Proton Dissociation Energies Research Articles

Hierarchically Macro–Microporous Covalent Organic Frameworks for Efficient Proton Conduction

AbstractAssembling molecular proton carriers into crosslinked networks is widely used to fabricate proton conductors, but they often suffer losses in conduction efficiency and stability accompanied by unclear causes. Covalent organic frameworks (COFs), with well‐defined crystal frameworks and excellent stability, offer a platform for exploring the proton transfer process. Herein, a strategy to construct proton conductors that induce conductivity and stability by introducing bottom‐up hierarchical structure, mass transport interfaces, and host–guest interactions into the COFs is proposed. The proton‐transport platforms are designed to possess hierarchically macro–microporous structure for proton storage and mass transport. The protic ionic liquids, with low proton dissociation energies investigated by DFT calculation, are installed at open channel walls for faster proton motion. As expected, the resultant proton conductors based on a covalent organic framework (PIL0.5@m‐TpPa‐SO3H) with hierarchical pores increase conductivity by approximately three orders of magnitude, achieving the value of 1.02 × 10−1 S cm−1 (90 °C, 100% RH), and maintain excellent stability. In addition, molecular dynamics simulations reveal the mechanism of “hydrogen‐bond network” for proton conduction. This work offers a fresh perspective on COF‐based material manufacturing for high‐performance proton conductors via a protocol of macro‐micropores.

Dynamics and Proton Conduction of Heterogeneously Confined Imidazole in Porous Coordination Polymers

AbstractThe nanoconfinement of proton carrier molecules may contribute to the lowing of their proton dissociation energy. However, the free proton transportation does not occur as easily as in liquid due to the restricted molecular motion from surface attraction. To resolve the puzzle, herein, imidazole is confined in the channels of porous coordination polymers with tunable geometries, and their electric/structural relaxations are quantified. Imidazole confined in a square‐shape channels exhibits dynamics heterogeneity of core‐shell‐cylinder model. The core and shell layer possess faster and slower structural dynamics, respectively, when compared to the bulk imidazole. The dimensions and geometry of the nanochannels play an important role in both the shielding of the blocking effect from attractive surfaces and the frustration filling of the internal proton carrier molecules, ultimately contributing to the fast dynamics and enhanced proton conductivity.

Dynamics and Proton Conduction of Heterogeneously Confined Imidazole in Porous Coordination Polymers.

The nanoconfinement of proton carrier molecules may contribute to the lowing of their proton dissociation energy. However, the free proton transportation does not occur as easily as in liquid due to the restricted molecular motion from surface attraction. To resolve the puzzle, herein, imidazole is confined in the channels of porous coordination polymers with tunable geometries, and their electric/structural relaxations are quantified. Imidazole confined in a square-shape channels exhibits dynamics heterogeneity of core-shell-cylinder model. The core and shell layer possess faster and slower structural dynamics, respectively, when compared to the bulk imidazole. The dimensions and geometry of the nanochannels play an important role in both the shielding of the blocking effect from attractive surfaces and the frustration filling of the internal proton carrier molecules, ultimately contributing to the fast dynamics and enhanced proton conductivity.

Zwitterionic Covalent Organic Frameworks: Attractive Porous Host for Gas Separation and Anhydrous Proton Conduction

Ionic covalent organic frameworks (COFs) consisting of an anionic or cationic skeleton and corresponding counterions have demonstrated great potential in many application fields such as ion conduction, molecular separation, and catalysis. However, arranging anionic and cationic groups into the same COF to form zwitterionic materials is still unexplored. Herein we design the synthesis of three zwitterionic COFs as attractive porous hosts for SO2/CO2 separation and anhydrous proton conduction. The separated cationic and anionic groups in zwitterionic COFs' channels can act as two different polar sites for SO2 adsorption, allowing zwitterionic COFs to achieve a high SO2 adsorption capacity (216 mL/g, 298 K) and impressive SO2/CO2 selectivity (118, 298 K). Furthermore, after loading with triazole/imidazole, the zwitterionic groups in COFs' channels can induce complete proton carrier deprotonation, producing more freely migrating protons. The free protons migrate along a continuous hydrogen-bonding network in zwitterionic COFs' channels, leading to outstanding anhydrous proton conductivity up to 4.38 × 10-2 S/cm, which is much higher than other N-heterocyclic-doped porous materials under anhydrous conditions. Proton dissociation energy calculations combined with frequency-dependent dielectric analysis give insight into the role of zwitterionic COFs for proton conductivity. Our work provides the possibility to design well-defined zwitterionic frameworks for gas separation and ion conduction.

