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.     

Structure and stability of glycine–(H2O)3 cluster and anion: Zwitterion vs. canonical glycine

Calculations are presented for the glycine–(H₂O) cluster anion, with glycine in canonical or zwitterionic form. The zwitterionic anions are predicted to be considerably lower in energy than the canonical anions, and the latter forms are found to be prone to isomerization to the zwitterionic anions. Therefore, we predict that the zwitterionic anions would be observed predominantly in the gas phase at low temperature. In contrast, calculated stability of neutral glycine–(H₂O)₃ clusters indicates that only the canonical forms of the anions would be observed in photoelectron experiments, if anions are produced from preformed neutrals. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

5水合甘氨酸复合体的结构和性能的理论研究

为了探讨“几个水分子导致甘氨酸两性离子复合体与其中性分子复合体等能”, 采用mp2/6-31＋＋g**//b3lyp/6- 31＋＋g**方法研究了甘氨酸水化的微观机理, 发现答案是5个水分子, 并证实了实验结果. 最稳定的5水合甘氨酸两性离子复合体具有双链结构, 与最稳定的5水合甘氨酸中性分子复合体能量接近(仅相差2.26 kJ/mol), 能够共存; 中性复合体和两性复合体之间可以通过直接质子迁移和桥助质子迁移等两种途径相互转化, 双链结构的甘氨酸复合体中质子迁移的能垒较低甚至无垒, 能够自发进行.     

The zwitterionic structure of 2‐hydroxy‐4‐[(2‐hydroxybenzylidene)amino]benzoic acid

The title compound, C₁₄H₁₁NO₄, exists in the solid phase in the zwitterionic form, 2‐{[(4‐carboxy‐3‐hydroxyphenyl)iminiumyl]methyl}phenolate, with the H atom from the phenol group on the 2‐hydroxybenzylidene ring transferred to the imine N atom, resulting in a strong intramolecular N—H...O hydrogen bond between the iminium H atom and the phenolate O atom, forming a six‐membered hydrogen‐bonded ring. In addition, there is an intramolecular O—H...O hydrogen bond between the carboxylic acid group and the adjacent hydroxy group of the other ring, and an intermolecular C—H...O contact involving the phenol group and the C—H group adjacent to the imine bond, connecting the molecules into a two‐dimensional network in the (10) plane. π–π stacking interactions result in a three‐dimensional network. This study is important because it provides crystallographic evidence, supported by IR data, for the iminium zwitterionic form of Schiff bases.<!?tpb=12pt>     

Tunable N-substitution in zwitterionic benzoquinonemonoimine derivatives: metal coordination, tandemlike synthesis of zwitterionic metal complexes, and supramolecular structures

Full details on a very efficient transamination reaction for the synthesis of zwitterionic N,N-dialkyl-2-amino-5-alcoholate-1,4-benzoquinonemonoiminium derivatives [C6H2(=NHR)2(=O)2] 5-16 are reported. The molecular structures of zwitterions 5 (R=CH3) in 5.H2O, 13 (R=CH2CH2OMe), 15 (R=CH2CH2NMe2), and of the parent, unsubstituted system [C6H2(=NH2)2(=O)2] 4 in 4.H2O have been established by single-crystal X-ray diffraction. This one-pot preparation can be carried out in water, MeOH, or EtOH and allows access to new zwitterions with N-substituents bearing functionalities such as -OMe (13), -OH (9-12), NR1R2 with R1 = or not equal R2 (14-16) or an alkene (8), leading to a rich coordination chemistry and allowing fine-tuning of the supramolecular arrangements in the solid state. As previously described for 15, which reacted with Zn(acac)2 to afford the octahedral Zn(II) complex [Zn[C6H2(NCH2CH2NMe2)O(O)(NHCH2CH2NMe2)]2] (20), ligands 13 and 16 with coordinating "arms" afforded with Zn(acac)2 the 2:1 adducts [Zn[C6H2(NCH2CH2X)O(=O)(NHCH2CH2NX)]2] 19 (X=OMe) and 21 (X=NHEt), with N2O4 and N4O2 donor sets around the octahedral Zn(II) center, respectively. Furthermore, zwitterions 15 and 16 reacted with ZnCl2 to give the stable, crystallographically characterized Zn(II) zwitterionic complexes [ZnCl2[C6H2(NCH2CH2NR1R2)O(=O)(NHCH2CH2NHR1R2)]] 22 (R1=R2=Me) and 23 (R1=Et, R2=H) by means of an unprecedented, tandemlike synthesis in which 1) the two pendant amino groups of the organic benzoquinonemonoimine zwitterionic precursor favor metal coordination and proton transfer and 2) the saturated linker prevents pi-conjugation between the charges. The nature of the structural arrangements in the solid state for both inorganic (20, 22, 23) and organic (5, 9, 13, and 15) molecules is determined by subtle variations in the nature of the N-substituent on the zwitterion precursor.     

