A computational study on pi and sigma modes of metal ion binding to heteroaromatics (CH)(5-m)X(m) and (CH)(6-m)X(m) (X = N and P): contrasting preferences between nitrogen- and phosphorous-substituted rings

The pi and sigma complexation energy of various heteroaromatic systems which include mono-, di-, and trisubstituted azoles, phospholes, azines and phosphinines with various metal ions, viz. Li(+), Na(+), K(+), Mg(2+), and Ca(2+), was calculated at the post Hartree-Fock MP2 level, MP2(FULL)/6-311+G(2d,2p)//MP2/6-31G. The azoles and azines were found to form stronger sigma complexes than the corresponding pi complexes, whereas the phospholes and phosphinines had higher pi complexation energy with Li(+), Mg(2+), and Ca(2+) while their pi and sigma complexation energies were very comparable with Na(+) and K(+). The strongest pi complex among the five-membered heteroaromatic system was that of pyrrole with all the metals except with Mg(2+), while benzene formed the strongest pi complex among the six-membered heterocyclic systems. The nitrogen heterocyclic system 4H-[1,2,4] triazole and pyridazine formed the strongest sigma complex among the five- and six-membered heteroaromatic systems considered. The complexation energy of the pi and sigma complexes of the azoles and azines was found to decrease with the increase in the heteroatom substitution in the ring, while that of phospholes and phosphinines did not vary significantly. The azoles and azines preferred to form sigma complexes wherein the metal had bidentate linkage, while the phospholes and phosphinines did not show binding mode preference. In the sigma complexes of both azoles and phospholes, the metal binds away form the electron-deficient nitrogen or phosphorus center.     

Vibrational analysis of complexes of urate with IA group metal cations (Li+, Na+ and K+)

Vibrational frequency analysis was performed for the complexes of alkali metal cations (Li+, Na+ and K+) with urate in the gas phase. The geometries of all possible metal cation-urate complexes were optimized at the B3LYP/6-311++G(d,p) level. The most stable complex corresponding to the each cation was used for the vibrational frequency analysis including the computation of % potential energy distribution (%PED). For comparison, the vibrational frequency analysis was also performed for the uric acid. The computed results are discussed in terms of the available experimental data. It was revealed that the characteristic stretching vibrational modes corresponding to the metal cation and the interacting nucleophilic sites of urate can be used to identify metals involved in the stone formation in the living system. Changes in different vibrational frequencies of uric acid consequent to the metal cation interactions are discussed.     

Interactions of metal ions with water: ab initio molecular orbital studies of structure, vibrational frequencies, charge distributions, bonding enthalpies, and deprotonation enthalpies. 2. Monohydroxides

The formation and properties of a wide range of metal ion monohydroxides, M(n)(+)[OH(-)], where n = 1 and 2, have been studied by ab initio molecular orbital calculations at the MP2(FULL)/6-311++G**//MP2(FULL)/6-311++G** and CCSD(T)(FULL)/6-311++G**//MP2(FULL)/6-311++G** computational levels. The ions M(n)()(+) are from groups 1A, 2A, 3A, and 4A in the second, third, and fourth periods of the Periodic Table and from the first transition series. Geometrical parameters, vibrational frequencies, atomic charge distributions, orbital occupancies, and bonding enthalpies are reported. The M(n)(+)-O distances are shorter in the hydroxides than in the corresponding hydrates (published previously as Part 1, Inorg. Chem. 1998, 37, 4421-4431) due to a greater electrostatic interaction in the hydroxides. The natural bond orbitals for most of the first-row transition metal ion hydroxides do not contain a formal metal-oxygen bonding orbital; nevertheless the atomic charge distributions show that for both n = 1 and 2 a significant amount of electron density is consistently transferred from the hydroxide ion to the bound metal ion. Deprotonation enthalpies for the hydrates have been evaluated according to the simple dissociation process, M(n)(+)[OH(2)] --> M(n)(+)[OH(-)] + H(+), and also via proton transfer to another water molecule, M(n)(+)[OH(2)] + H(2)O --> M(n)(+)[OH(-)] + H(3)O(+). The drastic reduction in these deprotonation enthalpies as H(2)O molecules are sequentially bonded in the first coordination shell of the metal ion (amounting to 71, 64, 85, and 91 kcal/mol for the bonding of six water molecules to Mg(2+), Ca(2+), Mn(2+), and Zn(2+), respectively) is found to be due to the greater decrease in the bonding enthalpies for the hydroxides relative to the hydrates. Proton transfer to bases other than water, for example side chain groups of certain amino acids, could more than offset the decrease in deprotonation energy due to the filling of the first coordination shell. Linear relationships have been found between the pK(a) values for ionization of the Mg(2+), Ca(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+) aquo ions, and Delta for the bonding of the first water molecule, for the bonding of the hydroxide ion, and for proton dissociation from the monohydrate. Similar relationships have also been found between the pK(a) values and the reciprocal of the M-O bond lengths in both the monohydrates and hydroxides. Thus the ionization of metal hydrates in water echoes the properties of the monomeric species M(n)(+)[OH(2)].     

