Fragmentation study of peptide acetals and aldehydes using in-source collision-induced dissociation

The fragmentation of peptide acetals and peptide diols, corresponding to the hydrated form of the peptide aldehyde, is dominated by the successive losses of two molecules of MeOH and water, respectively. Using model peptides, the fragmentation mechanism, with respect to the loss of methanol and water, was elucidated. The first loss was certainly charge-directed whereas the second probably occurred via the nucleophilic attack of the nitrogen of an amine on the C-terminal carbon leading to a cyclic ion.     

Influence of the nature of membrane ionogenic groups on water dissociation and electrolyte ion transport: A rotating membrane disk study

Polarization properties of electromembrane systems (EMS) consisting of a heterogeneous membrane, either the MK-41 phosphonic acid membrane or the MK-40 sulfonic acid membrane, and dilute sodium chloride solutions are investigated with the rotating membrane disk method. For the MK-41/0.01 M NaCl and MK-41/0.001 M NaCl EMS, effective ion transport numbers and partial current-voltage curves (CVC) are measured for sodium and hydrogen ions, and limiting-current densities and the diffusion-layer thickness are calculated as functions of the rotation rate of the membrane disk. With the theory of the overlimiting state of EMS, internal parameters of the systems under investigation—the diffusion-layer thickness, the space-charge distribution, and electric-field strengths in the diffusion layer and in the membrane—are calculated from experimentally obtained CVC and the dependence of effective transport numbers on current density. The catalytic influence of ionogenic groups on the dissociation rate of water is analyzed quantitatively. Partial CVC for H⁺ ions are calculated for the space-charge region in MK-40 and MK-41 membranes. Analogous CVC for bipolar membranes containing sulfonic acid and phosphonic acid groups are compared. The dissociation mechanism of water is the same in all EMS and is independent of the membrane type and the nature of the functional groups.     

Electrochemical study of isopoly and heteropoly anion transfer across the water | nitrobenzene interface. Part II. Vanadium-containing heteropolytungstate anions

The electrochemical transfer behaviour of vanadium-containing heteropolytungstate anions [PW_12−xV_xO₄₀]^(3+x)− (x = 1−4) across the water | nitrobenzene interface has been investigated by cyclic voltammetry and chronopotentiometry with cyclic linear current scanning. The transfer of PW₁₁V₁O⁴⁻₄₀, HPW₁₀V₂O⁴⁻₄₀, H₂PW₁₀V₂O³⁻₄₀, H₃PW₉V₃O³⁻₄₀ and H₄PW₈V₄O³⁻₄₀ across the water | nitrobenzene interface can be observed within the potential window. The effects were observed of pH in the water phase on the transfer behaviour and the formation of vanadium-containing heteropolytungstate anions in solution. Heteropolytungstate anions become more stable due to their involving the vanadium atom. The degree of protonation and the dissociation constant of the trivalent vanadium-containing heteropolytungstate anion of protonation increase with increasing vanadium content. The transfer processes are diffusion-controlled. The standard transfer potential, the standard Gibbs energy and the dissociation constant for vanadium-containing heteropolytungstate anions have been obtained and the transfer mechanisms are discussed.     

Irreversible dissociation of water molecules on the ion-exchange membrane-electrolyte solution interface in electrodialysis

The fluxes of hydrogen ions through the cation-exchange membrane and hydroxyl ions through the anion-exchange membrane in the electrodialysis were measured using the method of selective polarization. The experiments, which were conducted in a wide range of current densities, enabled us to obtain the results of ionic transport different from the literature data and to explain them on the basis of chemical reactions proceeding in the system. It is shown that a decrease in the hydration of ionogenic groups intensifies the ionic fluxes of the medium in the electrodialysis.     

Influence of basic residues on dissociation kinetics and dynamics of singly protonated peptides: time‐resolved photodissociation study

Product ion yields in postsource decay and time‐resolved photodissociation at 193 and 266 nm were measured for some peptide ions with lysine ([KF₆ + H]⁺, [F₆K + H]⁺, and [F₃KF₃ + H]⁺) formed by matrix‐assisted laser desorption ionization. The critical energy (E₀) and entropy (ΔS^?) were determined by RRKM fitting of the data. The results were similar to those found previously for peptide ions with histidine. To summarize, the presence of a basic residue, histidine or lysine, inside a peptide ion retarded its dissociation by lowering ΔS^?. On the basis of highly negative ΔS^?, presence of intramolecular interaction involving a basic group in the transition structure was proposed. Copyright © 2009 John Wiley & Sons, Ltd.     

