Françoise Rogalewicz

Fragmentation mechanisms of α-amino acids protonated under electrospray ionization: a collisional activation and ab initio theoretical study

Abstract The ionic complexes formed by electrospray of methanol/water solutions of all the α amino acids (AA) were studied by collisional activation in a triple quadrupole mass spectrometer. The fragmentation common to all protonated AA, except tryptophan, lysine, and arginine, is the well known sequential loss of H2O and CO yielding an immonium ion. For GlyH+ it is argued that formation of CH2NH2+ involves the most stable N-protonated form from which a proton is transferred to the hydroxy group. For the amino acids bearing a functional group on their side chain, formation of the immonium ion is in competition either with the loss of ammonia from the amino terminus or with the loss of a small molecule from the side chain. Extensive ab initio calculations at the MP2/6-31G∗ level have been carried out to determine the various fragmentation pathways of SerH+ and CysH+. These calculations are further used to validate an empirical determination of thermochemical data based on experimental heats of formation and Benson increments. Such approximate data are used to interpret the fragmentations of protonated Met, Thr, Asn, Asp, Gln, and Glu. They are in agreement with an initial protonation at the N terminus of these amino acids. On the other hand, side chain protonation is expected to occur for His, Trp, Lys, and Arg. With increasing collision energy, proton transfer to less basic sites X (X = SH, SCH3, OH, NH2 … ) can occur. All primary fragmentations start with an elongation of the C–+XH bond. This elongation may be assisted by a cyclisation stabilizing the incoming carbocation. The competitive fragmentations of each protonated amino acid are governed by a combination of enthalpic factors [bond dissociation energies (BDE) of the various C–+XH bonds and the energy of the final states associated with each HX loss] and activation barriers associated with rearrangements.

Interaction of neutral and zwitterionic glycine with Zn2+ in gas phase: ab initio and SIBFA molecular mechanics calculations

The interaction of Zn2+ with glycine (Gly) in the gas phase is studied by a combination of ab initio and molecular mechanics techniques. The structures and energetics of the various isomers of the Gly–Zn2+ complex are first established via high‐level ab initio calculations. Two low‐energy isomers are characterized: one in which the metal ion interacts with the carboxylate end of zwitterionic glycine, and another in which it chelates the amino nitrogen and the carbonyl oygen of neutral glycine. These calculations lead to the first accurate value of the gas‐phase affinity of glycine for Zn2+. Ab initio calculations were also used to evaluate the performance of various implementations of the SIBFA force field. To assess the extent of transferability of the distributed multipoles and polarizabilities used in the SIBFA computations, two approaches are followed. In the first, approach (a), these quantities are extracted from the ab initio Hartree–Fock wave functions of glycine or its zwitterion in its entirety, and for each individual Zn2+‐binding conformation. In the second, approach (b), they are assembled from the appropriate constitutive fragments, namely methylamine and formic acid for neutral glycine, and protonated methylamine and formate for the zwitterion; they undergo the appropriate vector or matrix rotation to be assembled in the conformation studied. The values of the Zn2+–glycine interaction energies are compared to those resulting from ab initio SCF and MP2 computations using both the all‐electron 6‐311+G(2d,2p) basis set and an effective core potential together with the valence CEP 4‐31G(2d) basis set. Approach (a) values closely reproduce the ab initio ones, both in terms of the total interaction energies and of the individual components. Approach (b) can provide a similar match to ab initio interaction energies as does approach (a), provided that the two constitutive Gly building blocks are considered as separate entities having mutual interactions that are computed simultaneously with those occurring with Zn2+. Thus, the supermolecule is treated as a three‐body rather than a two‐body system. These results indicate that the current implementation of the SIBFA force field should be adequate to undertake accurate studies on zinc metallopeptides.

Structures and fragmentations of zinc(II) complexes of amino acids in the gas phase. I. Electrosprayed ions which are structurally different from their liquid phase precursors

Abstract Zinc complexes of deprotonated amino acids (AA), denoted [AA − H + Zn]+, are readily formed in the gas phase by electrospray. Their fragmentations can be studied by low energy collisional activation, yielding structural information about the gaseous ions. In this article we show that such ions may not have a structure similar to that of their liquid phase precursors. The simple case of the deprotonated methanol complex [ZnOCH3]+ is first studied in detail. The precursor of this ion in the desolvation process is [(CH3OH)ZnOCH3]+. Accurate ab initio calculations show that the direct desolvation of this ion via methanol evaporation is a costly reaction, while rearrangement via β − H transfer to [(CH3OH)ZnH(OCH2)]+ is much more favorable. Competitive evaporation of either methanol or methanal from the rearranged ion is also more favorable. Thus the [Zn, O, C, H3]+ ion observed corresponds to a two-ligand, hydride complex [ZnH(OCH2)]+. These computational results are fully consistent with experiments, either direct source or collision induced dissociation (CID) spectra. Analogous observations hold for the zinc complex of deprotonated glycine [Gly − H + Zn]+. Detailed computations of the various possible structures of [Gly-H+Zn]+ and its precursor are consistent with the formation of rearranged structures, which are more stable than those of the initially formed species in solution. These structures are also adequate for explaining the low energy CID spectrum of [(Gly − H)Zn]+. It is concluded that for electrosprayed ions in general, whenever the last desolvation steps are energetically costly, rearrangements may occur before evaporation of the last solvent molecule. The common wisdom that electrospray ionization is a gentle process which produces faithful images of solution phase structures is shown not to apply to certain categories of metal ions.