In Situ-Doped Superacid in the Covalent Triazine Framework Membrane for Anhydrous Proton Conduction in a Wide Temperature Range from Subzero to Elevated Temperature.

Synthesis of solid-state proton-conducting membranes with low activation energy and high proton conductivity under anhydrous conditions is a great challenge. Here, we show a simple and convenient way to prepare covalent triazine framework membranes (CTF-Mx) with acid in situ doping for anhydrous proton conduction in a wide temperature range from subzero to elevated temperature (160 °C). The low proton dissociation energy and continuous hydrogen bond network in CTF-Mx make the membrane achieve high proton conductivity from 1.21×10-3 S cm-1 (-40 °C) to 2.08×10-2 S cm-1 (160 °C) under anhydrous conditions. Molecular dynamics and proton relaxation time analyses reveal proton hopping at low activation energies with greatly enhanced mobility in the CTF membranes.

Tuning proton dissociation energy in proton carrier doped 2D covalent organic frameworks for anhydrous proton conduction at elevated temperature

The proton dissociation degrees and dielectric properties of proton carriers doped COFs with neutral, polar, Lewis base and positively charged sites are investigated to get better understanding of structure–conductivity relationship.

(H2O)Zn(Imidazole)n]2+: the vital roles of coordination number and geometry in Zn–OH2acidity and catalytic hydrolysis

The Zn(ii)-(Imidazole(ate))n coordination motif occurs in numerous biochemical systems, including carbonic anhydrase and the matrix metalloproteinases (MMPs). Additionally, it has been used in synthetic materials, such as the zinc-based zeolitic imidazolate framework (ZIF) structures. Zinc centers in these systems typically act as Lewis acids that form complexes with small molecules, such as H2O, which is activated catalytically toward a number of important and useful hydrolysis reactions. The results reported herein from density functional theory (M05-2X) and ab initio (MP2 and CCSD(T)) calculations demonstrate that both the coordination number and the molecular geometry have a sizable impact on the binding strength, deprotonation energy, and acidity of the Zn(ii) coordinated water. Through a series of quantum mechanical calculations on [(ImH)nZn-OH2]2+ complexes (n = 1-5), both the solution-phase pKa and the gas-phase proton dissociation energy significantly increase as n increases. While this should not be too surprising, the Zn-OH2 bond dissociation energies and bond lengths don't necessarily undergo a concurrent decrease, and therefore would be of limited use as a prediction tool regarding Zn-OH2 acidity. In an effort to dissect the impacts of coordination number and molecular geometry on these thermodynamic parameters, we performed constrained geometry optimizations on the three- (n = 2) and four-coordinate (n = 3) complexes. These calculations surprisingly reveal a marked impact on the pKa and proton dissociation energy of the coordinated water, upon exclusive changes in the Zn(ii) coordination geometry, whether in the gas-phase or in aqueous solution. We discuss the relevance of these results to the catalytic peptide hydrolysis mechanism of the MMPs and possible implications for catalytic activity within or on the surfaces of ZIFs.

