On the stability of glycine-water clusters with excess electron: implications for photoelectron spectroscopy

Calculations are presented for the glycine-(H(2)O)(n) (-) (n=0-2) anionic clusters with excess electron, with the glycine core in the canonical or zwitterion form. A variety of conformers are predicted, and their relative energy is examined to estimate thermodynamic stability. The dynamic (proton transfer) pathways between the anionic clusters with the canonical and the zwitterion glycine core are examined. Small barrier heights for isomerization from the zwitterion glycine-(H(2)O)(2) (-) anion to those with canonical glycine core suggest that the former conformers may be kinetically unstable and unfavorable for detection of neutral glycine zwitterion-(H(2)O)(n) (n=1,2) clusters by photodetachment, in accordance with the photoelectron spectroscopic experiments by Bowen and co-workers [Xu et al., J. Chem. Phys. 119, 10696 (2003)]. The calculated stability of the glycine-(H(2)O)(n) (-) anion clusters with canonical glycine core relative to those with zwitterion core indicates that the observation of the anionic conformers with the canonical glycine core would be much more feasible, as revealed by Johnson and co-workers [Diken et al. J. Chem. Phys. 120, 9902 (2004)].     

Solvent effects on glycine II. Water-assisted tautomerization

The water-assisted tautomerization of glycine has been investigated at the B3LYP/6-31+G** level using supermolecules containing up to six water molecules as well as considering a 1:1 glycine-water complex embedded in a continuum. The conformations of the tautomers in this mechanism do not display an intramolecular H bond, instead the functional groups are bridged by a water molecule. The replacement of the intramolecular H bond by the bridging water reduces the polarity of the N-H bond in the zwitterion and increases that of the O-H bond in the neutral, stabilizing the zwitterion. Both the charge transfer effects and electrostatic interactions stabilize the nonintramolecularly H-bonded zwitterion conformer over the intramolecularly hydrogen bonded one. The nonintramolecularly H-bonded neutral is favored only by charge transfer effects. Although there is no strong evidence whether the intramolecularly hydrogen bonded or non hydrogen bonded structures are favored in the bulk solution represented as a dielectric continuum, it is likely that the latter species are more stable. The free energy of activation of the water-assisted mechanism is higher than the intramolecular proton transfer channel. However, when the presumably higher conformational energy of the zwitterion reacting in the intramolecular mechanism is taken into account, both mechanisms are observed to compete. The various conformers of the neutral glycine may form via multiple proton transfer reactions through several water molecules instead of a conformational rearrangement.     

Solvent effects on glycine. I. A supermolecule modeling of tautomerization via intramolecular proton transfer

The relative stabilities of glycine tautomers involved in the intramolecular proton transfer are investigated computationally by considering glycine-water complexes containing up to five water molecules. The supermolecule results are compared with continuum calculations. Specific solute-solvent interactions and solvent induced changes in the solute wave function are considered using the natural bond orbitals (NBO) method. The stabilization of the zwitterion upon solvation is explained by the changes in the wave functions localized on the forming and breaking bonds as well as by the different interaction energies in the zwitterionic and neutral clusters. Only the neutral species exist in mono- and dihydrated clusters and in the gas phase. In the smaller clusters, zwitterions are mainly stabilized by conformational effects, whereas in larger clusters, in particular when glycine is solvated on both sides of its heavy atom backbone, polarization effects dominate the stability of a given tautomer. Generally, the strength of the solute-solvent interactions is governed by the intermolecular charge transfer interactions. As the solvation progresses, the hypothetical gaseous zwitterion is better solvated than the gaseous neutral, making zwitterion to neutral tautomerization progressively less exothermic for clusters containing up to three water molecules, and endothermic for larger clusters. The neutral isomer does not exist for some solvent arrangements with five water molecules. Only solvent arrangements in which water molecules do not interact with the reactive proton are considered. Hence, the experimentally observed double well potential energy surface may be due to such an interaction or to a different reaction mechanism.     

