Size Metal Research Articles

Gas phase hydrogen deuterium exchange reactions of a model peptide: FT-ICR and computational analyses of metal induced conformational mutations

We utilized gas phase hydrogen/deuterium (H/D) exchange reactions and ab initio calculations to investigate the complexation between a model peptide (Arg-Gly-AspRGD) with various alkali metal ions. The peptide conformation is drastically altered upon alkali metal ion complexation. The associated conformational changes depend on both the number and type of complexing alkali metal ions. Sodium has a smaller ionic diameter and prefers a multidentate interaction that involves all three amino acids of the peptide. Conversely, potassium and cesium form different types of complexes with the RGD. The [RGD + 2Cs − H] + species exhibit the slowest H/D exchange reactivity (reaction rate constant of ∼6 × 10 −13 cm 3molecule −1s −1 for the fastest exchanging labile hydrogen with ND 3). The reaction rate constant of the protonated RGD is two orders of magnitude faster than that of the [RGD + 2Cs − H] +. Addition of the first cesium to the RGD reduces the H/D exchange reaction rate constant (i.e., D 0) by a factor of seven whereas sodium reduces this value by a factor of thirty. Conversely, addition of the second alkali metal ions has the opposite effect; the rate of D 0 disappearance for all [RGD + 2Met − H] + species (MetNa, K, and Cs) decreases with the alkali metal ion size.

Complexation of a four-strand tetraalcohol with labile metal ions probed by electrospray mass spectrometry

The saturated, stereodefined tetraalcohol 2,3,5,6-endo,endo,endo,endo-tetrakis(hydroxymethyl)bicyclo[2.2.1]heptane (tetol, L1) and the simple alcohol butane-1,3-diol (L2) form complexes with alkali metal ions (lithium, sodium, potassium, rubidium and caesium), alkali earth cations (magnesium, calcium, strontium and barium) and Ga(III) and Ce(IV) in aqueous solution, characterised by electrospray ionisation mass spectrometry (ESMS). Metal ion exchange between the Li+ complex of L1 and the other metal ions is rapid, with a range of M(L1)nm+ species detected, in addition to solvated species. With the alkal metal ions, M(L1)+ and M(L1)2+ are dominant, although speciation varies with metal ion size. For the alkaline earth ions, a range of complex ions up to n=8 are observed, although n=1–3 dominate. A preference for M(L1)22+ with Mg2+ versus M(L1)32+ with Ca2+ may again relate to a larger ion size. For the higher-charged Ga(III) and Ce(IV), hydroxo species M(OH)(L1)n(m−1)+ are dominant reflecting bulk solution behaviour, which the ESMS studies appear to map generally.

The Strain-Driven Pyrochlore to “Defect Fluorite” Phase Transition in Rare Earth Sesquioxide Stabilized Cubic Zirconias

The relationship between the ordering characteristic of the pyrochlore structure type and that characteristic of the “defect fluorite” structure type (immediately on either side of two phase regions separating the two structure types) in a range of rare eath sesquioxide stabilized cubic zirconias is investigated via electron diffraction and imaging. Systematic structural change as a function of composition and relative size of the constituent metal ions is highlighted and a multi-q to single-q=1/2 [111]* model proposed for the observed pyrochlore to defect fluorite phase transition. Strain introduced into the close-packed {111} metal ion planes of the defect fluorite average structure by the local cation and oxygen vacancy distribution is pointed to as the likely origin of the observed behavior.

Rapid kinetic and isotopic studies on dialkylglycine decarboxylase.

