Gas-phase hydrogen/deuterium exchange as a molecular probe for the interaction of methanol and protonated peptides

The gas-phase hydrogen/deuterium (HID) exchange kinetics of several protonated amino acids and dipeptides under a background pressure of CH₃OD were determined in an external source Fourier transform mass spectrometer. H/D exchange reactions occur even when the gas-phase basicity of the compound is significantly larger (> 20 kcal/mol) than methanol. In addition; greater deuterium incorporation is observed for compounds that have multiple sites of similar basicities. A mechanism is proposed that involves a structurally specific intermediate with extensive interaction between the protonated compound and methanol.     

Synthesis and Structure–Activity Correlation Studies of Secondary‐ and Tertiary‐Amine‐Based Glutathione Peroxidase Mimics

In this study, a series of secondary‐ and tertiary‐amino‐substituted diaryl diselenides were synthesized and studied for their glutathione peroxidase (GPx) like antioxidant activities with H₂O₂, cumene hydroperoxide, or tBuOOH as substrates and with PhSH or glutathione (GSH) as thiol cosubstrates. This study reveals that replacement of the tert‐amino groups in benzylamine‐based diselenides by sec‐amino moieties drastically enhances the catalytic activities in both the aromatic thiol (PhSH) and GSH assay systems. Particularly, the N‐propyl‐ and N‐isopropylamino‐substituted diselenides are 8–18 times more active than the corresponding N,N‐dipropyl‐ and N,N‐diisopropylamine‐based compounds in all three peroxide systems when GSH is used as the thiol cosubstrate. Although the catalytic mechanism of sec‐amino‐substituted diselenides is similar to that of the tert‐amine‐based compounds, differences in the stability and reactivity of some of the key intermediates account for the differences in the GPx‐like activities. It is observed that the sec‐amino groups are better than the tert‐amino moieties for generating the catalytically active selenols. This is due to the absence of any significant thiol‐exchange reactions in the selenenyl sulfides derived from sec‐amine‐based diselenides. Furthermore, the seleninic acids (RSeO₂H) derived from the sec‐amine‐based compounds are more stable toward further reactions with peroxides than their tert‐amine‐based analogues.     

Preparation and evaluation of novel chiral stationary phases based on quinine derivatives comprising crown ether moieties

The C9‐position of quinine was modified by meta‐ or para‐substituted benzo‐18‐crown‐6, and immobilized on 3‐mercaptopropyl‐modified silica gel through the radical thiol‐ene addition reaction. These two chiral stationary phases were evaluated by chiral acids, amino acids, and chiral primary amines. The crown ether moiety on the quinine anion exchanger provided a ligand‐exchange site for primary amino groups, which played an important role in the retention and enantioselectivity for chiral compounds containing primary amine groups. These two stationary phases showed good selectivity for some amino acids. The complex interaction between crown ether and protonated primary amino group was investigated by the addition of inorganic salts such as LiCl, NH₄Cl, NaCl, and KCl to the mobile phase. The resolution results showed that the simultaneous interactions between two function moieties (quinine and crown ether) and amino acids were important for the chiral separation.     

Echanges isotopiques d'hydrogène T-H,T-D,D-H et H-D de la diazoacétone et du diazoacétaldéhyde: Vitesse de protonation et effets isotopiques. Communication préliminaire

The velocities of four acid-catalysed hydrogen isotope exchange reactions between diazoacetone ( 1 ) and water (T → H, T → D, D → H, and H → D) and of three exchange reactions between diazoacetaldehyde ( 2 ) and water (T → H, T → D, and H → D) have been measured. The exchange D → H of 1 was found to be submit to general acid catalysis (α_B = 0.6). The rates of protonation of primary and secondary diazocarbonyl compounds of similar structure were found to be quite similar, in spite of difference in mecanism (A-2 and A-S_E2, respectively).     

Struktur und magnetische Eigenschaften von Bis{3‐amino‐1,2,4‐triazolium(1+)}pentafluoromanganat(III): (3‐atriazH)2[MnF5]

