Hydration of the alkali ions in the gas phase. Enthalpies and entropies of reactions M+(H2O)n-1 + H2O = M+(H2O)n

The thermochemistry of stepwise hydration of several potassiated amino acids was studied by measuring the gas-phase equilibria, AAK ϩ (H 2 O) nϪ1 ϩ H 2 O ϭ AAK ϩ (H 2 O) n (AA ϭ Gly, AL, Val, Met, Pro, and Phe), using a high-pressure mass spectrometer. The AAK ϩ ions were obtained by electrospray and the equilibrium constants K nϪ1,n were measured in a pulsed reaction chamber at 10 mbar bath gas, N 2 , containing a known partial pressure of water vapor. Determination of the equilibrium constants at different temperatures was used to obtain the ϩ and Na ϩ ions are the most abundant metal cations in living systems. They play an important role in several biochemical functions such as enzyme regulation, transfer of metal ions from intracellular to extracellular environments, electrical excitability of nerves, stabilization of DNA structures and so on [1]. Therefore, the interaction between alkali metal cations and various amino acids (AAs) has attracted considerable attention, and many experimental and theoretical studies have been conducted [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. These studies show that K ϩ binds to AAs quite differently than Na ϩ . For example, in the case of aliphatic AAs such as glycine (Gly), alanine (Ala), and valine (Val), the Na ϩ ion is coordinated to nitrogen and carbonyl oxygen (NO coordination) in the lowest energy conformers, while K ϩ prefers to bind to the carbonyl and hydroxyl oxygen atoms of the charge-solvated form (OO coordination) [6,8,10,16]. In proline (Pro), phenylalanine (Phe), and cysteine (Cys), the modes of K ϩ binding are similar to those of the corresponding Na ϩ complexes [3-5, 12, 20].Molecular dynamics simulations of a bacterial potassium ion channel reveal a significant selectivity filter for K ϩ compared with Na ϩ ions [21,22]. The different hydration state of Na ϩ and K ϩ in the channel inner pore is postulated to be a basic factor determining ionic selectivity [23,24]. Investigations into the relative energetics of noncovalent interactions between single amino acids, metal ions and water molecules could provide important information to explain ion channel function at a molecular level. Despite the considerable amount of experimental and theoretical studies that have been conducted on the binding of Na ϩ and K ϩ to amino acids, little data are available on the interactions of water molecules with metal ion/amino acid complexes. Because metal ions and biomolecules are normally surrounded by water in living systems, these interactions are of significant importance in understanding biological systems. Williams and coworkers [8,[25][26][27][28][29][30][31] studied the modes of metal ion and water binding in hydrated complexes of alkali metal cationized valine, glutamine, lysine, and ␣-methyl-proline using both black-␣ ␣ body infrared radiative dissociation (BIRD) experiments and theory. Armentrout et al. investigated the sequential bond energies of water to sodiated glycine [32] and proline [33] by threshold collision-induced dissociation ...

Section: Solvation Effect and Water Binding Energiesmentioning

confidence: 75%

Hydration of potassiated amino acids in the gas phase

Wincel

2007

“…It can be seen that the apparent thresholds for these systems are about 1 and 0 eV, respectively. Equilibrium measurements of the (H 2 O)Li ϩ -OH 2 and (H 2 O) 4 Li ϩ -OH 2 bond energies obtained by Dzidic and Kebarle [53] using HPMS provide 298 K bond energies of 108 Ϯ 4 and 58 Ϯ 4 kJ/mol. These values can be converted to 0 K bond energies for comparison to our threshold measurements using molecular constants calculated by ab initio theory [54].…”

Section: Internal Energy Of the Reactants: Vibrationalmentioning

confidence: 97%

“…One means to determine whether vibrational energy must be included in threshold modeling is to examine systems in which the thermochemistry is already known. An example of such a system is lithium ion water clusters, Li ϩ (H 2 O) x , which have been studied by both CID [52] and high pressure mass spectrometry (HPMS) [53]. Our CID results for the x ϭ 2 and 5 clusters are shown in Figure 7.…”

Section: Internal Energy Of the Reactants: Vibrationalmentioning

confidence: 99%

Mass spectrometry—Not just a structural tool: The use of guided ion beam tandem mass spectrometry to determine thermochemistry

