Scott Gronert

Estimation of effective ion temperatures in a quadrupole ion trap

A modified Finnigan LCQ quadrupole ion trap has been used to determine the equilibrium constant of the complexation reaction of thiophenolate with 2,2,2-trifluoroethanol. The process is particularly useful as a thermometer reaction because it has an exceptionally large temperature dependence. Using literature values for the thermochemistry, an effective ion temperature of 310 ± 20 K is indicated for the ion trap. This value is much lower than some earlier estimates for ion traps, but is consistent with a recent theoretical analysis and some previous interpretations of experimental data. The results suggest that quadrupole ion traps are suitable for studying gas phase reactions under nearly thermal conditions.

Gas phase acidities of the α amino acids

Abstract The gas phase acidities of 19 α amino acids have been determined using the kinetic method of Cooks and co-workers [R.G. Cooks and T.L. Kruger, J. Am. Chem. Soc., 99 (1977) 1279; S.A. McLuckey, D. Cameron and R.G. Cooks, J. Am. Chem. Soc., 103 (1981) 1313]. The range of gas phase acidities for this class of biomolecules spans from glycine, the least acidic (ΔG°acid = 1402.0 kJ mol−1), to histidine, the most acidic (ΔG°acid = 1356.0 kJ mol−1). For a few, simple amino acids (glycine, alanine, serine, and cysteine), ab initio theory at the HF/6-31 + G*//HF/3-21 (+)G* level was used to calculate gas phase acidities. There is good agreement between the theoretical and experimental acidities with the average deviation being ±8.5 kJ mol−1. Fully optimized structures are reported for these amino acids and their corresponding carboxylates.

The mechanism of C-terminal fragmentations in alkali metal ion complexes of peptides

Abstract A combination of mass spectrometry and ab initio calculations (MP2/6-31+G(d)//HF/6-31+G(d)) has been used to study the mechanism of C-terminal residue cleavage in gas phase peptide/alkali metal ion complexes. Although previous workers had suggested a mechanism relying on a concerted cleavage of an oxazolidin-5-one intermediate, the present calculations indicate that this pathway has a high barrier and is not competitive. Instead, it appears that the mechanism involves a rearrangement to an anhydride intermediate that fragments to give the observed products. The computational data indicates that this mechanism has a much lower activation energy than a concerted pathway and should be viable. Moreover, compelling evidence for the mechanism is found in experiments involving the lithium complexes of dipeptides. In the proposed mechanism, the two amino acids of a dipeptide are in equivalent positions in the anhydride intermediate (i.e., sequence information is lost) and therefore, fragmentation of either sequence of a dipeptide should give the same result. This was confirmed for eight pairs of dipeptides by collision-induced dissociation (CID) of their lithium complexes in a quadrupole ion trap mass spectrometer. Although the CID spectra are not identical, the yields of the products that would pass through the anhydride intermediate are nearly equivalent, independent of the original sequence. Finally, additional computational work shows that the mechanism does not rely on the presence of a metal and is also viable as a charge-remote fragmentation pathway.

Determining the gas-phase properties and reactivities of multiply charged ions

Approaches for analyzing kinetic and thermochemical data from the reactions of multiply charged ions are presented. A method for estimating the electrostatic repulsion in a multiply charged ion is described followed by examples of the potential energy surfaces for two representative reactions of multiply charged ions, proton transfer and nucleophilic substitution (S(N)2). The effect of electrostatic repulsion on reaction barriers is discussed and approaches for extracting thermochemical data from kinetic results are described. For reactions with small intrinsic barriers (i.e. proton transfer), considerable internal electrostatic repulsion is released at the transition state and multiply charged ions exhibit much greater reactivity than singly charged analogs. In contrast, relatively little electrostatic repulsion is released at the transition states of reactions with large intrinsic barriers (i.e. S(N)2) and multiply charged ions with moderate charge separations (>10 A) can exhibit reactivity that is very similar to that of singly charged analogs.

THE GAS-PHASE CONFORMATIONS OF VALINE : AN AB INITIO STUDY

Abstract A survey of 108 possible gas-phase conformations of valine led to 26 unique conformers at the HF/6-31G ∗ level. Relative energies are reported at the MP2/6-31+G ∗ //HF/6-31G ∗ level with corrections for zero-point vibrational energies. The results indicate that the preferred orientation of the isopropyl group depends on the interaction between the carboxyl and the amino group. In comparison to alanine, it does not appear that the larger size of the side chain significantly biases the conformational preferences.