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

Interactions between metal ions and amino acids are common both in solution and in the gas phase. The effect of metal ions and water on the structure of L-arginine is examined. The effects of metal ions (Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ni(2+), Cu(2+), and Zn(2+)) and water on structures of Arg x M(H2O)m , m = 0, 1 complexes have been determined theoretically by employing the density functional theories (DFT) and using extended basis sets. Of the three stable complexes investigated, the relative stability of the gas-phase complexes computed with DFT methods (with the exception of K(+) systems) suggests metallic complexes of the neutral L-arginine to be the most stable species. The calculations of monohydrated systems show that even one water molecule has a profound effect on the relative stability of individual complexes. Proton dissociation enthalpies and Gibbs energies of arginine in the presence of the metal cations Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ni(2+), Cu(2+), and Zn(2+) were also computed. Its gas-phase acidity considerably increases upon chelation. Of the Lewis acids investigated, the strongest affinity to arginine is exhibited by the Cu(2+) cation. The computed Gibbs energies DeltaG(o) are negative, span a rather broad energy interval (from -150 to -1500 kJ/mol), and are appreciably lowered upon hydration.     

Interactions of metal ions with monoaza crown ethers A15C5 and A18C6 carrying dansyl fluorophore as pendant in acetonitrile solution

The influence of metal cations Li(+), Na(+), K(+), Cs(+), Mg(2+), Ca(2+), Sr(2+), Ba(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Pb(2+) and Al(3+) on the spectroscopic properties of the dansyl (1-dimethylaminonaphthalene-5-sulfonyl) group covalently linked to monoaza crown ethers 1-aza-15-crown-5 (1,4,7,10,-tetraoxa-13-azacyclopentadecane) (A15C5) and 1-aza-crown-6 (1,4,7,10,13-pentaoxa-16-azacyclooctadecane) (A18C6) was investigated by means of absorption and emission spectrophotometry. Interaction of the alkali metal ions with both fluoroionophores is weak, while alkaline earth metal ions interact strongly causing 50 and 85% quenching of dansyl fluorescence of N-(5-dimethylamine-1-naphthalenesulfonylo)-1,4,7,10,-tetraoxa-13-azacyclopentadecane (A15C5-Dns) and N-(5-dimethylamine-1-naphthalenesulfonylo)-1,4,7,10,13-pentaoxa-16-azacyclooctadecane (A18C6-Dns), respectively. The Cu(2+), Pb(2+) and Al(3+) cations interact very strongly with dansyl chromophore, causing a major change in absorption spectrum of the chromophore and forming non-fluorescent complexes. The Co(2+), Ni(2+), Zn(2+), Mg(2+) cations interact moderately with both fluoroionophores causing quenching of dansyl fluorescence by several percent only.     

The significance of the alkene size and the nature of the metal ion in metal-alkene complexes: a theoretical study

Cation interactions with π-systems are a problem of outstanding contemporary interest and the nature of these interactions seems to be quite different for transition and main group metal ions. In this paper, we have systematically analyzed the contrast in the bonding of Cu(+) and main group metal ions. The molecular structures and energetics of the complexes formed by various alkenes (A = C(n)H(2n), n = 2-6; C(n)H(2n- 2), n = 3-8 and C(n)H(2n + 2), n = 5-10) and metal ions (M = Li(+), Na(+), K(+), Ca(2+), Mg(2+), Cu(+) and Zn(2+)) are investigated by employing ab initio post Hartree-Fock (MP2/6-311++G**) calculations and are reported in the current study. The study, which also aims to evaluate the effect of the size of the alkyl portion attached to the π-system on the complexation energy, indicates a linear relationship between the two. The decreasing order of complexation energy with various metal ion-alkene complexes follows the order Zn(2+)-A > Mg(2+)-A > Ca(2+)-A > Cu(+)-A > Li(+)-A > Na(+)-A > K(+)-A. The increased charge transfer and the electron density at (3,-1) intermolecular bond critical point corroborates well with the size of the π-system and the complexation energy. The observed deviation from the linear dependency of the Cu(+)-A complexes is attributed to the dπ→π* back bonding interaction. An energy decomposition analysis via the reduced variational space (RVS) procedure was also carried out to analyze which component among polarization, charge transfer, coulomb and exchange repulsion contributes to the increase in the complexation energy. The RVS results suggest that the polarization component significantly contributes to the increase in the complexation energy when the alkene size increases.     

