difficult to study the bridging dioxolenium ion intermediates because of their high reactivity and fleeting nature. Here we report an integrated approach, using infrared ion spectroscopy, DFT computations, and a systematic series of glycosylation reactions to probe these ions in detail. Our study reveals how distal acyl groups can play a decisive role in shaping the stereochemical outcome of a glycosylation reaction, and opens new avenues to exploit these species in the assembly of oligosaccharides and glycoconjugates to fuel biological research.

4.
J Am Soc Mass Spectrom ; 31(6): 1205-1211, 2020 Jun 03.
Artigo em Inglês | MEDLINE | ID: mdl-32383378

RESUMO

Hydroxyproline is a common variation of proline, with diverse biological roles. The hydroxylation of proline gives rise to several (natural and/or synthetic) isomeric forms, including both positional isomers and stereoisomers. While mass spectrometry is widely touted as a very selective analytical technique, the identification of closely related isomers often poses a challenge. In these cases, allied technologies become helpful in providing full characterization. Here, infrared multiple photon dissociation (IRMPD) spectroscopy is used to differentiate between three isomers, namely cis-3-hydroxyproline, cis-4-hydroxyproline, and trans-4-hydroxyproline. In contrast to the protonated species which show only minor variations in their IRMPD spectra, lithiated species were found to display significant spectral differences, making their differentiation more straightforward. The conformational origin of these spectral differences was investigated by complementary quantum-chemical calculations.

5.
Phys Chem Chem Phys ; 22(20): 11546-11557, 2020 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-32395733

RESUMO

The structures and properties of metal cationized complexes of 9-ethylguanine (9eG) and 1-methylcytosine (1mC), (9eG:1mC)M+, where M+ = Li+, Na+, K+, Rb+, Cs+ as well as the protonated complex, (9eG:1mC)H+, have been studied using a combination of IRMPD spectroscopy and computational methods. For (9eG:1mC)H+, the dominant structure is a Hoogsteen type complex with the proton covalently bound to N3 of 1mC despite this being the third best protonation site of the two bases; based on proton affinities N7 of 9eG should be protonated. However, this structural oddity can be explained considering both the number of hydrogen bonds that can be formed when N3 of 1mC is protonated as well as the strong ion-induced dipole interaction that exists between an N3 protonated 1mC and 9eG due to the higher polarizability of 9eG. The anomalous dissociation of (9eG:1mC)H+, forming much more (1mC)H+ than would be predicted based on the computed thermochemistry, can be explained as being due to the structural oddity of the protonation site and that the barrier to proton transfer from N3 of 1mC to N7 of 9eG grows dramatically as the base pair begins to dissociate. For the (9eG:1mC)M+; M = Li+, Na+, K+, Rb+, Cs+ complexes, single unique structures could not be assigned. However, the experimental spectra were consistent with the computed spectra. For (9eG:1mC)Li+, the lowest energy structure is one in which Li+ is bound to O6 of 9eG and both O2 and N3 of 1mC; there is also an interbase hydrogen bond from the amine of 1mC to N7 of 9eG. For Na+, K+, and Rb+, similar binding of the metal cation to 1mC is calculated but, unlike Li+, the lowest energy structure is one in which the metal cation is bound to N7 of 9eG; there is also an interbase hydrogen bond between the amine of 1mC and the carbonyl of 9eG. The lowest energy structure for the Cs complex is the Watson-Crick type base pairing with Cs+ binding only to 9eG through O6 and N7 and with three hydrogen bonds between 9eG and 1mC. It also interesting to note that the Watson-Crick base pairing structure gets lower in Gibbs energy relative to the lowest energy complexes as the metal gets larger. This indicates that the smaller, more densely charged cations have a greater propensity to interfere with Watson-Crick base pairing than do the larger, less densely charged metal cations.