Structures and fragmentations of zinc(II) complexes of amino acids in the gas phase. III. Rearrangement versus desolvation in the electrospray formation of the glycine-zinc complex

Abstract The zinc complex of deprotonated glycine (Gly), denoted [Gly−H+Zn] + , is readily formed in the gas phase by electrospray ionization. Low energy collisional activation of [Gly−H+Zn] + leads to three primary fragments, resulting from the losses of CO 2 , H 2 O+CO, and CO. Previous work has shown that the first two reactions require isomerization of the glycinate to nonclassical structures before the last desolvation step, and that loss of CO can only occur from a N-deprotonated glycine complex. It is shown herein, using accurate ab initio calculations, that such a structure does not pre-exist in solution, and that it is also formed in the electrospray process, during one of the last desolvation steps. Solvent molecules participate in this mechanism as proton relays between the two functional groups of Gly. These results provide a complete picture of the fragmentation of gaseous [Gly−H+Zn] + : each of the primary fragmentations arises from a specific precursor, none of which is the parent structure formed in solution.

Complexation of glycine by atomic metal cations in the gas phase

The interaction of glycine with 15 metal cations (M+ or M2+)in the gas phase has been studied by quantum chemical calculations. Three types of complexation have been considered: (i)chelation between nitrogen and the carbonyl oxygen, (ii)attachment to the carboxyl group of neutral glycine and (iii)attachment to the carboxylate group of zwitterionic glycine. It is found that the relative energies of these structures and, therefore, the nature of the lowest energy isomer, depend dramatically upon the metal ion. In several cases, metal ion attachment to glycine results in a switch from the neutral form (the most stable form of gaseous glycine)to the zwitterion (the most stable form of glycine in solution). This occurs with doubly-charged cations and, in some cases, with monocations. Several metal properties are invoked to explain these results: metal charge, size, electron affinity and polarizability. The role of metal–ion polarizability is illustrated by the computed geometries of M(CH3OH)2n+ complexes.

Structures and fragmentations of zinc(II) complexes of amino acids in the gas phase. II. Decompositions of glycine-Zn(II) complexes

Abstract The zinc complex of glycinate [Gly–H + Zn]+ has been formed by electrospray of a glycine/ZnCl2 mixture in a 50:50 vol. water/methanol solution. In this article, the precursors and the fragments of [Gly–H + Zn]+ ions are studied by means of collisional induced decomposition (CID) experiments including H/D exchanges and accurate ab initio calculations. Two precursors were identified: [Gly + CH3OH–H + Zn]+ (A) and [Gly + Gly–H + Zn]+ (B), A being much more abundant than B. The three main fragmentations of [Gly–H + Zn]+ are loss of carbon dioxyde, loss of carbon monoxyde, and successive losses of water and carbon monoxyde. To interpret these fragmentations four structures were chosen to describe [Gly–H + Zn]+. These structures are complexes between Zn(II) and glycine deprotonated either on the carboxylic group [NH2CH2COOZn]+ (1) or on the amine function [ZnNHCH2COOH]+ (2) or isomeric forms involving ZnH+ i.e. either [NH2CHCOOZnH]+ (3) or [HZn ⋯ NHCHCOOH]+ (4) respectively. None of the fragmentations is interpretable directly from structures 1 and 2. Loss of carbon dioxyde occurs from 3, loss of carbon monoxyde from a complex CX where HOZn+ interacts with CO and NHCH2, a rearranged form of 2. Successive losses of water and carbon monoxyde can take place from 4. The non occurrence of structures 1 and 2 during the fragmentation of [Gly–H + Zn]+ ions is interpreted by isomerizations within A before evaporation of the last molecule of solvent. These isomerizations are energetically easier than the last step of desolvation.

