Acid-base thermochemistry of gaseous oxygen and sulfur substituted amino acids (Ser, Thr, Cys, Met)

International audienceAcid-base thermochemistry of isolated amino acids containing oxygen or sulfur in their side chain (serine, threonine, cysteine and methionine) have been examined by quantum chemical computations. Density functional theory (DFT) was used, with B3LYP, B97-D and M06-2X functionals using the 6-31+G(d,p) basis set for geometry optimizations and the larger 6-311++G(3df,2p) basis set for energy computations. Composite methods CBS-QB3, G3B3, G4MP2 and G4 were applied to large sets of neutral, protonated and deprotonated conformers. Conformational analysis of these species, based on chemical approach and AMOEBA force field calculations, has been used to identify the lowest energy conformers and to estimate the population of conformers expected to be present at thermal equilibrium at 298 K. It is observed that G4, G4MP2, G3B3, CBS-QB3 composite methods and M06-2X DFT lead to similar conformer energies. Thermochemical parameters have been computed using either the most stable conformers or equilibrium populations of conformers. Comparison of experimental and theoretical proton affinities and Delta(acid)H shows that the G4 method provides the better agreement with deviations of less than 1.5 kJ mol-1. From this point of view, a set of evaluated thermochemical quantities for serine, threonine, cysteine and methionine may be proposed: PA = 912, 919, 903, 938; GB = 878, 886, 870, 899; Delta(acid)H = 1393, 1391, 1396, 1411; Delta(acid)G = 1363, 1362, 1367, 1382 kJ mol-1. This study also confirms that a non-negligible Delta(p)S° is associated with protonation of methionine and that the most acidic hydrogen of cysteine in the gas phase is that of the SH group. In several instances new conformers were identified thus suggesting a reexamination of several IRMPD spectra

Protonation Thermochemistry of Gaseous 2,2\u27-, 4,4\u27- and 2,4\u27-Bipyridines and 1,10-phenanthroline

Quantum chemical composite methods G3MP2B3, G3B3, G4MP2 and G4 have been used to calculate enthalpies of formation and gas phase basicities of the title compounds. Comparison of the results with the available experimental heats of formation values reveals correct agreement (to within ≈ 2 kJ/mol) with G3B3 and G4 methods. Systematic errors on the heats of formation of these aromatic molecules are detected when calculated using the G3MP2B3 and G4MP2 methods. Using G3B3 and G4 atomization energies, ΔfH°298 of 2,2’-bipyridine, 1, 1,10-phenanthroline, 2, 4,4’-bipyridine, 3, and 2,4’-bipyridine, 4, equal to 269, 316, 287 and 282 kJ/mol, respectively were obtained. Homodesmotic reactions confirm these ΔfH° estimates for the three isomeric bipyridines 1, 3 and 4. G3MP2B3, G3B3, G4MP2 and G4 methods lead to comparable proton affinities (PA) values for the four molecules 1−4, in particular because of error compensation in the case of G3MP2B3 and G4MP2 results. Excellent agreement is found with experimental PA values of reference nitrogen bases (within less than 1.2 kJ/mol) allowing us to safely predict PA values of 974, 999, 933 and 958 kJ/mol for 2,2’-bipyridine, 1, 1,10-phenanthroline, 2, 4,4’-bipyridine, 3, and 2,4’-bipyridine (protonated at the most basic site N(4\u27)), 4, respectively. Estimate of the corresponding gas phase basicities is also proposed after con-sideration of the entropy of hindered rotations: GB(1) = 943, GB(2) = 966, GB(3) = 900, GB(4) = 927 kJ/mol

From the mobile proton to wandering hydride ion: mechanistic aspects of gas-phase ion chemistry.

International audienceStructural characterization of molecular species by mass spectrometry supposes the knowledge of the type of ions generated and the mechanism by which they dissociate. In this context, a need for a rationalization of electrospray ionization(+)(-) mass spectra of small molecules has been recently expressed. Similarly, at the other end of the mass scale, efforts are currently made to interpret the major fragmentation processes of protonated and deprotonated peptides and their reduced forms produced in electron capture or electron transfer experiments. Most fragmentation processes of molecular and pseudo-molecular ions produced in the ion source of a mass spectrometer may be described by a combination of several key mechanistic steps: simple bond dissociation, formation of ion-neutral complex intermediates, hydrogen atom, hydride ion or proton migrations and nucleophilic attack. Selected crucial aspects of these elementary reactions, occurring inside positively charged ions, will be recalled and illustrated by examples taken in recent mass spectrometry literature. Emphasis will be given on the protonation process and its consequence in terms of structure and energetic

Heats of formation and protonation thermochemistry of gaseous benzaldehyde, tropone and quinone methides.

International audienceQuantum chemistry calculations using composite G3B3, G3MP2B3 and CBS-QB3 methods were performed for benzaldehyde, 1, tropone, 2, ortho-quinone methide, 3, para-quinone methide, 4, their protonated forms 1H(+)-4H(+) and the isomeric meta-hydroxybenzyl cation 5H(+). The G3B3 298 K heats of formation values obtained in this work are: -39, 61, 52, 39, 661, 679, 699, 680 and 733 kJ mol(-1) for 1-4, 1H(+)-5H(+), respectively. At the same level of theory, computed proton affinities are equal to 834, 916, 887 and 892 kJ mol(-1) for molecules 1-4. These results allow to correct discrepancies on the previously reported thermochemistry of molecules 2-4 and cations 2H(+)-5H(+)

Gas-phase basicities of polyfunctional molecules. Part 4: Carbonyl groups as basic sites

International audienceThis article constitutes the fourth part of a general review of the gas‐phase protonation thermochemistry of polyfunctional molecules (Part 1: Theory and methods, Mass Spectrom Rev 2007, 26:775–835, Part 2: Saturated basic sites, Mass Spectrom Rev 2012, 31:353–390, Part 3: Amino acids, Mass Spectrom Rev 2012, 31:391–435). This fourth part is devoted to carbonyl containing polyfunctional molecules. After a short reminder of the methods of determination of gas‐phase basicity and the underlying physicochemical concepts, specific examples are examined under two major chapters. In the first one, aliphatic and unsaturated (conjugated and cyclic) ketones, diketones, ketoalcohols, and ketoethers are considered. A second chapter describes the protonation energetic of gaseous acids and derivatives including diacids, diesters, diamides, anhydrides, imides, ureas, carbamates, amino acid derivatives, and peptides. Experimental data were re‐evaluated according to the presently adopted basicity scale. Structural and energetic information given by G3 and G4 quantum chemistry computations on typical systems are presented

Pierre Longevialle remembered

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