Theoretical Investigations on Proton Conductance and Degradation Mechanisms of Nafion Membrane in Fuel Cells

In this study, the low-humidity proton conductance and degradation mechanisms are theoretically investigated for Nafion®, which is the most frequently used proton exchange membrane. The proton conductance mechanism of Nafion membrane was first explored to reveal the cause for its high proton conductance under low-humidity conditions (Ref. 1). Nafion membrane gets an advantage in proton exchange membranes in fuel cells mainly due to its high low-humidity proton conductance. The actual proton conductance of Nafion at low humidity was explored by comparing to that of hydrocarbon multiblock sulfonated poly(arylene ether sulfone)s (SPE) membrane. Using long-range corrected density functional theory (LC-DFT) that one of the authors (TT) developed (Ref. 2), we calculated the proton dissociation energies from the sulfonic acid groups and investigated the subsequent proton conductance mechanism at low humidity. As a result, we found that these membranes have negligible difference in the proton dissociation ability inside the hydrated water clusters but show a remarkable difference in the conductivity of protonated water clusters in disconnected hydrogen bond networks. Note that low-humidity membranes are known to contain the regions of disconnected hydrogen bond networks (Ref. 3), in which Grotthuss mechanism assuming connected hydrogen bond networks does not proceed. Though vehicle mechanism assuming the diffusion of protonated water molecules (or clusters) seems to explain the proton conductance in the disconnected networks, we found that protonated water molecules and clusters need very large dissociation energies (about 400 kJ/mol). As an alternative, we suggested the relay mechanism (Fig. 1), in which protonated water clusters are relayed between the sulfonic acid groups through the doubly-hydrated structures. The calculated results showed that protonated water clusters require only about 100kJ/mol to dissociate in this relay mechanism. The vibrational spectra of hydrated Nafion membrane was also investigated to reveal the dependence of the proton conductance on humidity in further detail by a collaborative experimental and theoretical infrared (IR) spectroscopy study (Ref. 4). The experimental time-resolved attenuated total reflection Fourier transform IR spectroscopic results showed a unique IR peak intensity dependence on the hydration number. Based on the experimental results, we theoretically explored the dependence of the vibrational spectra of Nafion membrane on the hydration number by using LC-DFT. As a result, we found that this unique IR peak intensity dependence is correctly reproduced for the singly-hydrated but for the doubly-hydrated Nafion, in which protons are initially detached from the sulfonic acid groups in the geometry optimizations. This strongly supports the above-mentioned relay mechanism of the proton conductance. The degradation mechanism of Nafion membrane was also explored focusing on the H2O2-induced dissociation of the ether-linkages of the side chains. Many conventional studies on the degradation of membranes in fuel cells have assumed the principal mode of the degradation involves OH radicals. However, recent experimental studies have raised questions concerning this assumption. For example, a Fenton test, which is supposed to provide OH radicals, have revealed that the ratio of the degradation fragment molecules is considerably different from that of actual operating conditions (Ref. 5). Using LC-DFT, we found an H2O2-induced degradation mechanism, which is consistent with experimental results. In this mechanism, the ether-linkage of the side chains of Nafion is decomposed with a relatively low activation energy to produce (H2O)lHO3S-CF2-CF2-O-OH (l is the hydration number) as a key fragment. We also elucidated the subsequent decomposition reaction mechanisms, though these mechanism need metal ions to proceed. The vibrational spectra for the decomposed Nafion are found to be consistent with the experimental IR spectra. This mechanism is supposed to proceed together with OH radical-induced degradation mechanism. This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cells (SPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. T. Tsuneda, R. K. Singh, K. Miyatake, and M. Watanabe, Chem. Phys. Lett., 608, 11 (2014).H. Iikura, T. Tsuneda, T. Yanai, and K. Hirao, J. Chem. Phys., 115, 3540 (2001).G. Gebel, Polymer, 41, 5829 (2000); Macromolecules, 46, 6057 (2013).R. K. Singh, K. Kunimatsu, K. Miyatake, and T. Tsuneda, Macromolecules, 49, 6621 (2016).T. Kinumoto, M. Inaba, Y. Nakayama, K. Ogata, R. Umebayashi, A. Tasaka, Y. Iriyama, T. Abe, and Z. Ogumi, J. Pow. Sour., 158, 1222 (2006). Figure 1. Schematic diagram for the proton conductance in Nafion membrane in fuel cells. Figure 1

Theoretical investigation of proton-transfer in different membranes for PEMFC applications in low humidity conditions