The two half-reactions of the pyridoxal 5'-phosphate (PLP)-dependent enzyme dialkylglycine decarboxylase (DGD) were studied individually by multiwavelength stopped-flow spectroscopy. Biphasic behavior was found for the reactions of DGD-PLP, consistent with two coexisting conformations observed in steady-state kinetics [Zhou, X., and Toney, M. D. (1998) Biochemistry 37, 5761--5769]. The half-reaction kinetic parameters depend on alkali metal ion size in a manner similar to that observed for steady-state kinetic parameters. The fast phase maximal rate constant for the 2-aminoisobutyrate (AIB) decarboxylation half-reaction with the potassium form of DGD-PLP is 25 s(-1), while that for the transamination half-reaction between DGD-PMP and pyruvate is 75 s(-1). The maximal rate constant for the transamination half-reaction of the potassium form of DGD-PLP with L-alanine is 24 s(-1). The spectral data indicate that external aldimine formation with either AIB or L-alanine and DGD-PLP is a rapid equilibrium process, as is ketimine formation from DGD-PMP and pyruvate. Absorption ascribable to the quinonoid intermediate is not observed in the AIB decarboxylation half-reaction, but is observed in the dead-time of the stopped-flow in the L-alanine transamination half-reaction. The [1-(13)C]AIB kinetic isotope effect (KIE) on k(cat) for the steady-state reaction is 1.043 +/- 0.003, while a value of 1.042 +/- 0.009 was measured for the AIB half-reaction. The secondary KIE measured for the AIB decarboxylation half-reaction with [C4'-(2)H]PLP is 0.92 +/- 0.02. The primary [2-(2)H]-L-alanine KIE on the transamination half-reaction is unity. Small but significant solvent KIEs are observed on k(cat) and k(cat)/K(M) for both substrates, and the proton inventories are linear in each case. NMR measurements of C2--H washout vs product formation give ratios of 105 and 14 with L-alanine and isopropylamine as substrates, respectively. These results support a rate-limiting, concerted C alpha-decarboxylation/C4'-protonation mechanism for the AIB decarboxylation reaction, and rapid equilibrium quinonoid formation followed by rate-limiting protonation to the ketimine intermediate for the L-alanine transamination half-reaction. Energy profiles for the two half-reactions are constructed.

A Schiff-base bibracchial lariat ether selective receptor for lanthanide(III) ions

The bibracchial lariat ether N,N′-bis(2-salicylaldiminobenzyl)-4,13-diaza-18-crown-6 (L), in its di-deprotonated form, behaves as a selective receptor for lanthanide(III) ions . It only forms stable complexes with the three lightest members of the lanthanide series, lanthanum(III), cerium(III) and praseodymium(III). The X-ray crystal structures of these three complexes show the metal ion ten-coordinated and deeply buried in the cavity of the dianionic receptor. In all three structures the lariat ether presents a syn arrangement. However, the different size of the encapsulated metal ion modifies the conformation of the receptor, particularly in the fold of the crown, resulting in a very different coordination polyhedron for La(III) (distorted tetracapped trigonal prism) with respect to that of Ce(III) and Pr(III) (bicapped square antiprism). Spectrophotometric titration of the dianionic lariat ether with anhydrous La(ClO4)3 in acetonitrile allowed us to calculate log K[La(L − 2H)] = 4.65(4).

Phthalocyanines: structure and vibrations

Raman scattering for a range of metal phthalocyanines using excitation frequencies from 457.9 to 1064 nm have been interpreted in the light of the recent DFT calculation on zinc phthalocyanine. The intention was to determine the features of the spectra that might be used for in situ analysis of specific phthalocyanines. B1 bands were found to be the most intense. The frequency of one band with a large C–N–C ring displacement is particularly sensitive to the metal ion size. The effect is determined not only by the size of the ion but also by the effect it has on ring shape. First and second order overtone bands of zinc and copper phthalocyanines are broadly similar, with some coupling differences at about 2000 cm−1. The region between 1350 and 1550 cm−1 has been little studied previously. It shows a remarkable sensitivity to the metal ion present and provides a specific signature for each phthalocyanine studied. In contrast, a study of α-, β-, γ- and e-copper phthalocyanines using 514.5 nm excitation showed very few differences, suggesting that intra- rather than intermolecular markers are most efficiently determined by Raman scattering. The study enables the interpretation of the Raman spectra of the phthalocyanines in terms of molecular structure and due to the resonant enhancement involved will enable the in situ identification of specific phthalocyanines in matrices.