Low energy fragmentation of protonated glycine. An ab initio theoretical study

Abstract A common fragmentation of protonated α-amino acids is the loss of 46 u corresponding to the formation of an immonium ion. From protonated glycine, the fragmentation pathways leading to the loss of 46 u were investigated by means of ab initio calculations at various levels of theory: B3LYP/6-31G∗, MP2(FC)/6-31G∗, MP2(FC)/6-311+G(2d,2p)//MP2(FC)/6-31G∗, and MP2(FC)/6-311+G(2d,2p)//MP2(FC)/6-31G∗+ZPVE(MP2(FC)/6-31G∗. Several neutral species may correspond to 46 u: formic acid (HCOOH), carbon dioxide and dihydrogen (CO2 + H2) from N-protonated glycine; dihydroxycarbene [C(OH)2] from the CO-protonated isomer; and water and carbon monoxide (H2O + CO) from the OH-protonated form. The difference in energy between the N-, CO-, and OH-protonated forms is calculated to be 0, 112, and 122 kJ mol−1 at the highest level of theory. The fragmentation of lowest critical energy is the consecutive loss of water and carbon monoxide that was the mechanism previously admitted. For ions having long lifetimes this reaction is in competition with a loss of CO. This fragmentation and the consecutive losses of H2O + CO arise through the same determining step that is the isomerization of N-protonated glycine [GlyH+(N)] into OH-protonated glycine located 153 kJ mol−1 higher than GlyH+(N). At high energy, the loss of dihydroxycarbene may occur. Its formation from N-protonated glycine requires 313 kJ mol−1. The fragmentation is preceded by an isomerization of N-protonated glycine into CO-protonated glycine. Elimination of formic acid is ruled out by the present calculations.

Structures and fragmentations of zinc(II) complexes of amino acids in the gas phase: IV. Solvent effect on the structure of electrosprayed ions

Abstract A combination of mass spectrometric and quantum chemical techniques is used to study solvent effects on the structure of desolvated [AAH+Zn] + complexes (AA: amino acid), formed in the gas phase by electrospray ionization. By studying the collision-induced fragmentations of such complexes, we show that the solvent used may have a direct impact on the structure of the gaseous ions formed. In the case of [GlyH+Zn] + , four isomers may be potentially formed, and ions extracted from methanol/water and acetonitrile/water solutions show different fragmentations patterns. The formation of different isomers when using these two solvents is further evidenced by the fragmentation spectra of [AsnH+Zn] + , and [AspH+Zn] + . These cases illustrate the fine tuning which can sometimes be exerted on the structures of electrosprayed ions. Ab initio calculations are used to show the decisive role of the solvent binding energies on the last steps of the electrospray process. It is concluded that both the solvent and the cone voltage in the electrospray source may have a strong influence on the structure of fully desolvated, gaseous ions.

Adsorption–desorption effects in ion trap mass spectrometry using in situ ionization

Quadrupole mass spectrometers were compared for the GC-MS analysis of six molecules frequently encountered in analytical toxicology: diazepam, alprazolam, triazolam, LSD (lysergic acid diethylamide), trimethylsilylated LSD and trimethylsilylated buprenorphine. Experiments performed with ion trap detectors using in situ ionization led to important chromatographic peak tailing for the most polar compounds; it was assumed to result from adsorption-desorption of neutral molecules in the mass spectrometer. This study showed that the degree of peak tailing is correlated with analyte polarity, with materials coating ion trap surfaces and with analysis temperature and that this anomaly can be greatly reduced using passivated surfaces and a high temperature of analysis.

Structures and fragmentations of electrosprayed Zn(II) complexes of carboxylic acids in the gas phase: Isomerisation versus desolvation during the last desolvation step

Abstract Zn 2+ –carboxylate ions formed in methanol/water solutions are transferred in the gas phase by electrospray. At low cone voltage, species observed in the source spectra correspond to solvated Zn 2+ –carboxylates: [RCOOZn, (CH 3 OH) n ] + (R=H, CH 3 ; n =1–3) ions. Under low energy collisions, all ions with n >1 lose exclusively methanol, mimicking some of the last steps of ion desolvation. However, ions with only one molecule of solvent behave differently: [HCOOZn, CH 3 OH] + eliminates carbon dioxide and [CH 3 COOZn, CH 3 OH] + either loses the last molecule of methanol or fragments to give the acylium ion [CH 3 CO] + . Labelling experiments as well as accurate molecular orbital calculations are used to explain this different behaviour of [RCOOZn, CH 3 OH] + ions which fragment (totally or partially) instead of losing the last molecule of solvent. It appears that the loss of the last molecule of solvent from [HCOOZn, CH 3 OH] + requires more energy than does its isomerisation into [CO 2 , HZn, CH 3 OH] + , precursor for the loss of CO 2 . For [CH 3 COOZn, CH 3 OH] + , isomerisation processes and direct loss of methanol require very similar energies. In both cases, part of the gaseous ions formed after complete desolvation are chemically different from their precursors in solution.