6.
Anal Chem ; 92(10): 7282-7288, 2020 05 19.
Artigo em Inglês | MEDLINE | ID: mdl-32286052

RESUMO

Infrared ion spectroscopy (IRIS), a mass-spectrometry-based technique exploiting resonant infrared multiple photon dissociation (IRMPD), has been applied for the identification of novel psychoactive substances (NPS). Identification of the precise isomeric forms of NPS is of significant forensic relevance since legal controls are dependent on even minor molecular differences such as a single ring-substituent position. Using three isomers of fluoroamphetamine and two ring-isomers of both MDA and MDMA, we demonstrate the ability of IRIS to distinguish closely related NPS. Computationally predicted infrared (IR) spectra are shown to correspond with experimental spectra and could explain the molecular origins of their distinctive IR absorption bands. IRIS was then used to investigate a confiscated street sample containing two unknown substances. One substance could easily be identified by comparison to the IR spectra of reference standards. For the other substance, however, this approach proved inconclusive due to incomplete mass spectral databases as well as a lack of available reference compounds, two common analytical limitations resulting from the rapid development of NPS. Most excitingly, the second unknown substance could nevertheless be identified by using computationally predicted IR spectra of several potential candidate structures instead of their experimental reference spectra.

7.
J Phys Chem A ; 2020 Mar 16.
Artigo em Inglês | MEDLINE | ID: mdl-32119552

RESUMO

Experimental IR spectra in the 500-1850 cm-1 fingerprint frequency range are presented for the isolated, gaseous redox pair ions [Ru(bpy)3]2+, and [Ru(bpy)3]+, where bpy = 2,2'-bipyridine. Spectra are obtained using the FELIX free-electron laser and a quadrupole ion trap mass spectrometer. The 2+ complex is generated by electrospray ionization and the charge-reduced radical cation is produced by gas-phase one-electron reduction in an ion-ion reaction with the fluoranthene radical anion. Experimental spectra are compared against computed spectra predicted by density functional theory (DFT) using different levels of theory. For the closed-shell [Ru(bpy)3]2+ ion, the match between experimental and computed IR spectra is very good; however, this is not the case for the charge-reduced [Ru(bpy)3]+ ion, which demands additional theoretical investigation. When using the hybrid B3LYP functional, we observe that better agreement with experiment is obtained upon reduction of the Hartree-Fock exact-exchange contribution from 20% to about 14%. Additionally, calculations using the M06 functional appear to be promising in terms of the prediction of IR spectra; however, it is unclear if the correct electronic structure is obtained. The M06 and B3LYP functionals indicate that the added electron in [Ru(bpy)3]+ is delocalized over the three bpy ligands, while the long-range corrected LC-BLYP and the CAM-B3LYP functionals show it to be more localized on a single bpy ligand. Although these latter levels of theory fail to reproduce the experimentally observed IR frequencies, one may argue that the unusually large bandwidths observed in the spectrum are due to the fluxional character of a complex with the added electron not symmetrically distributed over the ligands. The experimental IR spectra presented here can serve as benchmark for further theoretical investigations.

8.
Phys Chem Chem Phys ; 22(5): 2999-3007, 2020 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-31957771

RESUMO

Infrared multiple photon dissociation (IRMPD) spectroscopy has been used to probe the structures of the three protonated base-pair mismatches containing 9-ethylguanine (9eG) in the gas phase. Computational chemistry has been used to determine the relative energies and compute the infrared spectra of these complexes. By comparing the IRMPD spectra with the computed spectra, in all cases, it was possible to deduce that what was observed experimentally were the lowest energy computed structures. The protonated complex between 9eG and 1-methylthymine (1mT) is protonated at N7 of 9eG-the most basic site of all three bases in this study-and bound in a Hoogsteen type structure with two hydrogen bonds. The experimental IRMPD spectrum for the protonated complex between 9eG and 9-methyladenine (9mA) is described as arising from a combination of the two lowest energy structures, both which are protonated at N1 of adenine and each containing two hydrogen bonds with 9eG being the acceptor of both. The protonated dimer of 9eG is protonated at N7 with an N7-H+-N7 ionic hydrogen bond. It also contains an interaction between a C-H of protonated guanine and the O6 carbonyl of neutral guanine which is manifested in a slight red shift of that carbonyl stretch. The protonated 9eG/9mA structures have been previously identified by X-ray crystallography in RNA and are contained within the protein database.