Comprehensive study on the solvation of mono- and divalent metal cations: Li+, Na+, K+, Be2+, Mg2+ and Ca2+

Hydration of mono- and divalent metal ions (Li(+), Na(+), K(+), Be(2+), Mg(2+) and Ca(2+)) has been studied using the DFT (B3LYP), second-order M?ller-Plesset (MP2) and CCSD(T) perturbation theory as well as the G3 quantum chemical methods. Double-zeta and triple-zeta basis sets containing both (multiple) polarization and diffuse functions were applied. Total and sequential binding energies are evaluated for all metal-water clusters containing 1-6 water molecules. Total binding energies predicted at lower levels of theory are compared with those from the high level G3 calculations, whereas the sequential binding energies are compared with available experimental values. An increase in the quality of the basis set from double-zeta to triple-zeta has a significant effect on the sequential binding energies, irrespective of the geometries used. Within the same group (I or II), the sequential binding energy predictions at the MP2 and B3LYP vary appreciably. We noticed that, for each addition of a water molecule, the change of the M-O distance in metal-water clusters is higher at the B3LYP than at the MP2 level. The charge of the metal ion decreases monotonically as the number of water molecules increase in the complex.     

Many-body potentials for aqueous Li(+), Na(+), Mg(2+), and Al(3+): comparison of effective three-body potentials and polarizable models

Many-body potentials for the aqueous Li(+), Na(+), Mg(2+), and Al(3+) ions have been constructed from ab initio cluster calculations. Pure pair, effective pair, effective three-body, and effective polarizable models were created and used in subsequent molecular dynamics simulations. The structures of the first and second solvation shells were studied using radial distribution functions and angular-radial distribution functions. The effective three-body and polarizable potentials yield similar first-shell structures, while the contraction of the O-O distances between the first and second solvation shells is more pronounced with the polarizable potentials. The definition of the tilt angle of the water molecules around the ions is discussed. When a proper definition is used, it is found that for Li(+), Mg(2+), and Al(3+) the water molecules prefer a trigonal orientation, but for Na(+) a tetrahedral orientation (ion in lone-pair direction) is preferred. The self-diffusion coefficients for the water molecules and the ions were calculated; the ionic values follow the order obtained from experiment, although the simulated absolute values are smaller than experiment for Mg(2+) and Al(3+).     

Poly(amine ester) dendrimer with naphthyl units as a fluorescent chemosensor for Al(III), Cu(II), and Zn(II).

A poly(amine ester) dendrimer with naphthyl units (G1N6) has been synthesized as a fluorescent chemosensor for metal ions. We investigated the metal-ion recognition of G1N6 by adding each of Ag(+), Al(3+), Ba(2+), Ca(2+), Cd(2+), Co(2+), Cu(2+), Fe(3+), Mg(2+), Ni(2+), and Zn(2+) in acetonitrile solution. Large changes were observed in the fluorescence spectra of G1N6 upon the addition of Al(3+), Cu(2+), and Zn(2+).     

Effect of solvation on ion binding to imidazole and methylimidazole

Quantum chemical [MP2(FULL)/6-311++G-(d,p)] calculations are done on the binding of hydrated Li(+), Na(+), K(+), Mg(2+), Cu(+), and Zn(2+) metal ions with biologically relevant heteroaromatics such as imidazole and methylimidazole. The computed interaction energies are found to be in good agreement with the available experimental data. The effect of hydration on hydrogen bonding has been studied in detail and it shows that the hydrogen bond strength between H(2)O···H-N(1) substantially increases in the presence of metal ions. The present study quantifies the cooperativity between M···imidazole (M = Li(+), Na(+), K(+), Mg(2+), Cu(+), and Zn(2+)) and N(1)-H···OH(2) interactions. Topological atoms in molecules (AIM) analysis and charge analysis support the variation in hydrogen-bonding strength and the variation in M···imidazole binding strength. Effect of hydration on N(1)-H stretching frequency is studied, and it shows a clear shift in the stretching frequency after sequential hydration of metal ion as well as the N(1) of imidazole. The present study provides a detailed account on the biologically important M-histidine motif interaction with metal ions, where histidine is modeled by imidazole and methylimidazole.     