Theoretical study of CH…O hydrogen bond in proton transfer reaction of glycine

Density functional theory (DFT) calculations are used to study the strength of the CH…O H‐bond in the proton transfer reaction of glycine. Comparison has been made between four proton transfer reactions (ZW1, ZW2, ZW3, SCRFZW) in glycine. The structural parameters of the zwitterionic, transition, and neutral states of glycine are strongly perturbed when the proton transfer takes place. It has been found that the interaction of water molecule at the side chain of glycine is high in the transition state, whereas it is low in the zwitterionic and neutral states. This strongest multiple hydrogen bond interaction in the transition state reduces the barrier for the proton transfer reaction. The natural bond orbital analysis have also been carried out for the ZW2 reaction path, it has been concluded that the amount of charge transfer between the neighboring atoms will decide the strength of H‐bond. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

配合物生成反应速率常数与平衡常数之间的直线自由能关系 II: 镍(II)-N-(对位取代苯基)氨基乙酸配合物的生成反应动力学研究

应用断流分光光度计(Stopped-Flow Spectrophotometer)研究了镍(II)与N-(对位取代苯基)氨基乙酸-(ρ-RC6H4NHCH2COOH, R=CH3O, CH3, H, Cl, 简写为NROH或HL)在25℃及30%(v/v)乙醇溶液中生成配合物的反应动力学. 实验结果表明, 不仅氨基酸配体的负离子(NRO^-或L^-)具有较高的反应活性, 而且两性离子(HN^+RO^-或^+HL^-)也是有效的反应配体. 反应按双途径进行, 即按Eigen-Tamm机理进行的NRO^-途径和以质子迁移为速率控制步骤的两性离子途径. 两性离子的反应活性(以log kHL^±表示)与配体的碱性强度(pK2)之间呈现直线自由能关系. 并发现镍(II)配合物的离解反应速率常数(log k-L^-)与配体的碱性强度(pK2)和配合物的稳定常数(log KNiL^N^i)之间均存在直线自由能关系.     

Ab initio molecular dynamics free-energy study of microhydration effects on the neutral-zwitterion equilibrium of phenylalanine.

The structures and relative energies of the most stable conformers of both naked and microsolvated phenylalanine, Phe.(H(2)O)(n)(n=0-3), are calculated by density functional theory. For selected structures, coordination-constrained ab initio molecular dynamics simulations determine the proton-transfer mechanism connecting neutral and zwitterionic forms of Phe. The associated free-energy profiles are calculated by thermodynamic integration. While no zwitterionic free-energy minimum is found for naked Phe, microsolvation is found to stabilize the zwitterionic form. For cluster sizes n > or = 3, the proton-transfer equilibrium shifts towards the zwitterionic structure for specific proton-transfer pathways. The energetically most favourable interconversion path between the neutral and zwitterionic forms is through a H(2)O bridge with free-energy barriers as low as 14.4 kJ mol(-1) for Phe.(H(2)O)(3). The free energy required for breaking a carboxylic OH bond involved in intramolecular hydrogen bonding is typically lower than in the water-assisted case. However, the resulting zwitterion turns out to be unstable with respect to the backward proton-transfer reaction.     

Glycine and L-Lysine Ionization in a Mixed Aqueous Solution

In mixed aqueous solution of glycine and L-lysine the ionization of both amino acids increases due to the proton transfer from glycine zwitterion to lysine zwitterion. This is manifested by a considerable nonlinear (pairwise interaction of amino acids) increase in the mixed solution electroconductivity at high amino acid concentrations. The solution pH varies negligibly, as the amino acids in aqueous solutions demonstrate buffering. The refraction coefficient additively depends on glycine and L-lysine concentrations.     

Grotthus-type and diffusive proton transfer in 7-hydroxyquinoline.(NH(3))(n) clusters.