Synthesis, aggregation, and binding behavior of synthetic amphiphilic receptors.

Amphiphilic bowl-shaped receptor molecules have been synthesized starting from diphenylglycoluril. Upon dispersion in water, these molecules self-assemble to form vesicles that bind neutral guests and alkali metal ions. In the case of bis(alkylester)-modified receptor compound 4, electron microscopy reveals that an increase in the size of the alkali metal ion (from Na(+) or K(+) to Rb(+) and to Cs(+)) leads to a change in the shape of the aggregates, viz. from vesicles to tubules. Monolayer experiments suggest that this behavior is due to a change in the conformation of this amphiphilic receptor. In water, molecules of 4 have an elongated conformation that changes to a sandwich-like one upon binding of alkali metal ions. Binding studies with vesicles from the bis-ammonium receptors 6 and 9 and the guest 4-(4-nitrophenylazo)resorcinol (Magneson) reveal that below the critical aggregation concentration (CAC) of the amphiphile 1:1 host-guest complexes are formed with high host-guest association constants. Above the CAC, a host-guest ratio of 2:1 was observed that indicates that only the cavities on the outside of the vesicle can be occupied. In the case of the naphthalene walled compound 8 changes in the vesicle structure are induced by the organic guest Magneson.

Removal of Cu(II), Pb(II), and Ni(II) by Adsorption onto Activated Carbon Cloths

The adsorption of three metal ions, Cu(II), Ni(II), and Pb(II), is performed by activated carbon cloths (ACC). Two adsorbents, CS 1501 (with more than 96% of micropore volume) and RS 1301 (with 32% of mesopore volume), are studied. Batch experiments are carried out to assess kinetic and equilibrium parameters. They allow kinetic data, transfer coefficients, and maximum adsorption capacities to be computed. These parameters show the fast external film transfer of metal ions on fibers, because of their low diameter (10 μm). Intraparticular diffusion coefficients are lower than those obtained with a granular activated carbon, but maximum adsorption capacities agree with literature values for GAC. They show the dependency of adsorption on metal ion size and ACC porosity, the largest cation Pb(II) being more adsorbed by the mesoporous cloth. The pH effect is studied, and pH adsorption edges are determined. They are short, only 2 pH units, and located below the precipitation edges. A decrease of equilibrium pH ...

NiII and ZnII complexes of the hexadentate macrocyclic ligand cis-6, 13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine

The title pendent-arm macrocyclic hexa­amine ligand binds stereospecifically in a hexadentate manner, and we report here its isomorphous NiII and ZnII complexes (both as perchlorate salts), namely (cis-6,13-di­methyl-1,4,8,11-tetra­aza­cyclo­tetra­decane-6,13-di­amine-κ6N)­nickel(II) di­per­chlorate, [Ni(C12H30N6)]­­(ClO4)2, and (cis-6,13-di­methyl-1,4,8,11-tetraaza-cyclo­tetra­decane-6,13-di­amine-κ6N)­zinc(II) di­per­chlorate, [Zn(C12H30N6)]­(ClO4)2. Distortion of the N—M—N valence angles from their ideal octahedral values becomes more pronounced with increasing metal-ion size and the present results are compared with other structures of this ligand.

The NiII, HgII and CuII complexes of 12-membered-ring mixed-donor macrocycles

The structures of di­aqua(1,7-dioxa-4-thia-10-aza­cyclo­do­decane)­nickel dinitrate, [Ni(C8H17NO2S)(H2O)2](NO3)2, (I), bis­(nitrato-O,O′)(1,4,7-trioxa-10-aza­cyclo­do­decane)­mercury, [Hg(NO3)2(C8H17NO3)], (II), and aqua­(nitrato-O)(1-oxa-4,7,10-tri­aza­cyclo­do­decane)copper nitrate, [Cu(NO3)(C8H19N3O)(H2O)]NO3, (III), reveal each macrocycle binding in a tetradentate manner. The conformations of the ligands in (I) and (III) are the same and distinct from that identified for (II). These differences are in agreement with molecular-mechanics predictions of ligand conformation as a function of metal-ion size.

