Home About us Contact
|
Home About us Contact | ||
|
|
||
Gas-phase Basicity (gas-phase + basicity)
Selected AbstractsGas-phase basicities for ions from bradykinin and its des-arginine analoguesJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 8 2001Nigel P. Ewing Abstract Apparent gas-phase basicities (GBapps) for [M + H]+ of bradykinin, des-Arg1 -bradykinin and des-Arg9 -bradykinin have been assigned by deprotonation reactions of [M + 2H]2+ in a Fourier transform ion cyclotron resonance mass spectrometer. With a GBapp of 225.8 ± 4.2 kcal mol,1, bradykinin [M + H]+ is the most basic of the ions studied. Ions from des-Arg1 -bradykinin and des-Arg9 -bradykinin have GBapp values of 222.8 ± 4.3 kcal mol,1 and 214.9 ± 2.3 kcal mol,1, respectively. One purpose of this work was to determine a suitable reaction efficiency ,break point' for assigning GBapp values to peptide ions using the bracketing method. An efficiency value of 0.1 (i.e. approximately 10% of all collisions resulting in a deprotonation reaction) was used to assign GBapps. Support for this criterion is provided by the fact that our GBapp values for des-Arg1 -bradykinin and des-Arg9 -bradykinin are identical, within experimental error, to literature values obtained using a modified kinetic method. However, the GBapps for bradykinin ions from the two studies differ by 10.3 kcal mol,1. The reason for this is not clear, but may involve conformation differences produced by experimental conditions. The results may be influenced by salt-bridge conformers and/or by conformational changes caused by the use of a proton-bound heterodimer in the kinetic method. Factors affecting the basicities of these peptide ions are also discussed, and molecular modeling is used to provide information on protonation sites and conformations. The presence of two highly basic arginine residues on bradykinin results in its high GBapp, while the basicity of des-Arg1 -bradykinin ions is increased by the presence of two proline residues at the N-terminus. The proline residue in the second position folds the peptide chain in a manner that increases intramolecular hydrogen bonding to the protonated N-terminal amino group of the proline at the first position. Copyright © 2001 John Wiley & Sons, Ltd. [source] Gas-phase basicities of polyfunctional molecules.MASS SPECTROMETRY REVIEWS, Issue 6 2007Part 1: Theory, methods Abstract The experimental and theoretical methods of determination of gas-phase basicities, proton affinities and protonation entropies are presented in a tutorial form. Particularities and limitations of these methods when applied to polyfunctional molecules are emphasized. Structural effects during the protonation process in the gas-phase and their consequences on the corresponding thermochemistry are reviewed and classified. The role of the nature of the basic site (protonation on non-bonded electron pairs or on ,-electron systems) and of substituent effects (electrostatic and resonance) are first examined. Then, linear correlations observed between gas-phase basicities and ionization energies or substituent constants are recalled. Hydrogen bonding plays a special part in proton transfer reactions and in the protonation characteristics of polyfunctional molecules. A survey of the main properties of intermolecular and intramolecular hydrogen bonding in both neutral and protonated species is proposed. Consequences on the protonation thermochemistry, particularly of polyfunctional molecules are discussed. Finally, chemical reactions which may potentially occur inside protonated clusters during the measurement of gas-phase basicities or inside a protonated polyfunctional molecule is examined. Examples of bond dissociations with hydride or alkyl migrations, proton transport catalysis, tautomerization, cyclization, ring opening and nucleophilic substitution are presented to illustrate the potentially complex chemistry that may accompany the protonation of polyfunctional molecules. © 2007 Wiley Periodicals, Inc., Mass Spec Rev 26:775,835, 2007 [source] Effect of buffer cations and of H3O+ on the charge states of native proteins.JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 6 2003Significance to determinations of stability constants of protein complexes Abstract The progressive reduction of charge in charge states of non-denatured proteins (lysozyme, ubiquitin, and cytochrome c), observed with nanospray in the positive ion mode, when the buffer salt ammonium acetate is replaced by ethylammonium acetates (EtNH3Ac, Et2NH2Ac and Et3NHAc) is rationalized on the basis of the charge residue model (CRM). The charge states of the multiply protonated protein are shown to be controlled by the increasing gas-phase basicities, GB(B), of the bases(B) NH3, EtNH2, Et2NH and Et3N. Charge states derived from evaluated apparent gas-phase basicities GBapp of the basic side-chains of the protein and the known GB(B) of the above bases are found to be in agreement with the experimentally observed charge states. This is a requirement of the CRM, because in this model the small positive ions (the buffer cations in the present