Proton translocation along ammonia wires is investigated in 7-hydroxyquinoline.(NH(3))(n) clusters, both experimentally by laser spectroscopy and theoretically by Hartree-Fock and density functional (DFT) calculations. These clusters serve as realistic finite-size models for proton transfer along a chain of hydrogen-bonded solvent molecules. In the enol tautomer of 7-hydroxyquinoline (7-HQ), the OH group acts as a proton injection site into the (NH(3))(n)cluster. Proton translocation along a chain of three NH(3) molecules within the cluster can take place, followed by reprotonation of 7-HQ at the quinolinic N atom, forming the 7-ketoquinoline tautomer. Exoergic proton transfer from the OH group of 7-HQ to the closest NH(3) molecule within the cluster giving a zwitterion 7-HQ-.(NH(3))(6)H+ (denoted PT-A) occurs at a threshold cluster size of n = 6 in the DFT calculations and at n = 5 or 6 experimentally. Three further locally stable zwitterion clusters denoted PT-B, PT-B', and PT-C, the keto tautomer, and several transition structures along the proton translocation path were characterized theoretically. Grotthus-type proton-hopping mechanisms occur for three of the proton transfer steps, which have low barriers and are exoergic or weakly endoergic. The step with the highest barrier involves a complex proton transfer mechanism, involving structural reorganization and large-scale diffusive motions of the cluster.     

Structures and isomerization of neutral and zwitterion serine–water clusters: Computational study

Calculations are presented for the structure and the isomerization reaction of various conformers of the bare serine, neutral serine–(H₂O)_n and serine zwitterion–(H₂O)_n (n = 1, 2) clusters. The effects of binding water molecules on the relative stability and the isomerization processes are examined. Hydrogen bonding between serine and the water molecule(s) may significantly affect the relative stability of conformers of the neutral serine–(H₂O)_n (n = 1, 2) clusters. The sidechain (OH group) in serine is found to have a profound effect on the structure and isomerization of serine–(H₂O)_n (n = 1, 2) clusters. Conformers with the hydrogen bonding between water and the hydroxyl group of serine are predicted. A detailed analysis is presented of the isomerization (proton transfer) pathways between the neutral serine–(H₂O)₂ and serine zwitterion–(H₂O)₂ clusters by carrying out the intrinsic reaction coordinate analysis. At least two water molecules need to bind to produce the stable serine zwitterion–water cluster in the gas phase. The isomerization for the serine–(H₂O)₂ cluster proceeds by the concerted double and triple proton transfer mechanism occurring via the binding water molecules, or via the hydroxyl group. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Directional dependence of the mean excitation energy and spectral moments of the dipole oscillator strength distribution of glycine and its zwitterion

In this contribution, we consider the interaction of glycine, a small, model biomolecule, and its zwitterion with fast ion radiation. The object of the study is to determine the differences in properties among various conformers and orientations of the neutral molecule and the zwitterion and to determine if these differences will have implications in terms of radiation protection and radiation therapy. To this end, quantum mechanical calculations were carried out on three conformers of the neutral molecule and two of the zwitterion to determine both the isotropic and directional components of the moments of the dipole oscillator strength distribution in each case. It is these moments that determine the interaction of swift radiation with a molecule.     

Molecular dynamics simulation of glycine zwitterion in aqueous solution

A classical molecular dynamics method was used to study the modifications of the solution structure and the properties of glycine zwitterion in aqueous solution due to the increase of glycine zwitterion concentration and the incorporation of Na(+) and Cl(-) ions to the solution. The glycine zwitterion had fundamentally a hydrophilic behavior at infinite dilution, establishing around six hydrogen bonds with the water molecules that surrounded it, which formed a strong hydration layer. Because of the increase of glycine zwitterion concentration, the hydration structure became more compact and the quantity of water molecules bound to this molecule decreased. The Na(+) ion bound to the CO(2) group of glycine, while the Cl(-) ion bound mainly to the NH(3) group of this molecule. The integration of the ions to the hydration layer of the glycine zwitterion produced modifications in the orientational correlation between atoms of glycine zwitterion and water that surrounded them and an increase of the approaches between the glycine zwitterion molecules. The incorporation of ions to the solution also produced changes in the water-water orientational correlation. Decreases of the water-water hydrogen bonds and diffusion coefficient of all molecules were observed when the glycine zwitterion concentration increased and when the ions were incorporated to the solution.     