Assuntos
Gases/química , Guanina/análogos & derivados , Espectrofotometria Infravermelho , Adenina/análogos & derivados , Adenina/química , Adenina/metabolismo , Pareamento Incorreto de Bases , Cristalografia por Raios X , Guanina/química , Guanina/metabolismo , Ligação de Hidrogênio , Modelos Moleculares , Fótons , Timina/análogos & derivados , Timina/química , Timina/metabolismo
9.
Br J Radiol ; 93(1107): 20190598, 2020 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-31782941

RESUMO

OBJECTIVES: To describe the measurements and to present the results of the beam commissioning and the beam model validation of a compact, gantry-mounted, spot scanning proton accelerator system with dynamic layer-by-layer field collimation. METHODS: We performed measurements of depth dose distributions in water, spot and scanned field size in air at different positions from the isocenter plane, spot position over the 20 × 20 cm2 scanned area, beam monitor calibration in terms of absorbed dose to water and specific field collimation measurements at different gantry angles to commission the system. To validate the beam model in the treatment planning system (TPS), we measured spot profiles in water at different depths, absolute dose in water of single energy layers of different field sizes and inversely optimised spread-out Bragg peaks (SOBP) under normal and oblique beam incidence, field size and penumbra in water of SOBPs, and patient treatment specific quality assurance in homogeneous and heterogeneous phantoms. RESULTS: Energy range, spot size, spot position and dose output were consistent at all gantry angles with 0.3 mm, 0.4 mm, 0.6 mm and 0.5% maximum deviations, respectively. Uncollimated spot size (one sigma) in air with an air-gap of 10 cm ranged from 4.1 to 16.4 mm covering a range from 32.2 to 1.9 cm in water, respectively. Absolute dose measurements were within 3% when comparing TPS and experimental data. Gamma pass rates >98% and >96% at 3%/3 mm were obtained when performing 2D dose measurements in homogeneous and in heterogeneous media, respectively. Leaf position was within ±1 mm at all gantry angles and nozzle positions. CONCLUSIONS: Beam characterisation and machine commissioning results, and the exhaustive end-to-end tests performed to assess the proper functionality of the system, confirm that it is safe and accurate to treat patients. ADVANCES IN KNOWLEDGE: This is the first paper addressing the beam commissioning and the beam validation of a compact, gantry-mounted, pencil beam scanning proton accelerator system with dynamic layer-by-layer multileaf collimation.


Assuntos
Ciclotrons , Terapia com Prótons/instrumentação , Absorção de Radiação , Ar , Calibragem , Certificação , Desenho de Equipamento , Humanos , Países Baixos , Imagens de Fantasmas , Terapia com Prótons/métodos , Radiometria/métodos , Reprodutibilidade dos Testes , Água
10.
Chemistry ; 2019 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-31724235

RESUMO

Invited for the cover of this issue is Valentine P. Ananikkov and co-workers. The image depicts the dynamic behaviour of a Pd/NHC catalytic system with easy transition from molecular to ionic complex. Read the full text of the article at 10.1002/chem.201903221.

11.
J Phys Chem A ; 123(38): 8226-8233, 2019 Sep 26.
Artigo em Inglês | MEDLINE | ID: mdl-31490692

RESUMO

Gas-phase infrared multiple-photon dissociation (IRMPD) spectra are recorded for the protonated dye molecules indigo and isoindigo by using a quadrupole ion trap (QIT) mass spectrometer coupled to the free electron laser for infrared experiments (FELIX). From their fingerprint IR spectra (600-1800 cm-1) and comparison with quantum-chemical calculations at the density functional level of theory (B3LYP/6-31++G(d,p)), we derive their structures. We focus particularly on the question of whether trans-to-cis isomerization occurs upon protonation and transfer to the gas phase. The trans-configuration is energetically favored in the neutral forms of the dyes in solution and in the gas phase. Instead, the cis-isomer is lower in energy for the protonated forms of both species, but indigo is also notorious for not undergoing double-bond trans-to-cis isomerization, in contrast to many other conjugated systems. The IR spectra suggest that protoisomerization from trans to cis indeed occurs for both dyes. To estimate the extent of isomerization, on-resonance kinetics are measured on diagnostic and common vibrational frequencies to determine the ratio of cis-to-trans isomers. We find ratios of 65-70% cis and 30-35% trans for indigo versus 75-80% cis and 20-25% trans for isoindigo. Transition-state calculations for the isomerization reactions have been carried out, which indeed suggest a lower barrier for protonated isoindigo, qualitatively explaining the more efficient isomerization.