Structure and Magnetic Properties of Bis{3‐amino‐1,2,4‐triazolium(1+)}pentafluoromanganate(III): (3‐atriazH)₂[MnF₅] The crystal structure of (3‐atriazH)₂[MnF₅], space group P1, Z = 4, a = 8.007(1) Å, b = 11.390(1) Å, c = 12.788(1) Å, α = 85.19(1)°, β = 71.81(1)°, γ = 73.87(1)°, R = 0.034, is built by octahedral trans‐chain anions [MnF₅]^2–_∞ separated by the mono‐protonated organic amine cations. The [MnF₆] octahedra are strongly elongated along the chain axis (<Mn–F_ax> 2.135 Å, <Mn–F_eq> 1.842 Å), mainly due to the Jahn‐Teller effect, the chains are kinked with an average bridge angle Mn–F–Mn = 139.3°. Below 66 K the compound shows 1D‐antiferromagnetism with an exchange energy of J/k = –10.8 K. 3D ordering is observed at T_N = 9.0 K. In spite of the large inter‐chain separation of 8.2 Å a remarkable inter‐chain interaction with |J′/J| = 1.3 · 10^–5 is observed, mediated probably by H‐bonds. That as well as the less favourable D/J ratio of 0.25 excludes the existence of a Haldene phase possible for Mn³⁺ (S = 2).     

Gas-phase H/D exchange reactions of polyamine complexes: (M + H)+, (M + alkali metal+), and (M + 2H)2+

Gas-phase hydrogen/deuterium exchange reactions between noncovalent polyamine complexes and D2O, CH3OD, or ND3 are undertaken in a quadrupole ion trap mass spectrometer. Structural features of the protonated polyamines can be differentiated by the rates and overall extent of exchange, specifically the presence of propylene units and/or a cyclic structure noticeably decreases exchange compared to the exchange observed for acyclic polyamines with only ethylene bridges between amino groups. Significant differences are observed for singly protonated vs. doubly protonated complexes, where the doubly protonated complexes undergo more efficient exchange at a higher rate than the analogous singly protonated complexes. Molecular modeling calculations suggest that more diffuse conformations may exist for the higher charge states, thus facilitating H/D exchange. In addition, H/D exchange reactions between the alkali metal cationized complexes and ND3 are nearly quenched, compared to the significant exchange seen for singly protonated complexes. A conformational change or the loss of a low energy reaction pathway may explain the limited exchange reactions seen when a bulky cation replaces a proton in the complex.     

Methyl guanine isomer distinction by hydrogen / deuterium exchange using a fourier transform mass spectrometer

Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Differentiation of the seven isomers of methyl guanine has been accomplished by monitoring gas-phase hydrogen/deuterium (H/D) exchange reactions of the protonated molecular ions with deuterium oxide (D₂O) in a Fourier transform mass spectrometer. In each case a distinctive reaction rate for the first H/D exchange was observed, and exchanges of up to three deuterium atoms occurred with characteristic ion abundances that could be used to differentiate the isomers. O⁶-Methyl guanine, for example, showed only one slow H/D exchange with D₂O, whereas l-methyl guanine exchanged two hydrogen atoms at a significantly faster rate. On comparison of the possible resonance structures of each protonated isomer with the experimental information about the number and rate of H/D exchanges observed, a reaction mechanism involving a concerted proton abstraction-deuterium cation donation was proposed.     

H/D exchange reactions of protonated diglycine; an electrospray ionization–flow tube reactor experiment

The H/D exchange reactions of protonated diglycine, GLY₂H⁺, with ND₃ were studied under thermal conditions with a combination of an electrospray ion source and a flow tube reactor. Consecutive exchange of the five labile hydrogens is observed with increasing flow rate of ND₃. Collision complexes corresponding to the consecutive H/D exchanges are monitored for the first time. The role of multiple exchanges in a single collision event with ND₃ is probed. Results will be discussed in the light of previously suggested mechanisms of H/D exchange of GLY₂H⁺ with deuterated ammonia.     

Hydrogen migrations in mass spectrometry. VI—the chemical ionization mass spectra of substituted benzoic acids and benzyl alcohols

The H₂ and CH₄ chemical ionization mass spectra of a series of series of substituted benzoic acids and substituted benzyl alcohols have been determined. For the benzoic acids the major fragmentation reactions of the protonated molecule involve elimination of H₂O or elimination of CO₂, the latter reaction involving migration of the carboxylic hydrogen to the aromatic ring. For the benzyl alcohols the major fragmentation reactions of [MH]⁺ involve loss of H₂O or CH₂O, analogous to the CO₂ elimination reaction for the benzoic acids. It is shown that the CO₂ and CH₂O elimination reactions occur only when a conjugated aromatic ring system is present, and that for the carboxylic acid systems, methyl groups and, to a lesser extent, phenyl groups are capable of migrating. The only discernible effect of substituents on the fragmentation of [MH]⁺ is an enhancement of the H₂O loss reaction in the benzoic acid system when an amino, hydroxyl, or halogen substituent is ortho to the carboxyl function. This ‘ortho’ effect, which differs in scope from that observed in electron impact mass spectra, is attributed to an intramolecular catalysis by the ortho substituent of the 1,3 hydrogen migration in the carbonyl protonated acid followed by H₂O elimination. Apparently, this route is favoured over the direct elimination of H₂O from the carbonyl protonated acid, since the latter has a high activation energy barrier because of unfavourable orbital symmetry restrictions.     