Glycine Dimers: Structure,Stability, and Medium Effects

We report a study on different ionization states and conformations of the bimolecular (Gly)₂ system by means of quantum mechanical calculations. Optimized geometries for energy minima of the glycine dimer, as well as relative energies and free energies were computed as a function of the medium: gas phase, nonpolar polarizable solvent, and aqueous solution. The polarizable continuum model was employed to account for solvation effects. Energy calculations were done using the MP2/aug‐cc‐pVTZ and B3LYP/6‐311+G(2df,2p) methods on B3LYP/6‐31+G(d,p) optimized structures (some single‐point energy calculations were also done using the B3PW91 and PBE1KCIS methods). Ionized forms of the glycine dimer (either zwitterion–zwitterion or neutral–zwitterion) are predicted to exist in all media, in contrast to amino acid monomers. In aqueous solution, dimerization is an exergonic process (?4 kcal mol^?1). Thus, according to our results, zwitterion–zwitterion Gly dimers might be abundant in supersaturated glycine aqueous solutions, a fact that has been connected with the structure of α‐glycine crystals but that remains controversial in the literature. Another noticeable result is that zwitterion–zwitterion interactions are substantially underestimated when computed using methods based on density functional theory. For comparison, some calculations for the dimer of the simplest chiral amino acid alanine were done as well and differences to the glycine dimer are discussed.     

A theoretical investigation of the relative stability of hydrated glycine and methylcarbamic acid--from water clusters to interstellar ices

We have theoretically investigated how the low-energy conformers of the neutral and the zwitterionic forms of glycine as well as methylcarbamic acid are stabilized by the presence water. The MP2/6-311++G(d,p) method was utilized to conduct calculations on glycine and methylcarbamic acid in both isolated clusters and in clusters embedded in the conductor-like polarizable continuum model (C-PCM), where the clusters explicitly contain between one and ten water molecules. The neutral forms of glycine and methylcarbamic acid were found to have similar hydration energies, whereas the neutral methylcarbamic acid was determined to be approximately 32 kJ mol(-1) more stable than the neutral glycine in the isolated clusters and 30 kJ mol(-1) more stable in the C-PCM embedded clusters. Both the number and strength of the hydrogen bonding interactions between water and the zwitterions drive the stability. This lowers the relative energy of the glycine zwitterion from 50 kJ mol(-1) above neutral glycine, when there are two water molecules in the clusters to 11 kJ mol(-1) below for the clusters containing ten water molecules. For the methylcarbamic acid clusters with two water molecules, the zwitterion is 51 kJ mol(-1) higher in energy than the neutral form, but it remains 13 kJ mol(-1) above the neutral methylcarbamic acid in the clusters containing ten water molecules. When the bulk water environment is simulated by the C-PCM calculations, we find both the methylcarbamic acid and glycine zwitterionic forms have similar energies at 20 kJ mol(-1) above the neutral methylcarbamic acid energy and 10 kJ mol(-1) lower than the neutral glycine energy. Although neither methylcarbamic acid nor glycine have been detected in the interstellar medium yet, our findings indicate that methylcarbamic acid is the more stable product from methylamine and carbon dioxide reactions in a water ice. This suggests that methylcarbamic acid likely plays a role in the intermediate steps if glycine is formed in the interstellar medium.     

