Effect of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) and water coordination on the structure of glycine and zwitterionic glycine

Interactions between metal ions and amino acids are common both in solution and in the gas phase. Here, the effect of metal ions and water on the structure of glycine is examined. The effect of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) and water on structures of Gly.Mn+(H2O)m and GlyZwitt.Mn+(H2O)m (m = 0, 2, 5) complexes have been determined theoretically by employing the hybrid B3LYP exchange-correlation functional and using extended basis sets. Selected calculations were carried out also by means of CBS-QB3 model chemistry. The interaction enthalpies, entropies, and Gibbs energies of eight complexes Gly.Mn+ (Mn+ = Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) were determined at the B3LYP density functional level of theory. The computed Gibbs energies DeltaG degrees are negative and span a rather broad energy interval (from -90 to -1100 kJ mol(-1)), meaning that the ions studied form strong complexes. The largest interaction Gibbs energy (-1076 kJ mol(-1)) was computed for the NiGly2+ complex. Calculations of the molecular structure and relative stability of the Gly.Mn+(H2O)m and GlyZwitt.Mn+(H2O)m (Mn+ = Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+; m = 0, 2, and 5) systems indicate that in the complexes with monovalent metal cations the most stable species are the NO coordinated metal cations in non-zwitterionic glycine. Divalent cations Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+ prefer coordination via the OO bifurcated bonds of the zwitterionic glycine. Stepwise addition of two and five water molecules leads to considerable changes in the relative stability of the hydrated species. Addition of two water molecules at the metal ion in both Gly.Mn+ and GlyZwitt.Mn+ complexes reduces the relative stability of metallic complexes of glycine. For Mn+ = Li+ or Na+, the addition of five water molecules does not change the relative order of stability. In the Gly.K+ complex, the solvation shell of water molecules around K+ ion has, because of the larger size of the potassium cation, a different structure with a reduced number of hydrogen-bonded contacts. This results in a net preference (by 10.3 kJ mol(-1)) of the GlyZwitt.K+H2O5 system. Addition of five water molecules to the glycine complexes containing divalent cations Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+ results in a net preference for non-zwitterionic glycine species. The computed relative Gibbs energies are quite high (-10 to -38 kJ mol(-1)), and the NO coordination is preferred in the Gly.Mn+(H2O)5 (Mn+ = Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) complexes over the OO coordination.     

Interaction of methyl beta-D-xylopyranoside with metal ions: density functional theory study of cationic and neutral bridging and pendant complexes

Density functional theory calculations on complexes of 4C1, 1C4 and 2SO ring conformations of methyl beta-D-xylopyranoside 1 with divalent metal cations, M = Mg2+, Ca2+, Zn2+, and Cd2+, are presented. Bridging and pendant cationic, [M(H2O)41]2+ and [M(H2O)(5)1]2+, as well as neutral complexes, [M(OH)2(H2O)(2)1] and [M(OH)2(H2O)(3)1], and neutral complexes involving a doubly deprotonated sugar, [M(H2O)(4)1(2-)], are considered. In aqueous and chloroform solution the stability of cationic and pendant neutral complexes is greatly diminished compared with gas-phase results. In contrast, bridging neutral complexes [M(OH)2(H2O)(2)1] and those of type [M(H2O)(4)1(2-)], are stabilized with increasing solvent polarity. Solvation also profoundly influences the preferred binding position and ring conformation. Compared with complexes of bare metal cations, additional ligands, e.g., H2O or OH-, significantly reduce the stability of 1C4 ring complexes. Irrespective of the cation, the most stable structure of bridging complexes [M(H2O)(4)1]2+ results from coordination of the metal to O3 and O4 of methyl beta-D-xylopyranoside in its 4C1 ring conformation.     

Speciation of cyclo(Pro-Gly)<Subscript>3</Subscript> and its divalent metal-ion complexes by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) was used to study the binding of selected group II and divalent transition-metal ions by cyclo(Pro-Gly)3 (CPG3), a model ion carrier peptide. Metal salts (CatXn) were combined with the peptide (M) at a molar ratio of 1:10 M/Cat in aqueous solvents containing 50% vol/vol acetonitrile or methanol and 1 or 10 mM ammonium acetate (NH4Ac). Species detected include [M+H]+, [M+Cat-H]+, [M2+Cat]2+, [M+Cat+Ac]+, and [M+Cat+X]+. The relative stabilities of complexes formed with different cations (Mg2+, Ca2+, Sr2+, Mn2+, Co2+, Ni2+, Cu2+, and Zn2+) were determined from the abundance of 1:1 and 2:1 M/Cat species relative to that of the unbound peptide. The largest metal ions (Ca2+, Sr2+, and Mn2+) formed the most stable complexes. Reducing the buffer concentration increased the overall extent of metal binding. Results show that the binding specificity of CPG3 depends upon the size of the metal ion and its propensity for electrostatic interaction with oxygen atoms. Product ion tandem mass spectrometry of [M+H]+ and [M+Cu-H]+ confirmed the cyclic structure of the peptide, although the initial site(s) of metal attachment could not be determined.     