Effects of disulfide linkages on gas-phase reactions of small multiply charged peptide ions

Multiply protonated ions of disulfide-intact and -reduced peptides were generated by electrospray ionization and studied by Fourier transform ion cyclotron resonance mass spectrometry. The effects of disulfide bonds on gas-phase deprotonation reactions and hydrogen/deuterium (H/D) exchange were investigated. Insight into conformations was gained from molecular dynamics calculations. For ions from three small peptides containing 9–14 amino acid residues, H/D exchange is more sensitive to changes in conformation than deprotonation. However, with both gas-phase reactions the more diffuse forms of the peptides (as determined by molecular modeling) react more readily. The effects of disulfide cleavage on the conformations and on the reactions were found to depend upon the sequence of the peptide. For [M + 3H]³⁺ of TGF-α (34–43), reduction of the disulfide linkage leads to a greatly extended structure and a dramatic increase in the rate and extent of H/D exchange. In contrast, [M + 2H]²⁺ of Arg⁸ -vasopressin becomes slightly more compact upon cleavage of the disulfide bond; these reduced ions are slower to react. For [M + 3H]³⁺ of somatostatin-14, reduction of the disulfide bond has little effect on conformation or gas-phase reactivity. Overall, these results indicate that there is no general rule on how cleavage of a disulfide bond will effect a peptide ion’s gas-phase reactivity.     

The gas-phase H/D exchange mechanism of protonated amino acids

A mass spectrometry and Density Functional Theory study of gas-phase H/D exchange in protonated Ala, Cys, Ile, Leu, Met, and Val is reported. Site-specific rate constants were determined and results identify the alpha-amino group as the protonation site. Lack of exchange on the Cys thiol group is explained by the absence of strong intramolecular hydrogen bonding within the reaction complex. In aliphatic amino acids the presence of a methyl group at the beta-C atom was found to lower the site-specific H/D exchange rate for amino hydrogens. Study of the exchange mechanism showed that isotopic exchange occurs in two independent reactions: in one, only the carboxylic hydrogen is exchanged and in the other, both carboxylic and amino group hydrogens exchange. The proposed reaction mechanisms, calculated structures of various species, and a number of structural findings are consistent with experimental data.     

Reactive collisions in quadrupole cells. 3: H/D exchange reactions of protonated aromatic amines with ND3

The H/D exchange reactions of a variety of protonated aromatic amines with ND₃ m the collision cell of a hybrid BEqQ tandem mass spectrometer have been studied. The MH⁺ ions were prepared by CH₄, t-C₄H₁₀, and NH₃ chemical ionization (CI) and, for some amines, by fast-atom bombardment (FAB). Evidence is presented that the kinetic energy of the incident ion as well as its internal energy must be dissipated by nonexchanging collisions before exchange occurs, once deactivated the MH⁺ ions exchange efficiently, which leads, in most cases, to [MHJ⁺ d _x ions m which all active hydrogens have been exchanged. The MH⁺ ion of 1,3-phenylenediamine formed by gas-phase CI exchanges only very slightly with ND₃ whereas a significant fraction of the MH⁺ ions formed by FAB exchange efficiently. This difference is rationalized in terms of dominant formation of the ring-protonated species in gas-phase CI reactions and significant formation of the N-protonated species by FAB with only the N-protonated species exchanging efficiently. Similar, although less pronounced, differences are observed for the MH⁺ ion of m-anisidine. In a number of cases apparent exchange of aromatic hydrogens also is observed. Evidence is presented for the interchange of ring and amine hydrogens in protonated aromatic amines and it is suggested that only the N-protonated species undergoes significant exchange with ND₃.     