Fragmentation and deformation mechanism of glycine isomers in gas phase: Investigations of charge effect

The structural parameters, relative stability, proton transfer energy barriers of four typical and life related isomers and conformers of different charged (n=0,+/-1,+/-2) glycine species have been investigated using B3LYP, BHLYP, and CCSD(T) methods. Results indicate that those neutral and (+/-1)-charged species are stable. For the (+2)-charged cases, all four triplet-state glycine species and only the singlet-state zwitterionic one are stable. On the other hand, only the singlet-state zwtterionic glycine ((1)GlyZW(-2)) and the corresponding neutral form counterpart ((1)Gly(-2)) are stable for the (-2)-charged cases. Either of the two stable structures holds a proton lying in the position (2-3 A) of being separated from its corresponding parental species. Those unstable divalent glycine species are dissociated into different smaller species spontaneously according to the characters of their different structures and electron spins. The presented fragmentation and deformation mechanisms can effectively predict and satisfactorily explain some experimental phenomena, which had been puzzling the mass spectrometry chemists. Also, the mechanisms should be suitable for any other similar molecule systems. Comparisons of the relative energies of the four (+1)-charged glycine species show that doublet-state glycine III ((2)GlyIII1) is more stable in energy by 12.1 kcal/mol than the (+1)-charged glycine Gly ((2)Gly1). This is consistent with the energy ordering of their corresponding mono-valence metal ion-bound derivatives. In addition, calculations show that an intramolecular proton transfer of (2)Gly(-1) to become its zwitterionic counterpart is preferred due to its least activation energy barrier (5.8 kcal/mol) among four discussed processes.     

One-electron oxidation of a hydrogen-bonded phenol occurs by concerted proton-coupled electron transfer

The hydrogen-bonded phenol 2-(aminodiphenylmethyl)-4,6-di-tert-butylphenol (HOAr-NH2) was prepared and oxidized in MeCN by a series of one-electron oxidants. The product is the phenoxyl radical in which the phenolic proton has transferred to the amine, *OAr-NH3+. The reaction of HOAr-NH2 and tris(p-tolyl)aminium ([N(tol)3]*+) to give *OAr-NH3+ + N(tol)3 has Keq = 2.0 +/- 0.5, follows second-order kinetics with k = (1.1 +/- 0.2) x 105 M-1 s-1 (DeltaG = 11 kcal mol-1), and has a primary isotope effect kH/kD = 2.4 +/- 0.4. Oxidation of HOAr-NH2 with [N(C6H4Br)3]*+ is faster, with k congruent with 4 x 107 M-1 s-1. The isotope effect, thermochemical arguments, and the dependence of the rate on driving force (DeltaDeltaG/DeltaDeltaG degrees = 0.53) all indicate that electron transfer from HOAr-NH2 must occur concerted with intramolecular proton transfer from the phenol to the amine (proton-coupled electron transfer, PCET). The data rule out stepwise paths that involve initial electron transfer to form the phenol radical cation *+HOAr-NH2 or that involve initial proton transfer to give the zwitterion -OAr-NH3+. The dependence of the electron-transfer rate constants on driving force can be fit with the adiabatic Marcus equation, yielding a large intrinsic barrier: lambda = 34 kcal mol-1 for reactions of HOAr-NH2 with NAr3*+.     

The enthalpies of interaction of glycine with some amides and ureas in water at 25°C

Measurement of solution enthalpyiesof glycine in aqueous solutions of formamide (F), N-methylformamide (NMF), N,N-dimethylformamide (DMF), monomethylurea (MMU), 1,3-diethylurea (DEU) and tetramethylurea (TMU) at 25°C have been undertaken. On the basis of the results, enthalpic coefficients of heterotactic interactions between a glycine zwitterion and a molecule of organic substance in aqueous solutions have been calculated. Using the additivity of groups concept by Savage and Wood (SWAG), contributions of each functional group of studied amides and ureas have been estimated. In this model a zwitterion of glycine has been considered as an individual equal to a single functional group.     