Factors governing the substitution of La3+ for Ca2+ and Mg2+ in metalloproteins: a DFT/CDM study

Trivalent lanthanide cations are extensively being used in biochemical experiments to probe various dication-binding sites in proteins; however, the factors governing the binding specificity of lanthanide cations for these binding sites remain unclear. Hence, we have performed systematic studies to evaluate the interactions between La3+ and model Ca2+ - and Mg2+ -binding sites using density functional theory combined with continuum dielectric methods. The calculations reveal the key factors and corresponding physical bases favoring the substitution of trivalent lanthanides for divalent Ca2+ and Mg2+ in holoproteins. Replacing Ca2+ or Mg2+ with La3+ is facilitated by (1) minimizing the solvent exposure and the flexibility of the metal-binding cavity, (2) freeing both carboxylate oxygen atoms of Asp/Glu side chains in the metal-binding site so that they could bind bidentately to La3+, (3) maximizing the number of metal-bound carboxylate groups in buried sites, but minimizing the number of metal-bound carboxylate groups in solvent-exposed sites, and (4) including an Asn/Gln side chain for sites lined with four Asp/Glu side chains. In proteins bound to both Mg2+ and Ca2+, La3+ would prefer to replace Ca2+, as compared to Mg2+. A second Mg2+-binding site with a net positive charge would hamper the Mg2+ --> La3+ exchange, as compared to the respective mononuclear site, although the La3+ substitution of the first native metal is more favorable than the second one. The findings of this work are in accord with available experimental data.     

Principal component analysis of the absorption and resonance Raman spectra of the metallochromic indicator antipyrylazo III

Metallochromic indicators, whose spectral properties are changed in the presence of metal cations, are used mainly in biological studies to monitor Ca2+ and Mg2+ ions. Antipyrylazo III is such indicator, employed for mid-range Ca2+ concentrations (10-1000 microM). The stoichiometry of the interactions of antipyrylazo III with Ca2+, Mg2+, Ba2+, Sr2+ and Zn2+ ions and the relevant binding constants were studied by principal component analysis (PCA) of the absorption spectral changes. The resonance Raman spectra of the above systems were measured as well, and the resolved Raman spectra of the various species were calculated and assigned. The vibrational spectra are more featured, more characteristic of the binding ions and exhibit stronger relative spectral changes upon binding the cations. The basis sets of Raman spectra could thus be used as an analytical tool for these divalent metallic cations.     

Evaluation of complexes of DNA duplexes and novel benzoxazoles or benzimidazoles by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry is used to compare the metal ion binding and metal-mediated DNA binding of benzoxazole (1, 2, 3, 4) and benzimidazole (5) compounds and to elucidate the putative binding modes and stoichiometries. The observed metal versus non-metal-mediated DNA binding, as well as the specificity of DNA binding, is correlated with the biological activities of the analogs. The ESI-MS spectra for the antibacterial benzoxazole and benzimidazole analogs 4 and 5 demonstrated non-specific and non-metal-mediated binding to DNA, with the appearance of DNA complexes containing multiple ligands. The anticancer analog 2 demonstrates a clear preference for metal-mediated DNA interactions, with an apparent selectivity for Ni2+ -mediated binding over the more physiologically relevant Mg2+ or Zn2+ cations. Complexation between DNA and the biologically inactive analog 1 was not observed, either in the absence or presence of metal cations.     