A comparative theoretical study was performed to explore the effect of polymer backbone on the proton exchange in different sulfonated membranes such as poly(phenylene oxide), poly(benzimidazole), polyimide, poly(oxadiazole) and Nafion 117 at low hydration state. Geometry optimization of the membranes in the presence of different numbers of water molecules (1–3 water molecules) revealed that proton dissociation energies and hydrogen bond interactions (which are of critical importance in proton transfer) depend strongly on the polymer structure’s functional groups. When the polymer dose not stabilize hydronium ion or sulfonate anion, water molecules are arranged around sulfonate group and form a two-ring hydrogen bonded structure; in addition, this polymer showed high proton transfer energy (25.0, 27.2 and 19.4kcal/mol for sulfonated poly(phenylene oxide), sulfonated meta-poly(benzimidazole) and Nafion 117). Theoretical studies also demonstrated that the position of sulfonate group and functional groups in the polymer structure was effective in proton dissociation. In this regard, proton transfer energy for para-sulfonated poly(benzimidazole) was about 17.7kcal/mol less than sulfonated meta-poly(benzimidazole); this energy difference could be attributed to the imidazolium cation formation and its stabilizing effect on sulfonate anion. The molecular graphs for sulfonated poly(oxadiazole) indicated that, although this polymer had the highest proton transfer energy among the polymers, it formed a continuous and strong hydrogen bond chain that facilitated proton transfer by hopping mechanism.

Methanethiol Binding Strengths and Deprotonation Energies in Zn(II)-Imidazole Complexes from M05-2X and MP2 Theories: Coordination Number and Geometry Influences Relevant to Zinc Enzymes.

Zn(II) is used in nature as a biocatalyst in hundreds of enzymes, and the structure and dynamics of its catalytic activity are subjects of considerable interest. Many of the Zn(II)-based enzymes are classified as hydrolytic enzymes, in which the Lewis acidic Zn(II) center facilitates proton transfer(s) to a Lewis base, from proton donors such as water or thiol. This report presents the results of a quantum computational study quantifying the dynamic relationship between the zinc coordination number (CN), its coordination geometry, and the thermodynamic driving force behind these proton transfers originating from a charge-neutral methylthiol ligand. Specifically, density functional theory (DFT) and second-order perturbation theory (MP2) calculations have been performed on a series of [(imidazole)nZn-S(H)CH3](2+) and [(imidazole)nZn-SCH3](+) complexes with the CN varied from 1 to 6, n = 0-5. As the number of imidazole ligands coordinated to zinc increases, the S-H proton dissociation energy also increases, (i.e., -S(H)CH3 becomes less acidic), and the Zn-S bond energy decreases. Furthermore, at a constant CN, the S-H proton dissociation energy decreases as the S-Zn-(ImH)n angles increase about their equilibrium position. The zinc-coordinated thiol can become more or less acidic depending upon the position of the coordinated imidazole ligands. The bonding and thermodynamic relationships discussed may apply to larger systems that utilize the [(His)3Zn(II)-L] complex as the catalytic site, including carbonic anhydrase, carboxypeptidase, β-lactamase, the tumor necrosis factor-α-converting enzyme, and the matrix metalloproteinases.

QSAR and mechanisms of radical scavenging activity of phenolic and anilinic compounds using structural, electronic, kinetic, and thermodynamic parameters

QSAR study was performed on the DPPH radical scavenging activity of a wide range (51 compounds) phenolic and anilinic compounds; many of them are widely distributed in biological systems. The structural diversity includes phenols (1-3 OH groups) and anilines (1-2 NH2 groups) with many electron donating and withdrawing groups, aminophenols (o, m, p), phenolic acids (o, m, p) or esters, enols, and phenolic terpenoids. Independents used were electronic Brown parameter and the structural parameters i.e., the indicator variable Q for forming a stable quinone-like product and the number of alkoxy (RO) and alkyl (R) groups as well as the thermodynamic parameters, bond dissociation energy (BDE), proton dissociation energy of O–H or N–H bond and HOMO of the parent compounds, and the resulted radicals or radical cations. Radical scavenging activity gave fair correlation with electronic Brown and the structural Q parameters, while it gave strong correlation with the structural parameters (R, RO, and Q) and BDE (R 2 = 0.979, N = 49, SE = 5.94, p < 0.001). Hydrogen-atom transfer and single-electron-transfer mechanisms were discussed in accordance with the resulted correlations. Correlating the rate constant (ks) with the same independents retained only the structural parameters; the weighting value of each parameter confirms that forming semiquinone radicals has the highest impact in stabilizing transition state and increasing the reaction rates followed by the presence of the electron donating groups alkoxy and alkyl groups.

