Hydration of potassiated amino acids in the gas phase

Abstract

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 ...

Gas-Phase Hydration Thermochemistry of Sodiated and Potassiated Nucleic Acid Bases

“…The water binding energies in the sodiated complexes are higher than in the potassiated ones (Tables 1 and 2) This observation is very similar to our previous studies [73–75] and is consistent with the dominating electrostatic interaction between the water molecule and metal ion in the NAB–M + ·(H 2 O) n = 1,2 complexes. Because the ionic radius of K + is larger than Na + , the charge density should be smaller and, therefore, the electrostatic interaction of H 2 O with NAB–K + is weaker compared with NAB–Na + .…”

“…This order can also be related to the binding strength between NAB–M + and H 2 O in the NAB–M + ·(H 2 O) systems, where water binds directly to the metal ion through oxygen [62, 64]. Such a correlation has been observed in our previous studies for the binding energies of water to cationized amino acids and monosaccharides [73–75]. As discussed above, the 1a structure of Ura–M + and Thy–M + , and 3c of Ade–M + solely or as the predominant tautomeric complexes, corresponds to the precursor ions for hydration in the present experiments.…”

Hydration energies of deprotonated amino acids from gas phase equilibria measurements

“…This study presents the thermochemical properties for the hydration of a number of gas-phase deprotonated amino acids, [AA Ϫ H] Ϫ , obtained from equilibrium experiments. The work is a continuation of previous studies on the hydration of protonated [12,13] and cationized [28,29] biomolecules in the gas phase.…”

“…It is established that side chains have a significant influence on the water binding energy to these cations [12,13,21,28,29].…”

Hydration Isomers of Protonated Phenylalanine and Derivatives: Relative Stabilities from Infrared Photodissociation

“…Hydration equilibrium, 8-12 blackbody infrared radiative dissociation (BIRD), 13-18 high pressure mass spectrometry, [19][20][21][22][23][24][25] ion beam experiments 26 have been used to measure enthalpies and/or entropies for hydration of metal cationized or protonated amino acids and peptides by a discrete number of water molecules. These studies reveal that the binding energy of the first water molecule to a protonated aliphatic or aromatic amino acid is ∼14-19 kcal/mol, and binding energies of subsequent water molecules decrease to values near the vaporization enthalpy of water (10.5 kcal/mol).…”