木质素模化物键离解能的理论研究

采用密度泛函理论B3P86方法,在6-31G(d,p)基组水平上,对木质素结构中的6种连接方式(β-O-4、α-O-4、4-O-5、β-1、α-1、5-5)的63个木质素模化物的醚键(C-O)和C-C键的键离解能E_B进行了理论计算研究。分析了不同取代基对键离解能的影响以及键长与键离解能的相关性。计算结果表明,C-O键的键离解能通常比C-C键的小,在各种醚键中C_α-O键的平均键离解能最小,为182.7 kJ/mol;其次是β-O-4连接中的C_β-O键,苯环和烷烃基上的取代基对醚键的键离解能有较强的弱化作用,C-O键的键长和键离解能的相关性较差。与C-O键相比,C-C键的键离解能受苯环上取代基的影响很小,而烷烃基上的取代基对C-C键的键离解能有较大的影响,C-C键的键离解能和键长之间存在较强的线性关系,C-C键的键长越长,其键离解能越小。     

Reactions of hydrated electrons (H2O)n- with carbon dioxide and molecular oxygen: hydration of the CO2- and O2- ions

The gas-phase reactions of hydrated electrons with carbon dioxide and molecular oxygen were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Both CO2 and O2 react efficiently with (H2O)n- because they possess low-lying empty pi* orbitals. The molecular CO2- and O2- anions are concurrently solvated and stabilized by the water ligands to form CO2(-)(H2O)n and O2(-)(H2O)n. Core exchange reactions are also observed, in which CO2(-)(H2O)n is transformed into O2(-)(H2O)n upon collision with O2. This is in agreement with the prediction based on density functional theory calculations that O2(-)(H2O)n clusters are thermodynamically favored with respect to CO2(-)(H2O)n. Electron detachment from the product species is only observed for CO2(-)(H2O)2, in agreement with the calculated electron affinities and solvation energies.     

Mechanisms of and effect of coadsorption on water dissociation on an oxygen vacancy of the MgO(100) surface

The dissociation mechanism of a water molecule at an oxygen vacancy on the MgO(100) surface was studied by using the embedded cluster method at the DFT/B3 LYP level, while the energetic information was refined by using the IMOMO method at the CCSD level. We found that a water molecule initially adsorbs on one of the magnesium ions surrounding the vacancy site with a binding energy of 15.98 kcal mol(-1). It then can dissociate on the MgO(100) surface along two possible dissociation pathways. One pathway produces a hydroxyl group bonded to the original magnesium with a proton filling the vacancy via a transition state with a barrier of 4.67 kcal mol(-1) relative to the adsorbed water configuration. The other pathway yields two hydroxy groups; the hydroxy group originally belonging to the water molecule fills the vacancy, while the hydrogen atom binds with the surface oxygen to form the other hydroxy group. Hydrogen atoms of these hydroxy groups can recombine to form a hydrogen molecule and the surface is healed. Although the barrier (14.09 kcal mol(-1)) of the rate-controlling step of the latter pathway is higher than that of the former one, the energies of all of its stationary points are lower than that of the separated reactants (H(2)O+cluster). The effects of water coadsorption are modeled by placing an additional water molecule near the active center, which suggests that the more coadsorbed water molecules further stabilize the hydroxy species and prevent the hydrogen molecule formation through the latter pathway. The results support the photoemission spectral evidence of water dissociation on the defective MgO(100) surface at low water coverage.     

Ab initio study of the reaction mechanism of water dissociation into the ionic species OH− and H3O+

A reaction mechanism of water dissociation is proposed where solvent effects are accounted for via a minimum stable model that considers the interaction of five water molecules. It is based on the fully self-consistent field (SCF) optimized structures of the reactant, product, and transition state, the calculations being at the Hartree–Fock and configuration interaction level [Møller–Plesset second-order perturbation (MP2) and coupled-cluster single and double excitations (CCSD)]. They were performed with four different basis sets that included polarized and diffuse orbitals. The dissociative mechanism leads to the ionic species OH⁻+H₃O⁺ as stable products and upon analysis of the energy hypersurface, a transition state is found which yields an activation barrier of 21.2 kcal/mol. This value is in good agreement with the experimentally determined enthalpy for the reaction. The contribution of the aggregation energy is emphasized. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 253–259, 1998     

Mechanism of protection of adenosine from sulphate radical anion and repair of adenosine radicals by caffeic acid in aqueous solution