12.
Chemistry ; 2019 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-31461192

RESUMO

N-Heterocyclic carbene (NHC) ligands are ubiquitously utilized in catalysis. A common catalyst design model assumes strong M-NHC binding in this metal-ligand framework. In contrast to this common assumption, we demonstrate here that lability and controlled cleavage of the M-NHC bond (rather than its stabilization) could be more important for high-performance catalysis at low catalyst concentrations. The present study reveals a dynamic stabilization mechanism with labile metal-NHC binding and [PdX3 ]- [NHC-R]+ ion pair formation. Access to reactive anionic palladium intermediates formed by dissociation of the NHC ligands and plausible stabilization of the molecular catalyst in solution by interaction with the [NHC-R]+ azolium ion is of particular importance for an efficient and recyclable catalyst. These ionic Pd/NHC complexes allowed for the first time the recycling of the complex in a well-defined form with isolation at each cycle. Computational investigation of the reaction mechanism confirms a facile formation of NHC-free anionic Pd in polar media through either Ph-NHC coupling or reversible H-NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.

13.
Phys Chem Chem Phys ; 21(30): 16591-16600, 2019 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-31317140

RESUMO

A charge-tagged phenyl pyruvic acid derivative was investigated by tandem-MS, infrared (IR) ion spectroscopy and theory. The tailor-made precursor ions efficiently lose CO2 in collision induced dissociation (CID) experiments, offering access to study the secondary decay reactions of the product ions. IR ion spectroscopy provides evidence for the formation of an enol acid precursor ion structure in the gas phase and indicates the presence of enol products formed after CO2 loss. Extensive DFT computations however, suggest intermediate generation of hydroxycarbene products, which in turn rearrange in a secondary process to the enol ions detected by IR ion spectroscopy. Quantum mechanical tunneling of the hydroxycarbene can be excluded since no evidence for aldehyde product ion formation could be found. This finding is in contrast to the behavior of methylhydroxycarbene, which cleanly penetrates the energy barrier to form exclusively acetaldehyde at cryogenic temperatures in an argon matrix via quantum mechanical hydrogen tunneling. The results presented here are attributed to the highly excited energy levels of the product ions formed by CID in combination with different barrier heights of the competing reaction channels, which allow exclusive access over one energy barrier leading to the formation of the enol tautomer ions observed.

14.
Environ Sci Technol ; 53(13): 7604-7612, 2019 07 02.
Artigo em Inglês | MEDLINE | ID: mdl-31184875

RESUMO

Characterizing the chemical composition of organic aerosols can elucidate aging mechanisms as well as the chemical and physical properties of the aerosol. However, the high chemical complexity and often low atmospheric abundance present a difficult analytical challenge. Milligrams or more of material may be needed for speciated spectroscopic analysis. In contrast, mass spectrometry provides a very sensitive platform but limited structural information. Here, we combine the strengths of mass spectrometry and infrared (IR) action spectroscopy to generate characteristic IR spectra of individual, mass-isolated ion populations. Soft ionization combined with in situ infrared ion spectroscopy, using the tunable free-electron laser FELIX, provides detailed information on molecular structures and functional groups. We apply this technique, along with quantum mechanical modeling, to characterize organic molecules in secondary organic aerosol (SOA) formed from the ozonolysis of α-pinene. Spectral overlap with a standard is used to identify cis-pinonic acid. We also demonstrate the characterization of isomers for multiple SOA products using both quantum mechanical computations and analyses of fragment ion spectra. These results demonstrate the detailed structural information on isolated ions obtained by combining mass spectrometry with fingerprint IR spectroscopy.