A structural and theoretical study of the intermolecular interactions in 8‐hydroxyquinolinium‐7‐carboxylate monohydrate

In the title compound, C₁₀H₇NO₃·H₂O, the zwitterionic organic molecules and the water molecules are connected by N—H...O and O—H...O hydrogen bonds to form ribbons, and π–π stacking interactions expand these ribbons into a three‐dimensional net. The energies of these hydrogen bonds adopt values typical for mildly weak interactions (3.33–7.75 kcal mol⁻¹; 1 kcal mol⁻¹ = 4.184 kJ mol⁻¹). The total π–π stacking interactions between aromatic molecules can be classified as mildly strong (energies of 15.3 and 33.9 kcal mol⁻¹), and they are made up of multiple constituent π–π interactions between six‐membered rings. The short intermolecular C—H...O contact between two zwitterionic molecules is nonbonding in character.     

l‐Tryptophan formic acid solvate at 183 K

In the title compound, C₁₁H₁₂N₂O₂·CH₂O₂, at 183 K. l ‐­tryptophan appears in the zwitterionic form, while the formic acid molecule is neutral. The formic acid molecule is the donor in a strong O—H?O hydrogen bond to the carboxyl­ate group of the tryptophan mol­ecule, with a short O?O contact of 2.487 (2) Å.     

Metal aminocarboxylate coordination polymers with chain and layered structures

The synthesis and structures of metal aminocarboxylates prepared in acidic, neutral, or alkaline media have been explored with the purpose of isolating coordination polymers with linear chain and two-dimensional layered structures. Metal glycinates of the formulae [CoCl2(H2O)2(CO2CH2NH3)] (I), [MnCl2(CO2CH2NH3)2] (II), and [Cd3Cl6(CO2CH2NH3)4] (III) with one-dimensional chain structures have been obtained by the reaction of the metal salts with glycine in an acidic medium under hydro/solvothermal conditions. These chain compounds contain glycine in the zwitterionic form. 4-Aminobutyric acid transforms to a cyclic amide under such reaction conditions, and the amide forms a chain compound of the formula [CdBr2(C4H7NO)2] (IV). Glycine in the zwitterionic form also forms a two-dimensional layered compound of the formula [Mn(H2O)2(CO2CH2NH3)2]Br2 (V). 6-Aminocaproic acid under alkaline conditions forms layered compounds with metals at room temperature, the metal being coordinated both by the amino nitrogen and the carboxyl oxygen atoms. Of the two layered compounds [Cd{CO2(CH2)5NH2}2]2 H2O (VI) and [Cu{CO2(CH2)5NH2}2]2 H2O (VII), the latter has voids in which water molecules reside.     

Gas phase hydrogen/deuterium exchange of arginine and arginine dipeptides complexed with alkali metals

The hydrogen/deuterium (H/D) exchange of protonated and alkali-metal cationized Arg-Gly and Gly-Arg peptides with D(2)O in the gas phase was studied using electrospray ionization quadropole ion trap mass spectrometry. The Arg-Gly and Gly-Arg alkali metal complexes exchange significantly more hydrogens than protonated Arg-Gly and Gly-Arg. We propose a mechanism where the peptide shifts between a zwitterionic salt bridge and nonzwitterionic charge solvated conformations. The increased rate of H/D exchange of the alkali metal complexes is attributed to the peptide metal complexes' small energy difference between the salt-bridge conformation and the nonzwitterionic charge-solvated conformation. Implications for the applicability of this mechanism to other zwitterionic systems are discussed.     

Bonding and structure of glycine on ordered Al2O3 film surfaces

The interaction between glycine (NH2CH2COOH) layers and an ultrathin Al2O3 film grown epitaxially onto NiAl(110) was studied by temperature-programmed desorption, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, work function measurements, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. At monolayer coverages at 110 K, there are two coexisting molecular forms: the anionic (NH2CH2COO-) and the zwitterionic form (NH3+CH2COO-) of glycine. As deduced from the photoemission data, the buildup of multilayers at 110 K leads to a condensed phase predominantly in the zwitterionic state. In contrast to the monolayer at 110 K, the monolayer formed at 300 K consists primarily of glycine molecules in the anionic state. The latter species is adsorbed with the oxygen atoms of the carboxylic group pointing toward the substrate. The polarization-dependent C K- and O K-edge NEXAFS spectra indicate that the glycinate species in the monolayer at 300 K is oriented nearly perpendicular to the surface, with the amino group pointing away from the surface.     

A supramolecular approach to zwitterionic alkaline metal silanides and formation of heterobimetallic silanides

Incorporating pendant polydonor groups is key to the synthesis and isolation of a series of novel and truly zwitterionic alkaline metal silanides of formula {Si[SiMe(2)O(CH(2))(n)OMe](3)}M (n = 2, 3; M = Li, Na, K) that can easily be converted into heterobimetallic silanides.     

Synthesis of novel heterocyclic structures via reaction of isocyanides with S-trans-enones

The reaction of enone 1, bearing an internal nucleophilic moiety, i.e., furan or pyrrole (X = O, NR'), with isocyanides is presented. The formation of products resulting from the reaction of the zwitterionic intermediate 2 with a second equivalent of isocyanide prior to cyclization to give 3, as well as the direct formation of 4 from 2, is described.     