Enantioselective gas-phase ion–molecule reactions in a quadrupole ion trap

Abstract Ion trap mass spectrometry (ITMS) was employed to study chiral guest-exchange reactions in gas-phase complexes of amino acids and β-cyclodextrin. Enantioselectivity was obtained with a selected group of amino acids containing alkyl side chains. The selectivity for the amino acids increased with the size of the side chain except for Phe where the selectivity dropped off considerably. The results were consistent with those obtained earlier with Fourier-transform mass spectrometry (FTMS). The two mass analyzers were compared with regard to enantiomeric analysis.

Gas phase reactions of trimethyl borate with phosphates and their non-covalent complexes

Using a quadrupole ion trap mass spectrometer, trimethyl borate was allowed to react with dihydrogen phosphate, deprotonated O-phosphoserine, and a set of hydrogen bonded complexes involving dihydrogen phosphate and neutral acids (phosphoric acid, acetic acid, serine, and O-phosphoserine). The reactions show a consistent pattern in which the initial attack leads to addition with the loss of one or two CH3OH molecules. Collision-activated dissociation (CAD) experiments on the reaction products generally lead to the loss of an additional CH3OH molecule. In no case is a partner from the original hydrogen-bonded complex lost. The results indicate that the reactions lead to structures where the phosphate and its complex partner are covalently bound to the boron. For each of the reactions, rate constants were determined. In the course of CAD experiments (up to MS5), several novel borophosphate structures were identified. The work is supported by ab initio calculations on selected species.

COULOMB REPULSION IN MULTIPLY CHARGED IONS : A COMPUTATIONAL STUDY OF THE EFFECTIVE DIELECTRIC CONSTANTS OF ORGANIC SPACER GROUPS

Abstract Using dication and dianion models, effective dielectric constants have been calculated for several organic spacer groups (alkyl, poly-ether, and poly-ketone). Specifically, ab initio calculations at the MP2/6-31+G( d , p )//HF/6−31+G( d ) level were used to estimate the Δ H acid and the proton affinity (PA) values of a series of diammonium and dialkoxide ions, respectively. The variation in these values as a function of the distance between charges leads to the effective dielectric constant (e) of the organic spacer group. For all the spacer groups, effective dielectric constants near unity were obtained indicating that the spacer group does not reduce the Coulomb repulsion in the doubly charged ion (relative to a vacuum). The values obtained in this study are all slightly less than unity (∼0.85) because the definition of the charge separation as the distance between the formal charge centers (N or O) neglects delocalization and overestimates the true charge separation. For similar reasons, e values slightly less than unity are also appropriate for zwitterionic systems.

The need for additional diffuse functions in calculations on small anions: the G2(DD) approach

Abstract It is demonstrated that standard basis sets including a single set of diffuse functions are not adequate for calculations on small anions. A new set of diffuse sp-functions is presented and in combination with a simple additivity scheme, they provide very accurate ΔH acid values for the first- and second-row non-metal hydrides. A mean absolute deviation of ca. 2 kJ/mol is observed with the greatest error being 4 kJ/mol.

The gas phase acid/base properties of 1,3,-dimethyluracil, 1-methyl-2-pyridone, and 1-methyl-4-pyridone: relevance to the mechanism of orotidine-5′-monophosphate decarboxylase

Abstract A combination of experimental and theoretical approaches have been used to probe the gas phase acidity and basicity of 1,3-dimethyluracil, 1-methyl-2-pyridone, and 1-methyl-4-pyridone. For the acidity measurements, bracketing experiments were completed in an electrospray/quadrupole ion trap mass spectrometer. The conjugate bases of the title species were formed by collision activated decarboxylation of appropriate carboxylate precursors. The data indicates only a small variation in the acidities (Δ H acid = 369.9–377.0 kcal/mol) with the uracil derivative being ∼7 kcal/mol more acidic than both the pyridones. To determine the basicities of the title compounds, Cooks’ kinetic method was used and a much larger variation (19 kcal/mol) was observed in the proton affinities of the neutral species: 1,3-dimethyluracil, 213.7 ± 3.0 kcal/mol; 1-methyl-2-pyridone, 222.3 ± 2.9 kcal/mol; and 1-methyl-4-pyridone, 233.1 ± 3.0 kcal/mol. Δ H acid and proton affinity values were also computed at the MP2/6-31+G( d , p )//HF/6-31+G( d ), and B3LYP/6-31+G( d , p )//HF/6-31+G( d ) levels. There is very good agreement between the experimental values and those from both levels of theory.