The photooxidation of adenosine in presence of peroxydisulphate (PDS) has been studied by spectrophotometrically measuring the absorbance of adenosine at 260 nm. The rates of oxidation of adenosine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of adenosine suggesting that caffeic acid acts as an efficient scavenger of SO ₄ ^• and protects adenosine from it. Sulphate radical anion competes for adenosine as well as for caffeic acid. The quantum yields of photooxidation of adenosine have been calculated from the rates of oxidation of adenosine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (Φ_expt1) and the quantum yields calculated (Φ_cal) assuming caffeic acid acting only as a scavenger of SO ₄ ^• show that Φ_expt1 values are lower than Φ_expt1 values. The ǵf values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for ₄ ^• scavenging by caffeic acid, are also found to be greater than Φ_expt1 values. These observations suggest that the transient adenosine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.     

Study of kinetics and mechanism of the acid dissociation of copper(II) complex of novel C-functionalized macrocyclic dioxotetraamines

The kinetics of the acid dissociation of copper(II) complexes of novel C-functionalized macrocyclic dioxotetraamines has been studied by means of a stopped-flow spectrophotometer. The acid dissociation rate follows the law V_d = C_comkK₁K₂H ²/(1+K₁H+K₁K₂H ²). From the experimental facts we have obtained, the dissociation kinetics are interpreted by a mechanism involving the negatively charged carbonyl oxygen of the complex being rapidly protonated in a pre-equilibrium step, the rate-determining step being intramolecular hydrogen (enolic tautomer) migration (to imine nitrogen). The dissociation rate reached a plateau in the strongly acidic solution. By means of temperature coefficient method, ΔH ^φ, ΔS ^φ of the pre-equilibrium step and ΔH ^≠, ΔS^≠ of the rate-determining step were obtained. The results of 13-membered macrocyclic dioxotetraamines have been discussed. The influence of the substituents to the acid dissociation rates has also been discussed. The Bronsted type linear free energy relationships do also exist in these C-functionalized dioxotetraamine copper(II) complexes.     

Adsorption and dissociation of H2O on Cu2O(100):A computational study

The adsorption and dissociation of water on Cu2O(100) have been investigated by the density functional theory-generalized gradient approximation (DFT-GGA) method. The corresponding reaction energies, the structures of the transition states and the activation energies were determined. Calculations with and without dipole correction were both studied to get an understanding of the effect of the dipole moment on the adsorption and reaction of water on dipole surface Cu2O(100). When dipole correction was added, the adsorption energies of H2O on different sites generally decreased. The calculated activation barriers for HxO (x = 1, 2) dehydrogenation are 0.42 eV (1.01 eV without the dipole correction) and 1.86 eV, respectively, including the zero point energy correction. The first dehydrogenation outcome is energetically the most stable product.     

A theoretical study of five nitrates: Electronic structure and bond dissociation energies

Three density-functional methods (B3P86, B3PW91, and B3LYP) are employed to investigate the O–NO₂ bond lengths, frontier orbital energies, and O–NO₂ bond dissociation energies (BDEs) of n-propyl nitrate (NPN), isopropyl nitrate (IPN), 2-ethylhexyl nitrate (EHN), triethylene glycol dinitrate (Tri-EGDN), and tetraethylene glycol dinitrate (Tetra-EGDN). It is found that the O–NO₂ bond lengthens (destabilizes) in the order of IPN, NPN, EHN, Tetra-EGDN, and Tri-EGDN. From the data of frontier orbital energies (E_HOMO, E_LUMO), and energy gaps (ΔE), we estimate the relative thermal stability ordering of five nitrates and their corresponding radicals. The predicted BDEs of O–NO₂ bond in NPN, IPN, EHN, Tri-EGDN, and Tetra-EGDN, are 176.6, 174.5, 168.1, 156.1, and 159.3 kJ mol⁻¹, respectively. Based on the finding that the present results of BDEs are well coincident with the experimental results of apparent activation energies from the literature, we can draw a conclusion that the experimental thermolysis of five nitrates is only unimolecular homolytical cleavage of the O–NO₂ bonds.     