Armentrout

2002

175

180

Guided ion beam tandem mass spectrometry has proved to be a robust tool for the measurement of thermodynamic information. Over the past twenty years, we have elucidated a number of factors necessary to make such thermochemistry accurate. Careful attention must be paid to the reduction of the raw data, ion intensities versus laboratory ion energies, to a more useful form, reaction cross sections versus relative kinetic energy. Analysis of the kinetic energy dependence of cross sections for endothermic reactions can then reveal thermodynamic data for both bimolecular and collision-induced dissociation (CID) processes. Such analyses need to include consideration of the explicit kinetic and internal energy distributions of the reactants, the effects of multiple collisions, the identity of the collision partner in CID processes, the kinetics of the reaction being studied, and competition between parallel reactions. This work provides examples illustrating the need to consider this multitude of effects along with details of the procedures developed in our group for handling each of them. ( W hen does a mass spectrometer become an ion beam instrument? Is this defined by the instrument, by whether the operator is an analytical or physical chemist, or by the experiments done? All mass spectrometers utilize ion beams, but the unspoken definition of an ion beam instrument is an apparatus that can be used to make quantitative physical measurements. In this regard, the capabilities of the instrumentation and the intent and training of the operator are all important factors. Whereas a mass spectrometer is often used as an identification tool through the use of mass spectral patterns, by taking advantage of the ability to easily change the kinetic energy of ions, it can also be a powerful instrument for the determination of thermodynamic data.In this article, I lay down some of the founding principles behind our use of tandem mass spectrometry, and in particular, so-called guided ion beam mass spectrometry to elucidate thermochemical information. This is achieved primarily by examining the kinetic energy dependence of endothermic ion-molecule reactions and determining their energy thresholds.Although many types of instruments have been used to perform such measurements, the link between such studies and guided ion beam techniques is a strong one. Indeed, the NIST Webbook [1] states in its section on Determinations of Reaction Endothermicity that "more commonly a so-called guided-beam apparatus is utilized for such determinations." Considering that there are only about a dozen such laboratories worldwide, this is a testament to the ability of this particular kind of apparatus to provide a plethora of high quality information. However, such information requires attention to myriad details, which are outlined in this article.Two fundamental types of reactions can be used to acquire thermodynamic information: Bimolecular exchange reactions, process 1, and collision-induced dissociation (CID), process 2.Reactions 1 can be either ex...

“…The sixth water binding energies of M(H 2 O) 6 ϩ , M ϭ Li, Na, and K, are 12.1, 10.7, and 10.0 kcal/mol, respectively [58]. Interestingly, the fourth water binding energies of M(H 2 O) 4 ϩ for M ϭ Na, Mg, and Al, are 13.1, 11.5, and 12.5 kcal/mol [60].…”

Section: Internal Energy Depositionmentioning

confidence: 99%

Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

Leib

Donald

Bush

et al. 2007

118

Hydrated divalent magnesium and calcium clusters are used as nanocalorimeters to measure the internal energy deposited into size-selected clusters upon capture of a thermally generated electron. The infrared radiation emitted from the cell and vacuum chamber surfaces as well as from the heated cathode results in some activation of these clusters, but this activation is minimal. No measurable excitation due to inelastic collisions occurs with the low-energy electrons used under these conditions. Two different dissociation pathways are observed for the divalent clusters that capture an electron: loss of water molecules (Pathway I) and loss of an H atom and water molecules (Pathway II). For Ca(H 2 O) n 2ϩ , Pathway I occurs exclusively for n Ն 30 whereas Pathway II occurs exclusively for n Յ 22 with a sharp transition in the branching ratio for these two processes that occurs for n Ϸ 24. The number of water molecules lost by both pathways increases with increasing cluster size reaching a broad maximum between n ϭ 23 and 32, and then decreases for larger clusters. From the number of water molecules that are lost from the reduced cluster, the average and maximum possible internal energy is determined to be ϳ4.4 and 5.2 eV, respectively, for Ca(. This value is approximately the same as the calculated ionization energies of M(H 2 O) n ϩ , M ϭ Mg and Ca, for large n indicating that the vast majority of the recombination energy is partitioned into internal modes of the ion and that the dissociation of these ions is statistical. For smaller clusters, estimates of the dissociation energies for the loss of H and of water molecules are obtained from theory. For Mg(H 2 O) n 2ϩ , n ϭ 4 -6, the average internal energy deposition is estimated to be 4.2-4.6 eV. The maximum possible energy deposited into the n ϭ 5 cluster is Ͻ7.1 eV, which is significantly less than the calculated recombination energy for this cluster. There does not appear to be a significant trend in the internal energy deposition with cluster size whereas the recombination energy is calculated to increase significantly for clusters with fewer than 10 water molecules. These, and other results, indicate that the dissociation of these smaller clusters is nonergodic. . The effectiveness of the bottom-up method for complex samples can be enhanced by using multidimensional separations. Clemmer and coworkers elegantly demonstrated that combining on-line liquid chromatography (LC) with ion mobility spectrometry and MS can greatly improve separations without increasing analysis times over LC/MS alone [2][3][4]. In contrast, the "top-down" approach to protein characterization has the advantage that de novo sequencing, including the identification and structural localization of labile posttranslational modifications, can be done directly on protein mixtures without proteolysis [5,6]. This top-down approach has greatly benefited from the development of electron capture dissociation (ECD), a method pioneered by McLafferty and coworkers [6][7][8][9]. In a typical ECD experim...