The chameleon-like nature of zwitterionic micelles. Control of anion and cation binding in sulfobetaine micelles. Effects on acid equilibria and rates

The rate of specific hydrogen ion catalyzed hydrolysis of 2-(p-heptoxyphenyl)-1,3-dioxolane and equilibrium protonation of 1-hydroxy-2-naphthoate ion in zwitterionic micelles of SB3-14, C14H29NMe2+(CH2)3SO3-, are increased markedly by NaClO4 which induces anionic character and uptake of H3O+ in the micelles. Other salts, for example, NaNO3, NaBr, and NaCl, have similar but much smaller effects on this uptake of H3O+.     

Nucleophilic fluorination reactions starting from aqueous fluoride ion solutions

The sulfonium borane 2(+) reacts with fluoride anions in MeOH/H(2)O mixtures to afford the zwitterionic fluoroborate 2-F as an easily isolable nonhygroscopic solid. In dry acetonitrile, 2-F reacts with PhS(-) to afford the anionic fluoroborate 1-F(-). The latter is very labile and acts as a nucleophilic fluorination reagent toward a variety of substrates including alkylhalides and electron-deficient aromatic compounds. This approach may become broadly applicable to nucleophilic fluorination procedures that involve wet fluoride sources.     

Zwitterionic pyrrolidine‐2,2‐diylbis(phosphonic acid) at 100, 150 and 293 K

The title compound, C₄H₁₁NO₆P₂, reveals a two‐dimensional network of P—O—H?O=P and N—H?O=P hydrogen‐bond interactions, forming molecular slabs parallel with the (010) plane. One O—H?O interaction is distinct within these sets: whilst forming the shortest intermolecular hydrogen bond, it possesses a short P—O(H) bond of 1.5291 (10) Å. Weak C—H?O contacts link individual stacks to produce a three‐dimensional array. The compound is zwitterionic: one H atom from a P—O—H group has transferred to the pyrrolidine ring N atom.     

含烯烃配体的过渡金属卡宾配合物的研究VI: 两个新奇的铁内盐配合物的合成和晶体结构

肉桂醛缩(2,4-二甲基苯胺)三羰基铁(1)与苯基锂和对氯苯基锂在低温下反应,所生成的加合物继后Et3OBF4在CH2Cl2中于-60℃烷基化, 分别生成两个组成为C19H22N(CO)3FeC6H5(2)和C19H22N(CO)3FeC6H4Cl(3)的新奇的铁内盐配合物. 2晶体属单斜晶系, 空间群为P21/n, 晶胞参数为: α=10.624(3), b=25.155(6),c=19.375(9)A, β=101.13(3)°, V=5042.2A^3, Z=8, Dc=1.27g/cm^3. 2的结构用直接法解出, 并用矩阵最小二乘法修正后得最后偏离因子R为0.071.     

Isolation and characterisation of transition and main group metal complexes supported by hydrogen-bonded zwitterionic polyphenolic ligands

Reaction of Zr(O(i)Pr)4 or Sn[N(SiMe3)2]2 with the tris-phenol amine ligand H3L(Me/Me) results in the formation of zirconium or tin complexes containing the new C3-symmetric zwitterionic ammonium-trisphenolate ligand HL2-, while increasing the steric bulk of the ligand results in the isolation of a zirconium complex containing the known trianionic ligand L3-.     

O‐Phenyl (triphenylphosphoniomethyl)phosphonate phenol solvate: supramolecular structure generated by O—H...O,C—H...O and C—H...π(arene) hydrogen bonds

The title compound, C₂₅H₂₂O₃P₂·C₆H₆O, has a zwitterionic betaine‐like structure and crystallizes as a phenol solvate. The two molecular components are held together by an almost linear intermolecular O—H...O hydrogen bond. The structure also contains three weak C—H...O and two C—H...π(arene) interactions.     

UV光引发表面接枝两性离子水凝胶层的制备及抗细菌黏附性能

采用聚合和交联的SiO₂有机/无机杂化溶胶作为基材, 通过与两性离子单体层之间的过渡层, 在紫外光作用下引发杂化溶胶和两性离子单体溶液中的双键反应, 使生成的杂化层在基材和表面的两性离子聚合物之间形成辅助性黏接作用, 从而在基材表面构筑两性离子水凝胶层. 通过傅里叶红外光谱(FTIR)、 原子力显微镜(AFM)和接触角测试等方法对所制备的两性离子水凝胶层和杂化层的表面进行了表征. 以空白玻璃片为对照样品, 以金黄色葡萄球菌和大肠杆菌为试验菌, 研究了用两性离子凝胶层修饰的玻璃表面的抗细菌黏附性能. 结果表明, 在SiO₂杂化过渡层中引入线型-Si-(CH₂)₂-O-链段可有效提高杂化过渡层对基材的附着力, 并改善其柔韧性. 与对照样品相比, 用两性离子凝胶层修饰的玻璃表面具有优异的抗菌黏附性能.