16999-99-8Relevant articles and documents
Gas-phase basicity measurements of dipeptides that contain valine
Gorman, Greg S.,Amster
, p. 5729 - 5735 (2007/10/02)
Gas-phase basicities of 22 dipeptides that contain valine were measured by a double bracketing method in a Fourier transform ion cyclotron resonance spectrometer. Matrix-assisted laser desorption was used to generate protonated peptide ions which were reacted with reference compounds to bracket the gas-phase basicity. In addition, neutral peptide molecules were formed by substrate-assisted laser desorption and with protonated reference ions to confirm the assignment of the gas-phase basicity. The rate of proton transfer between the protonated molecule of alanylvaline and six reference compounds was measured to examine the behavior of both exoergic and endoergic reactions. Gas-phase basicities of most of the dipeptides were found to be nearly equal to that of their most basic amino acid residue. The results are consistent with an intramolecular hydrogen bond between the N-terminus nitrogen and the amide carbonyl oxygen of a dipeptide. Furthermore, the results suggest that inductive effects cause an increase in the strength of the intramolecular hydrogen bond that the in the basicity of the C-terminus amino acid residue. Dipeptides VF and VY are more basic than their constituent amino acids. These data and molecular mechanics calculations suggest that these two peptides are stabilized by an electrostatic interaction between the N-terminal ammonium ion and the polarizable electrons of the aromatic side chain of the C-terminus.
Chloronium Ions as Alkylating Agents in the Gas-Phase Ion-Molecule Reactions with Negative Temperature Dependence
Sharma, D. K. Sen,Kebarle, P.
, p. 19 - 24 (2007/10/02)
The kinetics of the reactions Me2Cl+ + B = MeB+ + MeCl and MeEtCl+ + B = MeB+ or (EtB+) + EtCl (or MeCl) were studied with a pulsed-electron-beam, high-pressure mass spectrometer.At room temperature the rate constants were found to increase in the order B = benzene, toluene, isopropylbenzene, EtOH, Me2O, Et2O.At this point k become equal to the orbiting capture rate constant k1 ca. 10-9 molecule-1 cm3 s-.NH3 and Me3N were alkylated at orbiting capture rates.The temperature dependence of the rate constants for B = toluene, Me2O, and Et2O was examined.The rate constants were found to increase with decrease of temperature.This increase continued until the rate constants reached the magnitude of the orbiting rate constant kL.The rates remained approximately independent of temperature below this temperature.At low temperatures the collision-stabilized Me2Cl+B and MeEtCl+B could be observed.The temperature dependence of the equilibrium Me2Cl+ + toluene = (Me2Cl-toluene)+ was measured and led to the corresponding ΔHo and ΔSo.The reaction Me2Cl+ + benzene = Me-benzene+ + MeCl was found to have positive temperature dependence.On the basis of the above data it is suggested that the reactions Me2Cl+ + B = MeB+ + MeCl have an internal barrier in the potential energy of the reaction coordinate.This barrier protrudes above the energy level of the reactants (Me2Cl+ + B) for B = benzene.This leads to positive temperature dependence.For all other B, the top of the internal barrier lies below the level of the reactants and sinks lower, roughly in the order of increasing basicity of B.This lead to negative temperature dependence (toluene, isopropylbenzene, Me2O, Et2O).For B = NH3, MeNH2, Me3N, the barrier is so low that the reactions have orbiting capture rates equal to kL.Alkylation of bases B by chloronium ions like Me2Cl+ might have considerable utility in mass spectrometric analysis by chemical ionization.Ethers can be distinguished from alcohols and tertiary amines from primary and secondary amines.The alkylated ethers and the tertiary amines have no protic hydrogens and therefore do not form strongly hydrogen-bonded adducts.
Proton Affinities and Photoelectron Spectra of Three-Membered-Ring Heterocycles
Aue, Donald H.,Webb, Hugh M.,Davidson, William R.,Vidal, Mariano,Bowers, Michael T.,at al.
, p. 5151 - 5157 (2007/10/02)
Proton affinities and photoelectron spectra have been measured for azirane, phosphorane, oxirane, and thiirane and for the corresponding dimethylamine, phosphine, ether, and sulphide.The photoelectron spectra have been fully assigned by the use of ab initio STO-431G calculations and structural correlations within these series of molecules.The lone-pair ionization potentials of the heterocycles are higher than those of their open-chain dimethyl analogues because of charge redistribution effects in the C-X bonds and increased lone-pair's character in azirane and phosphirane.The proton affinities are lower in the heterocycles than in their dimethyl analogues as a result of increases in lone-pair's character and, especially for phosphirane, an increase in the RXR angle strain on protonation.Ab initio calculations on the protonated heterocycles and XHn models are presented to help interpret the proton-affinity data.