case) at the surface of the electrospray droplets are the excess ions that provide the charge of the final small droplet that contains the protein molecule and on evaporation of the solvent transfer the charge to the protein. The observed charge states in the absence of buffer salts, i.e. pure water, are attributed to excess H3O+ ions produced by the electrolysis process that attends electrospray. A proposed extended mechanism provides predictions of factors that determine the sensitivity for detection of the multiply protonated proteins. Consideration of restraints imposed by the CRM lead to some simple predictions for conditions that should be present to obtain accurate determinations by electrospray and nanospray of stability constants for the protein,complex equilibrium in aqueous solution. Copyright © 2003 John Wiley & Sons, Ltd. [source] Gas-phase basicities for ions from bradykinin and its des-arginine analoguesJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 8 2001Nigel P. Ewing Abstract Apparent gas-phase basicities (GBapps) for [M + H]+ of bradykinin, des-Arg1 -bradykinin and des-Arg9 -bradykinin have been assigned by deprotonation reactions of [M + 2H]2+ in a Fourier transform ion cyclotron resonance mass spectrometer. With a GBapp of 225.8 ± 4.2 kcal mol,1, bradykinin [M + H]+ is the most basic of the ions studied. Ions from des-Arg1 -bradykinin and des-Arg9 -bradykinin have GBapp values of 222.8 ± 4.3 kcal mol,1 and 214.9 ± 2.3 kcal mol,1, respectively. One purpose of this work was to determine a suitable reaction efficiency ,break point' for assigning GBapp values to peptide ions using the bracketing method. An efficiency value of 0.1 (i.e. approximately 10% of all collisions resulting in a deprotonation reaction) was used to assign GBapps. Support for this criterion is provided by the fact that our GBapp values for des-Arg1 -bradykinin and des-Arg9 -bradykinin are identical, within experimental error, to literature values obtained using a modified kinetic method. However, the GBapps for bradykinin ions from the two studies differ by 10.3 kcal mol,1. The reason for this is not clear, but may involve conformation differences produced by experimental conditions. The results may be influenced by salt-bridge conformers and/or by conformational changes caused by the use of a proton-bound heterodimer in the kinetic method. Factors affecting the basicities of these peptide ions are also discussed, and molecular modeling is used to provide information on protonation sites and conformations. The presence of two highly basic arginine residues on bradykinin results in its high GBapp, while the basicity of des-Arg1 -bradykinin ions is increased by the presence of two proline residues at the N-terminus. The proline residue in the second position folds the peptide chain in a manner that increases intramolecular hydrogen bonding to the protonated N-terminal amino group of the proline at the first position. Copyright © 2001 John Wiley & Sons, Ltd. [source] Gas-phase basicities of polyfunctional molecules.MASS SPECTROMETRY REVIEWS, Issue 6 2007Part 1: Theory, methods Abstract The experimental and theoretical methods of determination of gas-phase basicities, proton affinities and protonation entropies are presented in a tutorial form. Particularities and limitations of these methods when applied to polyfunctional molecules are emphasized. Structural effects during the protonation process in the gas-phase and their consequences on the corresponding thermochemistry are reviewed and classified. The role of the nature of the basic site (protonation on non-bonded electron pairs or on ,-electron systems) and of substituent effects (electrostatic and resonance) are first examined. Then, linear correlations observed between gas-phase basicities and ionization energies or substituent constants are recalled. Hydrogen bonding plays a special part in proton transfer reactions and in the protonation characteristics of polyfunctional molecules. A survey of the main properties of intermolecular and intramolecular hydrogen bonding in both neutral and protonated species is proposed. Consequences on the protonation thermochemistry, particularly of polyfunctional molecules are discussed. Finally, chemical reactions which may potentially occur inside protonated clusters during the measurement of gas-phase basicities or inside a protonated polyfunctional molecule is examined. Examples of bond dissociations with hydride or alkyl migrations, proton transport catalysis, tautomerization, cyclization, ring opening and nucleophilic substitution are presented to illustrate the potentially complex chemistry that may accompany the protonation of polyfunctional molecules. © 2007 Wiley Periodicals, Inc., Mass Spec Rev 26:775,835, 2007 [source] Proton affinities of maingroup-element hydrides and noble gases: Trends across the periodic table, structural effects, and DFT validationJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2006Marcel Swart Abstract We have carried out an extensive exploration of the gas-phase basicity of archetypal neutral bases across the periodic system using