Gas Phase Conformations of Selenocysteine and Related Ions: A Comprehensive Theoretical Study

Extensive ab initio molecular calculations have been first performed to thoroughly characterize the gas-phase potential energy surfaces (PES) of the 21th amino acid selenocysteine and related ions (neutral, protonated and deprotonated). A wide range of trial structures generated by considering the combinations of all internal single-bond rotamers was surveyed at the BHandHLYP/6-31G(d) level, and then refined at the BHandHLYP/6-311++G(d,p) level. A total of 76, 23, 38, and 3 unique stable conformers respectively for neutral, protonated, deprotonated, and doubly deprotonated selenocysteine is identified, and neutral zwitterionic forms are found to be as local minima on the gas-phase PES. The properties of the low energy conformers, such as relative energies, dipole moments, rotational constants, and intramolecular hydrogen bonds, were determined and analyzed. The thermochemical properties of proton affinity (PA), gas-phase basicity (GB), proton dissociation energy (PDE), gas-phase acidity (GA), and the vertical ionization energies (VIEs) were computed by the theoretical approaches of BHandHLYP, B3LYP, MP2, and CCSD(T). Moreover, the conformational equilibrium effect (CEE) on thermochemical properties was analyzed. The statistical simulation predicts that the CEE generally yields a physical correction on about a 1 kBT scale in GA/GB calculations for multi-conformer systems.

Structures of protonated methanol clusters and temperature effects

The accurate evaluation of pKa's, or solvation energies of the proton in methanol at a given temperature is subject to the determination of the most favored structures of various isomers of protonated (H(+)(MeOH)n) and neutral ((MeOH)n) methanol clusters in the gas phase and in methanol at that temperature. Solvation energies of the proton in a given medium, at a given temperature may help in the determination of proton affinities and proton dissociation energies related to the deprotonation process in that medium and at that temperature. pKa's are related to numerous properties of drugs. In this work, we were interested in the determination of the most favored structures of various isomers of protonated methanol clusters in the gas phase and in methanol, at a given temperature. For this aim, the M062X/6-31++G(d,p) and B3LYP/6-31++G(d,p) levels of theory were used to perform geometries optimizations and frequency calculations on various isomers of (H(+)(MeOH)n) in both phases. Thermal effects were retrieved using our homemade FORTRAN code. Thus, we accessed the relative populations of various isomers of protonated methanol clusters, in both phases for temperatures ranging from 0 to 400 K. As results, in the gas phase, linear structures are entropically more favorable at high temperatures, while more compact ones are energetically more favorable at lower temperatures. The trend is somewhat different when bulk effects are taken into account. At high temperatures, the linear structure only dominates the population for n ≤ 6, while it is dominated by the cyclic structure for larger cluster sizes. At lower temperatures, compact structures still dominate the population, but with an order different from the one established in the gas phase. Hence, temperature effects dominate solvent effects in small cluster sizes (n ≤ 6), while the reverse trend is noted for larger cluster sizes.

Local Structures and Chemical Properties of Deprotonated Arginine

The potential energy surface of gaseous deprotonated arginine has been systematically investigated by first principles calculations. At the B3LYP/6-31G(d) level, apart from the identification of several stable local structures, a new global minimum is located which is about 6.56 kJ/mol more stable than what has been reported. The deprotonated arginine molecule has two distinct forms with the deprotonation at the carboxylate group (COO−). These two forms are bridged by a very high energy barrier and possess very different IR spectral profiles. Our calculated proton dissociation energy and gas-phase acidity of arginine molecule are found to be in good agreement with the corresponding experimental results. The predicted geometries, dipole moments, rotational constants, vertical ionization energies and IR spectra of low energy conformers will be useful for future experimental measurements.