Assuntos
Poluentes Atmosféricos , Monoterpenos , Aerossóis , Monoterpenos Bicíclicos , Íons , Espectrometria de Massas
15.
J Am Soc Mass Spectrom ; 30(9): 1565-1577, 2019 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-31183839

RESUMO

Infrared multiple photon dissociation action spectroscopy was performed on the AlaOrn b2+ and AlaAlaOrn b3+ fragment ions from ornithine-containing tetrapeptides. Infrared spectra were obtained in the fingerprint region (1000-2000 cm-1) using the infrared free electron lasers at the Centre Laser Infrarouge d'Orsay (CLIO) facility in Orsay, France, and the free electron lasers for infrared experiments (FELIX) facility in Nijmegen, the Netherlands. A novel terminal ornithine lactam AO+ b2+ structure was synthesized for experimental comparison and spectroscopy confirms that the b2+ fragment ion from AOAA forms a lactam structure. Comparison of experimental spectra with scaled harmonic frequencies at the B3LYP/6-31+G(d,p) level of theory shows that AO+ b2+ forms a terminal lactam protonated either on the lactam carbonyl oxygen or the N-terminal nitrogen atom. Several low-lying conformers of these isomers are likely populated following IRMPD dissociation. Similarly, a comparison of the experimental IRMPD spectrum with calculated spectra shows that AAO+ b3+-ions also adopt a lactam structure, again with multiple different protonation sites, during fragmentation. This study provides spectroscopic confirmation for the lactam cyclization proposed for the "ornithine effect" and represents an alternative bn+ structure to the oxazolone and diketopiperazine/macrocycle structures most often formed.

16.
Faraday Discuss ; 217(0): 434-452, 2019 07 18.
Artigo em Inglês | MEDLINE | ID: mdl-31012451

RESUMO

The molecular structures of six open-shell z3-ions resulting from electron transfer dissociation mass spectrometry (ETD MS) were investigated using infrared ion spectroscopy in the 800-1850 and 3200-3700 cm-1 spectral ranges in combination with density functional theory and molecular mechanics/molecular dynamics calculations. We assess in particular the question of whether the radical remains at the Cα-site of the backbone cleavage, or whether it migrates by H-atom transfer to another, energetically more favorable position. Calculations performed herein as well as by others show that radical migration to an amino acid side chain or to an α-carbon along the peptide backbone can lead to structures that are more stable, by up to 33 kJ mol-1 for the systems investigated here, by virtue of resonance stabilization of the radical in these alternative positions. Nonetheless, for four out of the six z3-ions considered here, our results quite clearly indicate that radical migration does not occur, suggesting that the radical is kinetically trapped at the site of ETD cleavage. For the two remaining systems, a structural assignment is less secure and we suggest that a mixture of migrated and unmigrated structures may be formed.

17.
Angew Chem Int Ed Engl ; 58(26): 8746-8751, 2019 06 24.
Artigo em Inglês | MEDLINE | ID: mdl-31017713

RESUMO

Uronic acids are important constituents of polysaccharides found on the cell membranes of different organisms. To prepare uronic-acid-containing oligosaccharides, uronic acid 6,3-lactones can be employed as they display a fixed conformation and a unique reactivity and stereoselectivity. Herein, we report a highly ß-selective and efficient mannosyl donor based on C-4 acetyl mannuronic acid 6,3-lactone donors. The mechanism of glycosylation is established using a combination of techniques, including infrared ion spectroscopy combined with quantum-chemical calculations and variable-temperature nuclear magnetic resonance (VT NMR) spectroscopy. The role of these intermediates in glycosylation is assayed by varying the activation protocol and acceptor nucleophilicity. The observed trends are analogous to the well-studied 4,6-benzylidene glycosides and may be used to guide the development of next-generation stereoselective glycosyl donors.

18.
J Phys Chem A ; 123(19): 4149-4157, 2019 May 16.
Artigo em Inglês | MEDLINE | ID: mdl-31021091

RESUMO

We report the fingerprint IR spectra of mass-isolated gaseous coordination complexes of 2,2'-bipyridine (bpy) and 1,4,8,11-tetra-azacyclotetradecane (cyclam) with a copper ion in its I and II oxidation states. Experiments are carried out in a quadrupole ion trap (QIT) mass spectrometer coupled to the FELIX infrared free-electron laser. Dications are prepared using electrospray ionization (ESI), while monocations are generated by charge reduction of the dication using electron transfer-reduction (ETR) in the QIT. Interestingly, [Cu(bpy)2]+ can also be generated directly using ESI, so that its geometries as produced from ETR and ESI can be compared. The effects of charge reduction on the IR spectra are investigated by comparing the experimental spectra with the IR spectra modeled by density functional theory. Reduction of Cu(II) to the closed-shell Cu(I) ion retains the square-planar geometry of the Cu-cyclam complex. In contrast, for the bis-bpy complex with Cu, charge reduction induces a conversion from a near-square-planar to a tetrahedral geometry. The geometry of [Cu(bpy)2]+ is identical to that of the complex generated directly from ESI as a native structure, which indicates that the ETR product ion thermalizes. For [Cu(cyclam)]+, however, the square-planar geometry of the 2+ complex is retained upon charge reduction, although a (distorted) tetrahedral geometry was predicted to be lower in energy. These differences are attributed to different barriers to rearrangement.