the generalized gradient approximation (GGA) of the density functional theory (DFT) at BP86/QZ4P//BP86/TZ2P. First, we validate DFT as a reliable tool for computing proton affinities and related thermochemical quantities: BP86/QZ4P//BP86/TZ2P is shown to yield a mean absolute deviation of 2.0 kcal/mol for the proton affinity at 298 K with respect to experiment, and 1.2 kcal/mol with high-level ab initio benchmark data. The main purpose of this work is to provide the proton affinities (and corresponding entropies) at 298 K of the neutral bases constituted by all maingroup-element hydrides of groups 15,17 and the noble gases, that is, group 18, and periods 1,6. We have also studied the effect of step-wise methylation of the protophilic center of the second- and third-period bases. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 1486,1493, 2006 [source] Fourier transform ion cyclotron resonance study of the gas-phase basicity of N -nitrosodimethylamineJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 11 2004Maria E. Crestoni [source] Density functional computations of proton affinity and gas-phase basicity of proline.JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 2 2002E. Tocci, M. Toscano., N. Russo, T. Marino The original article to which this Erratum refers was published in Journal of Mass Spectrometry36(3) 2001, 301,305 [source] Substituent effect on electron affinity, gas-phase basicity, and structure of monosubstituted propargyl radicals and their anions: a theoretical studyJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 2 2010Gab-Yong Lee Abstract The substituent effect of electron-withdrawing groups on electron affinity and gas-phase basicity has been investigated for substituted propargyl radicals and their corresponding anions. It is shown that when a hydrogen of the , -CH2 group or acetylenic CH in the propargyl system is substituted by an electron-withdrawing substituent, electron affinity increases, whereas gas-phase basicity decreases. The calculated electron affinities are 0.95,eV (CHCCH2,), 1.15,eV (CHCCHF,), 1.38,eV (CHCCHCl,), 1.48,eV (CHCCHBr,) for the isomers with terminal CH and 1.66,eV (CFCCH2,), 1.70,eV (CClCCH2,), 1.86,eV (CBrCCH2,) for the isomers with terminal CX at B3LYP level. The calculated gas-phase basicities for their anions are 378.4,kcal/mol (CHCCH2:,), 371.6,kcal/mol (CHCCHF:,), 365.1,kcal/mol (CHCCHCl:,), 363.5,kcal/mol (CHCCHBr:,) for the isomers with terminal CH and 362.6,kcal/mol (CFCCH2:,), 360.4,kcal/mol (CClCCH2:,), 356.3,kcal/mol (CBrCCH2:,) for the isomers with terminal CX at B3LYP level. It is concluded that the larger the magnitude of electron-withdrawing, the greater is the electron affinity of radical and the smaller is the gas-phase basicity of its anion. This tendency of the electron affinities and gas-phase bacisities is greater in isomers with the terminal CX than isomers with the terminal CH. Copyright © 2009 John Wiley & Sons, Ltd. [source] Using multivariate statistical methods to model the electrospray ionization response of GXG tripeptides based on multiple physicochemical parametersRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 14 2009M. A. Raji Response factors were determined for twelve GXG peptides (where G stands for glycine and X is any of alanine [A], arginine [R], asparagine [N], aspartic acid [D], glycine [G], histidine [H], leucine [L], lysine [K], phenylalanine [F], serine [S], tyrosine [Y], valine [V]) by electrospray ionization mass spectrometry (ESI-MS). The response factors were measured using a novel flow injection method. This new method is based on the Gaussian distribution of analyte concentration resulting from band-broadening dispersion experienced by the analyte upon passage through an extended volume of PEEK tubing. This method removes the need for preparing a discrete series of standard solutions to assess concentration-dependent response. Relative response factors were calculated for each peptide with reference to GGG. The observed trends in the relative response factors were correlated with several analyte physicochemical parameters, chosen based on current understanding of ion release from charged droplets during the ESI process. These include analyte properties: nonpolar surface area; polar surface area; gas-phase basicity; proton affinity; and Log D. Multivariate statistical analysis using multiple linear regression, decision tree, and support vector regression models were investigated to assess their potential for predicting ESI response based on the analyte properties. The support vector regression model was more versatile and produced the least predictive error following 12-fold cross-validation. The effect of variation in solution pH on the relative response factors is highlighted, as evidenced by the different predictive models obtained for peptide response at two pH values (pH,=,6.0 and 9.0). The relationship between physicochemical parameters and associated ionization efficiencies for GXG tripeptides is discussed based on the equilibrium partitioning model. Copyright © 2009 John Wiley & Sons, Ltd. [source] | ||||||||||||||||||||||