Cu^2＋诱导甘氨酸质子迁移机理的理论研究

采用CCSD/6-31＋＋G＊＊//B3LYP/6-31＋＋G＊＊方法研究了Cu2＋诱导甘氨酸质子迁移的机理.优化得到了7个中性配合物和1个两性配合物,其中两性配合物最稳定,结合能为215.93 kcal mol-1.中性构型间通过分子内单键的旋转相互转化,C-N、C-C和C-O键旋转的能垒范围分别为1.62～2.49、0.27～7.80和2.27～16.97 kcal mol-1;中性构型N6经质子迁移变为两性构型,能垒为33.82 kcal mol-1.Cu2＋作用于甘氨酸,使甘氨酸N5原子负电荷减少超过0.5,降低了N5对H6原子的库仑吸引,钝化了共价键B（O3–H6）,动力学上不利于H6质子迁移;但是H6质子迁移后,形成的两性构型Z1却是热力学最稳定体系.     

Double proton transfer behavior and one-electron oxidation effect in double H-bonded glycinamide-formic acid complex

The behavior of double proton transfer occurring in a representative glycinamide-formic acid complex has been investigated at the B3LYP/6-311 + + G( * *) level of theory. Thermodynamic and, especially, kinetic parameters, such as tautomeric energy, equilibrium constant, and barrier heights have been discussed, respectively. The relevant quantities involved in the double proton transfer process, such as geometrical changes, interaction energies, and intrinsic reaction coordinate calculations have also been studied. Computational results show that the participation of a formic acid molecule favors the proceeding of the proton transfer for glycinamide compared with that without mediate-assisted case. The double proton transfer process proceeds with a concerted mechanism rather than a stepwise one since no ion-pair complexes have been located during the proton transfer process. The calculated barrier heights are 11.48 and 0.85 kcal/mol for the forward and reverse directions, respectively. However, both of them have been reduced by 2.95 and 2.61 kcal/mol to 8.53 and -1.76 kcal/mol if further inclusion of zero-point vibrational energy corrections, where the negative barrier height implies that the reverse reaction should proceed with barrierless spontaneously, analogous to that occurring between glycinamide and formamide. Furthermore, solvent effects on the thermodynamic and kinetic processes have also been predicted qualitatively employing the isodensity surface polarized continuum model within the framework of the self-consistent reaction field theory. Additionally, the oxidation process for the double H-bonded glycinamide-formic acid complex has also been investigated. Contrary to that neutral form possessing a pair of two parallel intermolecular H bonds, only a single H bond with a comparable strength has been found in its ionized form. The vertical and adiabatic ionization potentials for the neutral complex have been determined to be about 9.40 and 8.69 eV, respectively, where ionization is mainly localized on the glycinamide fragment. Like that ionized glycinamide-formamide complex, the proton transfer in the ionized complex is characterized by a single-well potential, implying that the proton initially attached to amide N4 in the glycinamide fragment cannot be transferred to carbonyl O13 in the formic acid fragment at the geometry of the optimized complex.     

Proton transfer reaction of a new orthohydroxy Schiff base in some protic and aprotic solvents at room temperature and 77 K

The ground and excited state proton transfer reactions of a new orthohydroxy Schiff base, salicylidine-3,4,7-methyl amine (SMA) has been studied by means of absorption, emission and time resolved fluorescence spectroscopy in some polar protic and aprotic solvents at room temperature and 77K. The spectral behavior of SMA has been investigated both in neutral and basic conditions. The intramolecularly hydrogen bonded enol and zwitterion have been detected in pure methanol and ethanol, the anion is detected in basic condition. At 77K, SMA shows phosphorescence in pure methanol and ethanol. From nanosecond measurements and quantum yields of fluorescence, we have estimated the decay rates of proton transfer reaction in methanol and ethanol.