Internal reactions of ion–neutral complexes from some disubstituted protonated benzaldehydes and acetophenones

Protonated benzaldehydes ‘a’ and protonated acetophenones ‘b’, substituted by a methoxymethyl group, a hydroxy-methyl group and a mercaptomethyl group, respectively, in position 3, in addition to a methoxymethyl side chain at position 5, have been prepared by electron impact induced dissociation from the corresponding benzylic alcohols. The spontaneous fragmentations of metastable ions of ‘a’ and ‘b’ have been investigated with the aid of specifically deuterated derivatives. Large signals arc observed for the loss of methanol induced by a proton migration across the aromatic ring. The competing loss of H₂O and H₂S, respectively, from the second side chain is less abundant, in agreement with the smaller PA's of HO? and HS? groups. The elimination of HCOX and CH₃COX (X = OCH₃, OH, SH), respectively, from ‘a’ and ‘b’ is also observed. The label distributions for these reactions are in agreement with a mechanism corresponding to an internal reaction of [CHO] ⁺ and [CH₃CO] ⁺, respectively, with the functional group of the side chains in an intermediary ion–neutral complex. In addition, fragmentations are observed arising from reactions between the two side chains at positions 3 and 5. The D labelling proves specific reactions without any H/D exchange and thus reaction channels separated from the methanol loss. The results are explained by internal ion-molecule reactions in an intermediary ion-neutral complex of a methoxymethyl cation, a hydroxymethyl cation and a mercaptomethyl cation, respectively, formed by a protolytic bond cleavage of the side chains.     

Facile synthesis of controlled‐structure primary amine‐based methacrylamide polymers via the reversible addition‐fragmentation chain transfer process

We report here a novel direct method for the syntheses of primary aminoalkyl methacrylamides that requires mild reagents and no protecting group chemistry. The reversible addition‐fragmentation chain transfer polymerization (RAFT) of the aminoalkyl methacrylamide revealed to be highly efficient with 4‐cyanopentanoic acid dithiobenzoate (CTP) as chain transfer agent and 4,4′‐azobis(4‐cyanovaleric acid) (ACVA) as initiator. Cationic amino‐based homopolymers of reasonably narrow polydispersities (M_w/M_n < 1.30) and predetermined molecular weights were obtained without recourse to any protecting group chemistry. A range of block and random copolymers were also synthesized via the RAFT process. The homopolymers and copolymers were characterized by aqueous conventional and triple detection gel permeation chromatography systems. Furthermore, the primary amine‐based methacrylamide monomers and polymers revealed to be highly stable both with the primary amino group in the protonated and deprotonated form. We have also demonstrated that stabilized gold nanoparticles can be generated with the RAFT‐synthesized amine‐based polymers via a photochemical process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4984–4996, 2008     

Lewis Acids as Activators in CBS‐Catalysed Diels–Alder Reactions: Distortion Induced Lewis Acidity Enhancement of SnCl4

The effect of several Lewis acids on the CBS catalyst (named after Corey, Bakshi and Shibata) was investigated in this study. While ²H NMR spectroscopic measurements served as gauge for the activation capability of the Lewis acids, in situ FT‐IR spectroscopy was employed to assess the catalytic activity of the Lewis acid oxazaborolidine complexes. A correlation was found between the Δδ(²H) values and rate constants k_DA, which indicates a direct translation of Lewis acidity into reactivity of the Lewis acid–CBS complexes. Unexpectedly, a significant deviation was found for SnCl₄ as Lewis acid. The SnCl₄–CBS adduct was much more reactive than the Δδ(²H) values predicted and gave similar reaction rates to those observed for the prominent AlBr₃–CBS adduct. To rationalize these results, quantum mechanical calculations were performed. The frontier molecular orbital approach was applied and a good correlation between the LUMO energies of the Lewis acid–CBS–naphthoquinone adducts and k_DA could be found. For the SnCl₄–CBS–naphthoquinone adduct an unusual distortion was observed leading to an enhanced Lewis acidity. Energy decomposition analysis with natural orbitals for chemical valence (EDA‐NOCV) calculations revealed the relevant interactions and activation mode of SnCl₄ as Lewis acid in Diels–Alder reactions.     

Quantifying the Thermodynamics of Ligand Binding to CsPbBr3 Quantum Dots

Cesium lead halide perovskites are an emerging class of quantum dots (QDs) that have shown promise in a variety of applications; however, their properties are highly dependent on their surface chemistry. To this point, the thermodynamics of ligand binding remain unstudied. Herein, ¹H NMR methods were used to quantify the thermodynamics of ligand exchange on CsPbBr₃ QDs. Both oleic acid and oleylamine native ligands dynamically interact with the CsPbBr₃ QD surface, having individual surface densities of 1.2–1.7 nm^?2. 10‐Undecenoic acid undergoes an exergonic exchange equilibrium with bound oleate (K_eq=1.97) at 25 °C while 10‐undecenylphosphonic acid undergoes irreversible ligand exchange. Undec‐10‐en‐1‐amine exergonically exchanges with oleylamine (K_eq=2.52) at 25 °C. Exchange occurs with carboxylic acids, phosphonic acids, and amines on CsPbBr₃ QDs without etching of the nanocrystal surface; increases in the steady‐state PL intensities correlate with more strongly bound conjugate base ligands.     