Proton Transfer Reactions for Methanol and Water Containing Manganese Ion Complexes

Under considerations in the current study are reactions of the type [Mn(LOH)(2)](2+) → [Mn(LO)](+) + LOH(2)(+), where the ligand LOH represents water or/and methanol. Preferential proton transfer reactions and loss of any ligand fragments are discussed in the light of ligand polarizability, dipole moment, dissociation energy, proton affinity, differences in ligand-ion ionization energy, and ion radii. The results indicate the proton affinity and dissociation energy of the O-H bond are more important for the overall proton transfer reaction than differences in the first ionization energy of the ligand and the second ionization energy of the metal ion.

Comprehensive ab initio study on the conformations of L-Threonine and L-allo-Threonine and related species in gas phase

Systematic conformational searches have been performed for various species of L-Threonine (L-Thr) and L-allo-Threonine (L-allo-Thr) in gas phase. The trial structures were generated by allowing for all combinations of internal single-bond rotamers. The trial structures were optimized at the HF/3-21G(d) level and then subjected to further optimization at the B3LYP/6-311++G(d, p) level. A total of 74 canonical neutral, 12 protonated, 22 deprotonated and 4 secondary deprotonated conformers are found for the L-Thr species, while 75 canonical neutral, 14 protonated, 22 deprotonated, and 6 secondary deprotonated conformers are found for the L-allo-Thr species. However, no stable zwitterionic structures of isolated threonine may exist. The conformational energies are determined by the MP2/6-311+G(2df, p) calculations. The intramolecular hydrogen bonds were characterized. The equilibrium conformer distributions at different temperatures are shown. The proton affinities, gas phase basicities, proton dissociation energies and gas phase acidities are determined and compared with the experiments. The conformations of threonine hydrated with one water molecule were also systematically searched. The zwitterionic structures may be stabilized by one water molecule, but their energies are relatively high. The infrared spectra of the most stable structures of all species are presented.

A computational study of ultrafast acid dissociation and acid-base neutralization reactions. I. The model

Ultrafast, time-resolved investigations of acid-base neutralization reactions have recently been performed using systems containing the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) and various Bronsted bases. Two conflicting neutralization mechanisms have been formulated by Mohammed et al. [Science 310, 83 (2005)] and Siwick et al. [J. Am. Chem. Soc. 129, 13412 (2007)] for the same acid-base system. Herein an ab initio molecular dynamics based computational model is formulated, which is able to investigate the validity of the proposed mechanisms in the general context of ground-state acid-base neutralization reactions. Our approach consists of using 2,4,6-tricyanophenol (exp. pKa congruent with 1) as a model for excited-state HPTS( *) (pKa congruent with 1.4) and carboxylate ions for the accepting base. We employ our recently proposed dipole-field/quantum mechanics (QM) treatment [P. Maurer and R. Iftimie, J. Chem. Phys. 132, 074112 (2010)] of the proton donor and acceptor molecules. This approach allows one to tune the free energy of neutralization to any desired value as well as model initial nonequilibrium hydration effects caused by a sudden increase in acidity, making it possible to achieve a more realistic comparison with experimental data than could be obtained via a full-QM treatment of the entire system. It is demonstrated that the dipole-field/QM model reproduces correctly key properties of the 2,4,6-tricyanophenol acid molecule including gas-phase proton dissociation energies and dipole moments, and condensed-phase hydration structure and pKa values.

Combining ab initio quantum mechanics with a dipole-field model to describe acid dissociation reactions in water: First-principles free energy and entropy calculations