磷酸二甲酯阴离子(DMP~-)与一价、二价金属离子 相互作用的理论研究

在B3LYP、HF和MP2水平上运用全电子从头算(AE)和相对论有效实势(RECP)及6-311+G**和LanL2DZ基组计算Ⅰa、Ⅰb、Ⅱa和Ⅱb族金属离子与磷酸二甲酯阴离子(DMP-)的相互作用。 RECP用于除Li+、Be2+外所有的金属离子。 对Na+、K+、Cu+、Mg2+、Ca2+、Zn2+用AE和RECP 2种方法处理。 结果表明:RECP能可靠地用于重金属离子络合物; 二价金属离子络合物(DMP-—M2+)比一价金属离子络合物 (DMP-—M+)稳定;二价金属离子(M2+)可能比一价金属离子(M+)更易使多核苷酸折叠。     

NMR, UV-vis and CD study on the interaction of pradimicin BMY-28864 with divalent cations of alkaline earth metal

In order to clarify the mechanism of the calcium-activated anti-fungal action of pradimicin derivatives, we investigated the compatibility of four divalent metal cations, Ca2+, Mg2+, Sr2+ and Ba2+, when combined with pradimicin BMY-28864 in an aqueous solution. The 1H NMR studies suggest that all the tested cations can induce a chemical exchange between two types of aggregation of BMY-28864 in an aqueous solution. The exchange rate, however, varies according to the cation species. In descending order they are: Ba2+ > Sr2+ > Ca2+ > Mg2+. The differences in the binding ability of BMY-28864 to the cations and the solubility of the chloride salts are explained by the electronegativity of these cations. The UV-Vis and CD spectra of these solutions show isosbestic points that correspond to an exchange process in the cations' binding to BMY-28864. The results provide experimental support for the hypothesis that there is a common mechanism underlying their cations interactions with the antibiotic. The CD study also provided evidence about the stoichiometric relation of the divalent metal ions to the antibiotic, 1:2, which was obtained by NMR analyses.     

碱金属和碱土金属离子在反相高效液相色谱柱上的保留机理

发现以酒石酸和吡啶二甲酸等羧酸水溶液作淋洗剂时，钠，铵，钾，镁和钙等碱金属及碱土金属离子在ＯＤＳ反相高效液相色谱柱上有明显的保留，而且相互之间能达到一定程度的分离。单独用分配或疏水作用等反相高效液相色谱的保留机理难以解释其保留行为。为此，作者提出了动态包固定相机理，即认为羧酸根阴离子因其疏水性在ＯＤＳ固定相有保留，在固定相表面形成具有羧酸基阳离子交换树脂功能的动态包覆固定相。     

不同形态的氧氟沙星在凹凸棒土上的吸附特征

通过控制反应体系的pH值,探究了阳离子、兼性和阴离子形态的氧氟沙星(OFL,3种形态分别记为OFL~+,OFL~±和OFL~-)在凹凸棒土(ATP)上的吸附特征.实验结果表明,OFL~+主要通过与ATP表面的Ca~(2+),Mg~(2+)进行阳离子交换吸附于ATP上,当其吸附量较高时,会存在少量的氢键;OFL~±和OFL~-可与ATP表面的铁氧化物、铝氧化物进行表面络合,也可与溶液中从ATP中溶解出的Ca~(2+)和Mg~(2+)形成络合物,再通过静电作用吸附于ATP上.在中性至微碱性(pH=7.10~7.70)条件下,由于Ca的电负性小于Mg,[Ca~(2+)-OFL]+不能稳定地存在于溶液中,使得OFL±与Ca~(2+)进行阳离子交换而与Mg~(2+)形成络合物,再通过静电作用吸附于ATP上.当OFL主要以OFL~-形态存在于溶液中时(p H=9.00~10.00),Ca~(2+)和Mg~(2+)均可与OFL~-形成络合物,再通过静电作用吸附于ATP上.     

The special five-membered ring of proline: An experimental and theoretical investigation of alkali metal cation interactions with proline and its four- and six-membered ring analogues

The interaction of the alkali metal cations, Li+, Na+, and K+, with the amino acid proline (Pro) and its four- and six-membered ring analogues, azetidine-2-carboxylic acid (Aze) and pipecolic acid (Pip), are examined in detail. Experimentally, threshold collision-induced dissociation of the M+(L) complexes, where M = Li, Na, and K and L = Pro, Aze, and Pip, with Xe are studied using a guided ion beam tandem mass spectrometer. From analysis of the kinetic energy dependent cross sections, M(+)-L bond dissociation energies are measured. These analyses account for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Ab initio calculations for a number of geometric conformations of the M+(L) complexes were determined at the B3LYP/6-311G(d,p) level with single-point energies calculated at MP2(full), B3LYP, and B3P86 levels using a 6-311+G(2d,2p) basis set. Theoretical bond energies show good agreement with the experimental bond energies, which establishes that the zwitterionic form of the alkali metal cation/amino acid, the lowest energy conformation, is formed in all cases. Despite the increased conformational mobility in the Pip systems, the Li+, Na+, and K+ complexes of Pro show higher binding energies. A meticulous examination of the zwitterionic structures of these complexes provides an explanation for the stability of the five-membered ring complexes.     