Nitrogen-15 n.m.r. studies of 13C, 15N labeled arginine

¹⁵N n.m.r. spectra of [¹³C-2, 3-¹⁵N₂-guanidino]arginine and [¹³C, ¹⁵N₂] urea were obtained in D₂O and H₂O at a variety of pH values both with and without proton decoupling. The effects of the proton exchange rate are readily observable in the proton coupled ¹⁵N spectra. When the guanidino group is deprotonated (pK = 12.5), the terminal nitrogens give a single resonance 6.6 ppm downfield of the protonated species, indicating a rapid tautomeric exchange. The observed NH and CN couplings are compared with calculated values, and good agreement is found for ¹J(CN) using a Blizzard–Santry type calculation. The ramifications of the proton exchange on ¹⁵N n.m.r. spectra of amino acids and peptides are discussed.     

High‐resolution mass spectrometry and hydrogen/deuterium exchange study of mitorubrin azaphilones and nitrogenized analogues

Azaphilones represent numerous groups of wild fungal secondary metabolites that exhibit exceptional tendency to bind to nitrogen atoms in various molecules, especially those containing the amine group. Nitrogenized analogues of mitorubrin azaphilones, natural secondary metabolites of Hypoxylon fragiforme fungus, have been detected in the fungal methanol extract in very low concentrations. Positive electrospray ionization interfaced with high‐resolution mass spectrometry was applied for confirmation of the elemental composition of protonated species. Collision‐induced dissociation (CID) experiments have been performed, and fragmentation mechanisms have been proposed. Additional information regarding both secondary metabolite analogue families has been reached by application of gas‐phase proton/deuterium (H/D) exchanges performed in the collision cell of a triple quadrupole mass spectrometer. An incomplete H/D exchange with one proton less than expected was observed for both protonated mitorubrin azaphilones and their nitrogenized analogues. By means of the density functional theory, an appropriate explanation of this behavior was provided, and it revealed some information concerning gas‐phase H/D exchange mechanism and protonation sites. Copyright © 2012 John Wiley & Sons, Ltd.     

Gas Phase H/D Exchange of Sodiated Amino Acids: Why Do We See Zwitterions?

The gas-phase interaction of sodiated amino acids and sodiated amino acid methyl esters with various deuterium donors is investigated by combining results of H/D exchange reactions with those from density functional theory and molecular dynamics calculations. Discrepancy between experimentally and theoretically obtained structures for sodium cationized amino acids is explained by deuterium donor caused perturbation of the most stable amino acid conformation. Detailed study of H/D exchange mechanism on sodiated amino acids shows that the H/D exchange reaction is preceded by a multistep quasi-isoenergetic transition (perturbation) from a charge solvated to zwitterionic structure in the amino acid. Although the computation refers to the system AlaNa(+) and D(2)O, these mechanisms apply to all amino acids, except those where a functional side-chain group takes part in the perturbation process. The suggested perturbation mechanism applies also for other deuterium donors such as CD(3)OD or even ND(3) and indicates that a single water molecule suffices to convert the sodiated amino acid from charge solvated to zwitterionic form.     

O-Protonation of α-Diazoketones in Super-strong Acids

Primary diazoketones, R? CO? CHN₂, are O-protonated in HF? SbF₅? SO₂ or FSO₃H? SbF₅? SO₂ at ?60°, as observed by NMR. The OH-proton resonates at 9.3–9.6 δ and is coupled with H? C 1 (J = 1–2.5 Hz). Secondary diazoketones, R? CO? C(N₂)? R, when protonated, give an OH-singlet at 8.85 δ. The assignments are corroborated by use of deuterated diazoketones, R? CO? CDN₂, or deuterated acid, FSO₃D. Primary diazoketones react with FSO₃H at ?60° to ?15°, giving products assigned the fluorosulfate structure, R? CO? CH₂? OSO₂F; they do not exchange H? C 1 with solvent before or during decomposition. Intermediate C-protonated diazonium ions and α-oxo-carbonium ions (vinyl carbonium ions) have not been identified. 3-Diazo-4-methyl-2-pentanone (VIII) reacts with FSO₃H at ?15°, eliminating N₂ and giving protonated mesityl oxide by a strictly intramolecular hydride shift.