On the ion‐pair dissociation mechanisms in the small NaCl·(H2O)6 cluster: A perspective from reaction path search calculations

We performed reaction path search calculations for the NaCl·(H₂O)₆ cluster using the global reaction route mapping (GRRM) code to understand the atomic‐level mechanisms of the NaCl → Na⁺ + Cl⁻ ionic dissociation induced by water solvents. Low‐lying minima, transition states connecting two local minima and corresponding intrinsic reaction coordinates on the potential energy surface are explored. We found that the Na Cl distances at the transitions states for the dissociation pathways were distributed in a relatively wide range of 2.7–3.7 Å and that the Na Cl distance at the transition state did not correlate with the commonly used solvation coordinates. This suggests that the definition of the transition states with specific structures as well as good reaction coordinate is very difficult for the ionic dissociation process even in a small water cluster. © 2018 Wiley Periodicals, Inc.     

A computational study of bond dissociation enthalpies in haloethenes

Carbon–hydrogen bond dissociation enthalpies (BDEs) were computed for all haloethenes, C₂H_4−nX_n (n=0–3, X=F, Cl, Br, I), at the B3LYP/6-311+G(3df,2p) level using isodesmic reactions. It was found that C–H bond strengths in the monohaloethenes varied substantially, by as much as 18 kJ mol⁻¹, dependent upon the bond's stereochemical position relative to the halogen. BDEs in the dihaloethanes varied in the order CX₂CH–H>(E)-CHXCX–H>(Z)-CHXCX–H. Trends in the computed bond enthalpies were discussed and explained on the basis of relative steric repulsions and hyperconjugative delocalization interactions, as determined from Natural Bond Orbital analysis.     

The role of neutral defects in the structural chemistry of liquid water

For defective water associates with an extra hydrogen atom (n-defective associates), size and structure effects on ionization potentials compared to defectless (normal) associates of similar structures have been investigated by quantum chemical and molecular mechanics methods. The ionization potentials of small n defective water associates increase (from fractions of eV to 7 eV–8 eV) with the number of water molecules in the associate. These are most probably the source of a hydrated electron that are responsible for the ensuing equilibrium between all defects in liquid water: neutral n and p defects and ion defects (H _aq ⁺ , OH _aq ^– , e _aq ^– ). Delocalization of an odd electron in defective associates stabilizes the latter and promotes their recombinations, forming hydrated water molecules, hydrogen peroxide, and gaseous hydrogen. Structural instability of fullerene (H₂O)₂₀ relative to normal associates has been found. This compound is stabilized by the endo inclusion of a hydrogen atom, and exo fixation of the hydrogen atom gives rise to an extra source of hydrated electrons.Original Russian Text Copyright © 2004 by G. A. Domrachev, D. A. Selivanovskii, E. G. Domracheva, L. G. Domracheva, A. I. Lazarev, P. A. Stunzhas, S. F. Shishkanov, and V. L. VaksTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 4, pp. 670–677, July–August, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Preparation and chromatographic performance of polymer-based anion exchangers for ion chromatography: A review

In the last decade the developments in the field of ion chromatography (IC) were aimed at increasing the efficiency, sensitivity and rapidity of analysis, as well as on improving separation selectivity. Since selectivity and efficiency to the large extent depend on the surface chemistry of the stationary phase, the development of novel anion exchangers remains one of the priority tasks in modern IC. The exact chemistry of commercially available resins is not known and not many literature data devoted to the procedures of preparing anion exchangers for IC have become available in the last 10–15 years. However, the knowledge about the surface chemistry of anion exchangers can provide understanding of the trends in selectivity and efficiency changes, as well as help with the choice of the stationary phase type suitable for solving a particular analytical task. The current review is devoted to the methods of preparing anion exchangers based on polystyrene-divinylbenzene (PS-DVB) and ethylvinylbenzene-divinylbenzene (EVB-DVB) for IC of inorganic and small organic anions and is aimed at demonstrating the improvement of their performance over the years, which was brought by the development of the new types of stationary phase architecture.     

Kinetics and mechanism of protection of thymine from sulphate radical anion under anoxic conditions

The rates of photooxidation of thymine in presence of peroxydisulphate (PDS) have been determined by measuring the absorbance of thymine at 264 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of thymine suggesting that caffeic acid acts as an efficient scavenger of SO ₄ ^•- and protects thymine from it. Sulphate radical anion competes for thymine as well as for caffeic acid. The rate constant of sulphate radical anion with caffeic acid has been calculated to be 1.24 x 10¹⁰ dm³ mol^-1s^-1. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (φ_expt1) and the quantum yields calculated (φ_cl) assuming caffeic acid acting only as a scavenger of SO ₄ ^•- radicals show that φ_expt1 values are lower than φ_cl values. The φ ’ values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for SO ₄ ^•- scavenging by caffeic acid, are also found to be greater than φ_expt1 values. These observations suggest that the thymine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.