Factors governing the metal coordination number in isolated group IA and IIA metal hydrates

Many of the group IA and IIA metal ions, such as Na+, K+, Mg2+, and Ca2+, play crucial roles in biological functions. Previous theoretical studies generally focus on the number of water molecules bound to a particular (as opposed to all) alkali or alkaline earth cations and could not establish a single preferred CN for the heavier alkali and alkaline earth ion-water complexes. Crystal structures of hydrated Na+, K+, and Rb+ also cannot establish the preferred number of inner-shell water molecules bound to these cations. Consequently, it is unclear if the gas-phase CNs of group IA metal hydrates increase with increasing ion size, as observed for the group IIA series from the Cambridge Structural Database, and if the same factors govern the gas-phase CNs of both group IA and IIA ion-water complexes. Thus, in this work, we determine the number of water molecules directly bound to the series of alkali (Li+, Na+, K+, and Rb+) and alkaline earth (Be2+, Mg2+, Ca2+, Sr2+, and Ba2+) metal ions in the gas phase by computing the free energy for forming an isolated metal-aqua complex as a function of the number of water molecules at 298 K. The preferred gas-phase CNs of group IA hydrates appear insensitive to the ion size; they are all 4, except for Rb+, where a CN of 6 seems as likely. In contrast, the preferred gas-phase CNs of the group IIA dications increase with increasing ion size; they are 4 for Be2+, 6 for Mg2+ and Ca2+, and 7 for Sr2+ and Ba2+. An entropic penalty disfavors a gas-phase CN greater than 4 for group IA hydrates, but it does not dictate the gas-phase CNs of group IIA hydrates. Instead, interactions between the metal ion and first-shell water molecules and between first-shell and second-shell water molecules govern the preferred gas-phase CNs of the group IIA metal hydrates.     

Theoretical study of interaction of urate with li(+), na(+), k(+), be(2+), mg(2+), and ca(2+) metal cations

The geometries and energetics of complexes of Li(+), Na(+), K(+), Be(2+), Mg(2+), and Ca(2+)metal cations with different possible uric acid anions (urate) were studied. The complexes were optimized at the B3LYP level and the 6-311++G(d,p) basis set. Complexes of urate with Mg(2+), and Ca(2+)metal cations were also optimized at the MP2/6-31+G(d) level. Single point energy calculations were performed at the MP2/6-311++G(d,p) level. The interactions of the metal cations at different nucleophilic sites of various possible urate were considered. It was revealed that metal cations would interact with urate in a bi-coordinate manner. In the gas phase, the most preferred position for the interaction of Li(+), Na(+), and K(+) cations is between the N(3) and O(2) sites, while all divalent cations Be(2+), Mg(2+), and Ca(2+) prefer binding between the N(7) and O(6) sites of the corresponding urate. The influence of aqueous solvent on the relative stability of different complexes has been examined using the Tomasi's polarized continuum model. The basis set superposition error (BSSE) corrected interaction energy was also computed for complexes. The AIM theory has been applied to analyze the properties of the bond critical points (electron densities and their Laplacians) involved in the coordination between urate and the metal cations. It was revealed that aqueous solvation would have significant effect on the relative stability of complexes obtained by the interaction of urate with Mg(2+) and Ca(2+)cations. Consequently, several complexes were found to exist in the water solution. The effect of metal cations on different NH and CO stretching vibrational modes of uric acid has also been discussed.     

Metal ions-induced conformational change of P23 by using TNS as fluorescence probe

In 10 mM N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid (Hepes), pH 7.4, 25 degrees C, the conformational change of the truncated form of ciliate Euplotes Octocarinatus centrin (P23) induced by metal ions were investigated using 2-p-toluidinylnaphthalene-6-sulfonate (TNS) as a probe. The results show that upon metal ions binding, P23 undergo a conformational change and the contributions to the conformational change from the two EF-hands are different, and Tb3+ has more larger influence than Ca2+ with the same concentration metal ions, which provide possible the evidence that the different EF-hands play distinct biological functions. Meanwhile, the conditional binding constants of TNS and Ca2-loaded or Tb2-loaded P23 were obtained, K (Ca2-P23+TNS)=(7.49+/-0.88)x10(5) mol-1 L, K (Tb2-P23+TNS)=(8.24+/-0.49)x10(5) mol-1 L.     