19.
Eur J Mass Spectrom (Chichester) ; 25(1): 58-72, 2019 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-30773924

RESUMO

Electrospray ionization was used to generate species such as [ZnNO3(CH3OH)2]+ from Zn(NO3)2•XH2O dissolved in a mixture of CH3OH and H2O. Collision-induced dissociation of [ZnNO3(CH3OH)2]+ causes elimination of CH3OH to form [ZnNO3(CH3OH)]+. Subsequent collision-induced dissociation of [ZnNO3(CH3OH)]+ causes elimination of 47 mass units (u), consistent with ejection of HNO2. The neutral loss shifts to 48 u for collision-induced dissociation of [ZnNO3(CD3OH)]+, demonstrating the ejection of HNO2 involves intra-complex transfer of H from the methyl group methanol ligand. Subsequent collision-induced dissociation causes the elimination of 30 u (32 u for the complex with CD3OH), suggesting the elimination of formaldehyde (CH2 = O). The product ion is [ZnOH]+. Collision-induced dissociation of a precursor complex created using CH3-18OH shows the isotope label is retained in CH2 = O. Density functional theory calculations suggested that the "rearranged" product, ZnOH with bound HNO2 and formaldehyde is significantly lower in energy than ZnNO3 with bound methanol. We therefore used infrared multiple-photon photodissociation spectroscopy to determine the structures of both [ZnNO3(CH3OH)2]+ and [ZnNO3(CH3OH)]+. The infrared spectra clearly show that both ions contain intact nitrate and methanol ligands, which suggests that rearrangement occurs during collision-induced dissociation of [ZnNO3(CH3OH)]+. Based on the density functional theory calculations, we propose that transfer of H, from the methyl group of the CH3OH ligand to nitrate, occurs in concert with the formation of a Zn-C bond. After dissociation to release HNO2, the product rearranges with the insertion of the remaining O atom into the Zn-C bond. Subsequent C-O bond cleavage, with H transfer, produces an ion-molecule complex composed of [ZnOH]+ and O = CH2.

20.
Phys Chem Chem Phys ; 20(44): 28319-28330, 2018 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-30398499

RESUMO

The molecular family of nucleotide triphosphates (NTPs), with adenosine 5'-triphosphate (ATP) as its best-known member, is of high biochemical importance as their phosphodiester bonds form Nature's main means to store and transport energy. Here, gas-phase IR spectroscopic studies and supporting theoretical studies have been performed on adenosine 5'-triphosphate, cytosine 5'-triphosphate and guanosine 5'-triphosphate to elucidate the intrinsic structural properties of NTPs, focusing on the influence of the nucleobase and the extent of deprotonation. Mass spectrometric studies involving collision induced dissociation showed similar fragmentation channels for the three studied NTPs within a selected charge state. The doubly charged anions exhibit fragmentation similar to the energy-releasing hydrolysis reaction in nature, while the singly charged anions show different dominant fragmentation channels, suggesting that the charge state plays a significant role in the favorability of the hydrolysis reaction. A combination of infrared ion spectroscopy and quantum-chemical computations indicates that the singly charged anions of all NTPs are preferentially deprotonated at their ß-phosphates, while the doubly-charged anions are dominantly αß-deprotonated. The assigned three-dimensional structure differs for ATP and CTP on the one hand and GTP on the other, in the sense that ATP and CTP show no interaction between nucleobase and phosphate tail, while in GTP they are hydrogen bonded. This can be rationalized by considering the structure and geometry of the NTPs where the final three dimensional structure depends on a subtle balance between hydrogen bond strength, flexibility and steric hindrance.

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