Adsorption of tetracycline onto goethite in the presence of metal cations and humic substances

Adsorption of tetracycline, one of the most widely used antibiotics, onto goethite was studied as a function of pH, metal cations, and humic acid (HA) over a pH range 3-10. Five background electrolyte cations (Li(+), Na(+), K(+), Ca(2+), and Mg(2+)) with a concentration of 0.01 M showed little effect on the tetracycline adsorption at the studied pH range. While the divalent heavy metal cation, Cu(2+), could significantly enhance the adsorption and higher concentration of Cu(2+), stronger adsorption was found. The results indicated that different adsorption mechanisms might be involved for the two types of cations. Background electrolyte cations hardly interfere with the interaction between tetracycline and goethite surfaces because they only form weak outer-sphere surface complexes. On the contrary, Cu(2+) could enhance the adsorption via acting as a bridge ion to form goethite-Cu(2+)-tetracycline surface complex because Cu(2+) could form strong and specific inner-sphere surface complexes. HA showed different effect on the tetracycline sorption under different pH condition. The presence of HA increased tetracycline sorption dramatically under acidic condition. Results indicated that heavy metal cations and soil organic matters have great effects on the tetracycline mobility in the soil environment and eventually affect its exposure concentration and toxicity to organisms.     

Surface Complexation Modeling of K(+), NO(3)(-), SO(4)(2-), Ca(2+), F(-), Co(2+), and Cr(3+) Ion Adsorption on Silica Gel

Surface complex formation of K(+), NO(3)(-), SO(4)(2-), Ca(2+), F(-), Co(2+), and Cr(3+) ions was determined on the surface of silica gel. Experimental data obtained by acid-base titration of suspensions were interpreted in terms of the triple-layer model. The value of the deprotonation constant of surface OH could be determined precisely but the protonation constant was rather uncertain. The logarithms of ion pair formation constants for K(+), NO(3)(-), Ca(2+), and SO(4)(2-) adsorbed in the beta-plane are log K(ipM,X) approximately 0, therefore these species can be considered inert ions in the investigated pH range. F(-), Co(2+), and Cr(3+) ions were found to be strongly sorbed in the o-plane. In order to provide a good fit and to obtain parameters independent of their initial values, all possible equilibrium must be accounted for in the models. Copyright 2001 Academic Press.     

Fluorescence and NMR binding studies of N-aryl-N'-(9-methylanthryl)diaza-18-crown-6 derivatives

N-Aryl-N'-(9-methylanthryl)diaza-18-crown-6 derivatives perform as fluorescent photoinduced electron-transfer (PET) sensors with very selective response toward Ca(2+) versus Mg(2+), Na(+), and K(+). The fluorescence intensity was increased by a factor of up to 170 in the presence of Ca(ClO(4))(2). (1)H NMR studies show that metal cations affect these molecules very differently: Ca(2+) has a global effect on each molecule, while Mg(2+) affects part of each molecule, and K(+) and Na(+) affect each molecule moderately, which is very consistent with the fluorescence response.     

Inverse hydrogen bonds between SiH_4 and hydrides of Na,Mg and Be

The optimized geometries of the three complexes between MeHn (Me=Na,Mg,Be;n=1 or 2) and SiH4 have been calculated at the B3LYP/6-311++g**,MP2/6-311++g(3df,3pd) and MP2/aug-cc-pvtz levels,respectively.The red-shift inverse hydrogen bonds (IHBs) based on Si-H,an electron donor,were reported.The calculated binding energies with basis set super-position error (BSSE) correction of the three complexes are-5.98,-8.65 and-3.96 kJ mol-1 (MP2/6-311++g(3df,3pd)),respectively,which agree with the results obtained via M...     

Highly sensitive and selective detection of beryllium ions using a microcantilever modified with benzo-9-crown-3 doped hydrogel

A microcantilever sensor modified by chitosan/gelatin hydrogels that are doped with benzo-9-crown-3 has been developed for the sensitive and selective detection of beryllium ions in an aqueous solution. The microcantilever undergoes bending deflection upon exposure to Be(2+) due to selective absorption of Be(2+) in the hydrogel. The detection limit is 10(-11) M. Other metal ions, such as Li(+), Na(+), K(+), Mg(2+), and Ca(2+), have a marginal effect on the deflection of the microcantilever. The mechanism of the bending is discussed and the results showed that the microcantilever may be used for in situ detection of beryllium.     