Theoretical Studies on the Interactions of Cations with Diazine

1 INTRODUCTION Interaction between cations and π electron systems is a kind of common and important non-covalent inter- action[1～21]. It could be observed in many systems, such as gas ion-molecule complexes and biological macromolecular proteins. Especially, in biological macromolecular systems, this interaction is signi- ficant for revealing the structures and functions of proteins or disclosing the atomic essence of protein- ligand effect. Interaction between benzene and metal ions h…     

2'-硼酸基苯甲醛-7-(8-羟基-5-磺酸基)喹啉腙衍生物的合成及对Pb2+的识别

合成了2', 3'和4'-硼酸基苯甲醛-7-(8-羟基-5-磺酸基)喹啉腙衍生物(化合物1~3), 研究了硼酸基团取代位置对主体分子识别金属离子客体性能的影响, 比较了不同主体分子与Pb2+结合能力的差异. 研究结果表明, 在pH=7.0的KH2PO4-NaOH缓冲溶液中, 3种腙衍生物对Pb2+均具有选择性识别作用, 主客体分子间形成1∶1型的发光配合物. 其中邻位取代的化合物1与Pb2+的结合能力比化合物2和3强, 配合物1-Pb2+的最大发射波长为477 nm, 稳定常数为1.1×103 L/mol. 其它金属离子如Cu2+, Mn2+, Mg2+, Fe2+, Ca2+, Co2+, Ni2+, Hg2+, Cd2+和Ag+ 等对主体分子荧光光谱的影响较小. 同时, 荧光强度的变化值与Pb2+浓度在0.36~10 μmol/L范围内呈现良好的线性关系, 相关系数R=0.9976(n=16), 检出限为0.23 μmol/L. 将此方法用于环境水样中Pb2+的测定, 回收率为92%~108%.     

Experimental and theoretical study of the vibrational spectra of 12-crown-4-alkali metal cation complexes

The vibrational, Raman, and IR, spectra of the five 12-crown-4 (12c4) complexes with Li+, Na+, K+, Rb+, and Cs+ alkali metal cations were measured. Except for a small shift of the position of some bands in the vibrational spectra of the Li+ complex, the vibrational spectra of the five complexes are so similar that it is concluded that the five complexes exist in the same conformation. B3LYP/6-31+G* force fields were calculated for six of the eight predicted conformations in a previous report (J. Phys. Chem. A 2005, 109, 8041) of the 12c4-Li+, Na+, and K+ complexes that are of symmetries higher than the C1 symmetry. These six conformations, in energy order, are of C4, Cs, Cs, C(2v), C(2v), and Cs symmetries. Comparison between the experimental and calculated vibrational frequencies assuming any of the above-mentioned six conformations shows that the five complexes exist in the C4 conformation. This agrees with the fact that the five alkali metal cations are larger than the 12c4 ring cavity. The B3LYP/6-31+G* force fields of the C4 conformation of the Li+, Na+ and K+ complexes were scaled using a set of eight scale factors and the scale factors were varied so as to minimize the difference between the calculated and experimental vibrational frequencies. The root-mean-square (rms) deviations of the calculated frequencies from the experimental frequencies were 7.7, 5.6, and 5.1 cm(-1) for the Li+, Na+, and K+ complexes, respectively. To account for the earlier results of the Li+ complex that the Cs conformation is more stable than the C4 conformation by 0.16 kcal/mol at the MP2/6-31+G* level, optimized geometries of the complex were calculated for the C4 and Cs conformations at the MP2/6-311++G** level. The C4 conformation was calculated to be more stable than the Cs conformation by 0.13 kcal/mol.     