We introduce a novel approach to compute dissociation free energy and entropy values in simulations that employ a density functional theory description of the acidic moiety and of the solvent. The approach consists of utilizing an alchemical transformation of a weak acid A-COOH into the strong acid B-COOH, which makes it practical to employ alchemical free energy perturbation methods in the context of ab initio molecular dynamics simulations. The present alchemical transformation circumvents the need to tackle changes in the total number of electrons and atoms by replacing the chemical residue responsible for the change in acidity with an easily tunable external effective potential. Our investigation demonstrates that (1) a simple but effective class of external potentials that control acidity changes in the acetic/trifluoroacetic acid series can be achieved by replacing the methyl and trifluoromethyl substituents by screened dipoles. Using this dipole-field/quantum-mechanics (DF/QM) approach one can predict gas-phase geometries, proton dissociation energies, total dipole moments, and water binding energies in good agreement with full-QM values. (2) The resulting alchemical perturbation calculations are stable and well converged and allow one to compute absolute pK(a) values whose accuracy is limited primarily by the exchange-correlation functional employed: H-COOH=2.5+/-0.6 (full-QM calculation), 3.7 (exp); F(3)C-COOH=0.4+/-0.6 (DF/QM calculation), 0.5 (exp); H(3)C-COOH=3.1+/-0.7 (DF/QM calculation), 4.7 (exp); 3) Our DF/QM model predicts that the difference in acidity between H-COOH and H(3)C-COOH is dominated by solvent entropy effects, in excellent agreement with experimental observations. The calculated difference between the dissociation energies of these acids is DeltaDelta(d)U=0.0+/-0.26 kcal/mol while the experimental value is 0.0+/-0.1 kcal/mol.

DFT STUDY ON GAS-PHASE INTERACTION BETWEEN HISTIDINE AND ALKALI METAL IONS (Li+, Na+, K+); AND INFLUENCE OF THESE IONS ON HISTIDINE ACIDITY

The gas-phase metal affinities of histidine Li +, Na + and K + ions have been determined theoretically employing the hybrid B3LYP exchange–correlation functional and using 6-311++G** basis sets. All computations indicate that the metal ion affinity decreases on going from Li + to Na + and K + for the considered amino acid. Different types of M + coordinations on several histidine conformers/tautomers were considered in detail. The optimized structures indicate that Li + and Na + prefer a tri-dentate coordination, bonding with a nitrogen atom of imidazole ring ( N τ), – NH 2, and an oxygen atom of a carbonyl, while in the K +-histidine lowest-energy conformer, the cation appears to be bi-coordinated to both oxygen atoms of the zwitterionic form by the energy values not too far from that of tri-coordination. We also performed the DFT calculations for proton dissociation energy of histidine both in the presence and absence of alkali metal ions. Our results also reveal that the acidity of histidine dramatically increases upon metal ion complexation.

Protonation Processes and Electronic Spectra of Histidine and Related Ions

A full structural assignment of the neutral, protonated, and deprotonated histidine conformers in the gas phase is presented. A total of 3024 unique trial structures were generated by all combinations of internal single-bond rotamers of these species and optimized at the B3LYP/6-311G* level and further optimized at the B3LYP/6-311++G** level. A set of unique conformers is found, and their relative energies, free energies, dipole moments, rotational constants, electron affinities, ionization energies, and harmonic frequencies are determined. The population ratio of histidine and its tautomer is 1:0.16 at 298 K. Massive conformational changes are observed due to protonation and deprotonation, and the intramolecular H-bonds are characterized with the atoms in molecules theory. The calculated proton dissociation energy, gas-phase acidity, proton affinity, and gas-phase basicity are in excellent agreement with the experiments. The deprotonation and protonation of gaseous histidine both occur on the imidazole ring, explaining the versatile biofunctions of histidine in large biomolecules. The UV spectra of neutral and singly and doubly protonated histidine are investigated with the TDDFT/B3LYP/6-311+G(2df,p) calculations. The S0-S1, S0-S2, and S0-S3 excitations of histidine are mixed pipi*/npi* transitions at 5.37, 5.44, and 5.69 eV, respectively. The three excitation energies for histidine tautomer are 4.85, 5.47, and 5.52 eV, respectively. The three excitations for protonated histidine are mainly npi* transitions at 5.45, 5.67, and 5.82 eV, respectively. The S0-S1 excitation of protonated histidine produces ImH-CbetaH2-CalphaH(COOH)-NH2+, while the S0-S2 and S0-S3 transitions produce ImH-CbetaH2-CalphaH(NH2)-(COOH)+. These data may help to understand the mechanisms of the UV fragmentation of biomolecules.