Molecular-dynamics simulations of alkaline-earth metal cations in water by atom-bond electronegativity equalization method fused into molecular mechanics

Intermolecular potential for alkaline-earth metal (Be(2+), Mg(2+), and Ca(2+)) cations in water has been derived using the atom-bond electronegativity equalization method fused into molecular mechanics (ABEEM/MM), and it is consistent with what was previously applied to the hydration study of the monovalent cations. Parameters for the effective interaction between a cation and a water molecule were determined, reproducing the ab initio results. The static, dynamic, and thermodynamic properties of Be(2+)(aq), Mg(2+)(aq), and Ca(2+)(aq) were studied using these potential parameters. Be(2+) requires a more complicated form of the potential function than Mg(2+) and Ca(2+) in order to obtain better fits. Strong influences of the twofold charged cations on the structures of the hydration shells and some other properties of aqueous ionic solutions are discussed and compared with the results of a previous study of monovalent cations in water. At the same time, comparative study of the hydration properties of each cation is also discussed. This work demonstrates that ABEEM/MM provides a useful tool in the exploration of the hydration of double-charged cations in water.     

Interactions of diethylenetriaminepentaacetic acid (dtpa) and triethylenetetraaminehexaacetic acid (ttha) with major components of natural waters

The binding ability of diethylene triamine pentaacetate (dtpa(5-)) and triethylene tetraamine hexaacetate (ttha(6-)) ligands towards major components, H(+), Na(+), Mg(2+) and Ca(2+), of natural waters was studied in both single and mixed ionic media at different ionic strengths and at T=25 degrees C. Some measurements, performed in Mg(2+)-Ca(2+) mixtures, allowed us to find the formation of new mixed species MgCa(dtpa), MgCa(ttha) and MgCaH(ttha), here reported for the first time. All the complexes formed in the various systems were characterized in terms of both stoichiometry and stability, and an attempt was made to find general rules for the stability of mixed metal complexes in comparison with that of simple species. To obtain quantitative information on the complexing ability of dtpa and ttha in seawater, measurements have been carried out in artificial seawater ionic medium (Na(+), K(+), Ca(2+), Mg(2+), Cl(-) and SO(4)(2-)). Calculations, performed by considering the salt mixture as single salt BA, allowed us to find some quite stable B(i)H(j)L species. Under the natural seawater conditions [S(salinity)=35], we found for the most important species logbeta( B(dtpa))=9.64 and. Literature data comparison is also reported.     

CO2 adsorption in mono-, di- and trivalent cation-exchanged metal-organic frameworks: a molecular simulation study

A molecular simulation study is reported for CO(2) adsorption in rho zeolite-like metal-organic framework (rho-ZMOF) exchanged with a series of cations (Na(+), K(+), Rb(+), Cs(+), Mg(2+), Ca(2+), and Al(3+)). The isosteric heat and Henry's constant at infinite dilution increase monotonically with increasing charge-to-diameter ratio of cation (Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) < Al(3+)). At low pressures, cations act as preferential adsorption sites for CO(2) and the capacity follows the charge-to-diameter ratio. However, the free volume of framework becomes predominant with increasing pressure and Mg-rho-ZMOF appears to possess the highest saturation capacity. The equilibrium locations of cations are observed to shift slightly upon CO(2) adsorption. Furthermore, the adsorption selectivity of CO(2)/H(2) mixture increases as Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) ≈ Al(3+). At ambient conditions, the selectivity is in the range of 800-3000 and significantly higher than in other nanoporous materials. In the presence of 0.1% H(2)O, the selectivity decreases drastically because of the competitive adsorption between H(2)O and CO(2), and shows a similar value in all of the cation-exchanged rho-ZMOFs. This simulation study provides microscopic insight into the important role of cations in governing gas adsorption and separation, and suggests that the performance of ionic rho-ZMOF can be tailored by cations.     

Development and applications of fluorescent indicators for Mg2+ and Zn2+

In a study of the spectroscopic behavior of two Schiff base derivatives, salicylaldehyde salicylhydrazone (1) and salicylaldehyde benzoylhydrazone (2), Schiff base 1 has high selectivity for Zn(2+) ion not only in abiotic systems but also in living cells. The ion selectivity of 1 for Zn(2+) can be switched for Mg(2+) by swapping the solvent from ethanol-water to DMF (N,N-dimethylformamide)-water mixtures. Imine 2 is a good fluorescent probe for Zn(2+) in ethanol-water media. Many other ions tested, such as Li(+), Na(+), Al(3+), K(+), Ca(2+), Cr(3+), Mn(2+), Fe(3+), Co(2+), Ni(2+), Cu(2+), Ag(+), Cd(2+), Sn(2+), Ba(2+), Hg(2+), and Pb(2+), failed to induce any spectral change in various solvents. The selectivity mechanism of 1 and 2 for metal ions is based on a combinational effect of proton transfer (ESPT), C═N isomerization, and chelation-enhanced fluorescence (CHEF). The coordination modes of the complexes were investigated.