Selective interaction of lower rim calix[4]arene derivatives and bivalent cations in solution. Crystallographic evidence of the versatile behavior of acetonitrile in lead(II) and cadmium(II) complexes

The interaction of lower rim calix(4)arene derivatives containing ester (1) and ketone (2) functional groups and bivalent (alkaline-earth, transition- and heavy-metal) cations has been investigated in various solvents (methanol, N,N-dimethylformamide, acetonitrile, and benzonitrile). Thus, 1H NMR studies in CD3OD, C3D7NO, and CD3CN show that the interaction of these ligands with bivalent cations (Mg2+, Ca2+, Sr2+, Ba2+, Hg2+, Pb2+, Cd2+) is only observed in CD3CN. These findings are corroborated by conductance measurements in these solvents including benzonitrile, where changes upon the addition of the appropriate ligand (1 or 2) to the metal-ion salt only occur in acetonitrile. Thus, in this solvent, plots of molar conductance against the ligand/metal cation ratio reveal the formation of 1:1 complexes between these ligands and bivalent cations. Four metal-ion complex salts resulting from the interaction of 1 and 2 with cadmium and lead, respectively, were isolated and characterized by X-ray crystallography. All four structures show an acetonitrile molecule sitting in the hydrophobic cavity of the ligand. The mode of interaction of the neutral guest in the cadmium(II) complexes differs from each other and from that found in the lead(II) complexes and provides evidence of the versatile behavior of acetonitrile in binding processes involving calix(4)arene derivatives. The thermodynamics of complexation of these ligands and bivalent cations in acetonitrile is reported. Thus, the selective behavior of 1 and 2 for bivalent cations is for the first time demonstrated. The role of acetonitrile in the complexation process in solution is discussed on the basis of 1H NMR and X-ray crystallographic studies. It is suggested that the complexation of 1 and 2 with bivalent cations is likely to involve the ligand-solvent adducts rather than the free ligand. Plots of complexation Gibbs energies against the corresponding data for cation hydration show a selectivity peak which is explained in terms of the predominant role played by cation desolvation and ligand binding energy in complex formation involving metal cations and macrocycles in solution. A similar peak is found in terms of enthalpy suggesting that for most cations (except Mg2+) the selectivity is enthalpically controlled. The ligand effect on the complexation process is quantitatively assessed. Final conclusions are given highlighting the role of the solvent in complexation processes involving calix(4)arene derivatives and metal cations.     

Cation sensors containing a (bpy)Re(CO)3 group linked to an azacrown ether via an alkenyl or alkynyl spacer: synthesis, characterisation, and complexation with metal cations in solution

Two [(bpy)Re(CO)3L]+ complexes (bpy = 2,2'-bipyridine), where L contains an aza-15-crown-5 ether which is linked to Re via an alkenyl- or alkynyl-pyridine spacer, have been synthesised along with model complexes. Solutions of the complexes in acetonitrile have been studied by UV-Vis absorption spectroscopy, and by 1D and 2D 1H NMR spectroscopy. Strong UV-Vis bands, assigned to intraligand charge-transfer transitions localised at the L ligands, blue shift on protonation of the azacrown nitrogen atom or on complexation of alkali-metal (Li+, Na+ and K+) or alkaline-earth metal (Mg2+, Ca2+ and Ba2+) cations to the azacrown; the magnitude of the blue shift is dependent on the cation, with protonation giving the largest shift of ca. 100 nm. Cation binding constants in the range of log K= 1-4 depend strongly on the identity of the metal cation. Protonation or cation complexation causes downfield shifts in the 1H NMR resonances from most of the azacrown and L ligand protons, and their magnitudes correlate with those of the blue shifts in the UV-Vis bands; shifts in the azacrown 1H NMR resonances report on how the different metal cations interact with the macrocycle. UV-Vis and 1H NMR spectra of the free L ligands enable the effect of the Re centre to be assessed. Together, the data indicate that the alkene spacer gives a more responsive sensor than the alkyne spacer by providing stronger electronic communication across the L ligand.     

Calix[4]quinones derived from double calix[4]arenes: synthesis, complexation, and electrochemical properties toward alkali metal ions

Bis(calix[4]diquinones) 1 and 2 and double calix[4]diquinone 3 have been synthesized from their corresponding double calix[4]arenes 4, 5, and 6, respectively. Compounds 4-6 have been prepared from one-pot and stepwise syntheses under high pressure. Complexation studies of ligands 1-3 with alkali metal ions such as Li+, Na+, K+, and Cs+ were carried out by 1H NMR titrations. Receptors 1 can selectively form 1:1 complexes with Na+. Ligand 2 prefers to form 1:1 complexes with K+ and Cs+. Receptor 3 retained the cone conformation of the calix[4]arene unit upon binding K+ but changed the conformation when complexing Li+ and Na+. Electrochemical studies using cyclic voltammetry and square wave voltammetry showed significant changing of voltammograms of 2 and 3 in the presence of alkali metal ions. Receptor 3 showed the electrochemically switched binding property toward Na+ and K+.