The NiII, HgII and CuII complexes of 12-membered-ring mixed-donor macrocycles

The structures of di­aqua(1,7-dioxa-4-thia-10-aza­cyclo­do­decane)­nickel dinitrate, [Ni(C8H17NO2S)(H2O)2](NO3)2, (I), bis­(nitrato-O,O′)(1,4,7-trioxa-10-aza­cyclo­do­decane)­mercury, [Hg(NO3)2(C8H17NO3)], (II), and aqua­(nitrato-O)(1-oxa-4,7,10-tri­aza­cyclo­do­decane)copper nitrate, [Cu(NO3)(C8H19N3O)(H2O)]NO3, (III), reveal each macrocycle binding in a tetradentate manner. The conformations of the ligands in (I) and (III) are the same and distinct from that identified for (II). These differences are in agreement with molecular-mechanics predictions of ligand conformation as a function of metal-ion size.

Compatible-solute-supported periplasmic expression of functional recombinant proteins under stress conditions.

The standard method of producing recombinant proteins such as immunotoxins (rITs) in large quantities is to transform gram-negative bacteria and subsequently recover the desired protein from inclusion bodies by intensive de- and renaturing procedures. The major disadvantage of this technique is the low yield of active protein. Here we report the development of a novel strategy for the expression of functional rIT directed to the periplasmic space of Escherichia coli. rITs were recovered by freeze-thawing of pellets from shaking cultures of bacteria grown under osmotic stress (4% NaCl plus 0.5 M sorbitol) in the presence of compatible solutes. Compatible solutes, such as glycine betaine and hydroxyectoine, are low-molecular-weight osmolytes that occur naturally in halophilic bacteria and are known to protect proteins at high salt concentrations. Adding 10 mM glycine betaine for the cultivation of E. coli under osmotic stress not only allowed the bacteria to grow under these otherwise inhibitory conditions but also produced a periplasmic microenvironment for the generation of high concentrations of correctly folded rITs. Protein purified by combinations of metal ion affinity and size exclusion chromatography was substantially stabilized in the presence of 1 M hydroxyecotine after several rounds of freeze-thawing, even at very low protein concentrations. The binding properties and cytotoxic potency of the rITs were confirmed by competitive experiments. This novel compatible-solute-guided expression and purification strategy might also be applicable for high-yield periplasmic production of recombinant proteins in different expression systems.

Towards achieving selectivity in metal ion binding by fixing ligand–chelator complex geometry in polymers

The synthesis of polymers with pre-organized metal ion binding sites for optimal binding of selected metal ions is reported. An acyclic N-vinylbenzyl substituted chelator, triethylenetetramine, is complexed with copper(II) ions and crosslinked with matrix monomer TRIM [2-ethyl-2-(hydroxymethyl) propane-1, 3-diol trimethacrylate] to form the polymer. UV–visible spectra indicate that the coordination geometry of the monomeric metal–ligand complex is highly conserved in the polymer. Unlike macrocyclic ligands, the acyclic ligating molecules demonstrate flexibility in forming chelation rings that are specific to metal-ion size and geometry. The complexation step is used as a means to optimize the chelator conformation for copper ions. The resulting polymers, after removal of metal ions, show selectivity towards the templated metal ions. The small size of the metal ions facilitated their access to the templated sites embedded in the porous polymers. This scheme has the potential to become a generalized procedure for making metal ion selective polymers.

Complexation equilibria of oxy-acid–2-amino-2-deoxy- d-gluconic acid–metal(II) ion ternary systems in aqueous solution as studied by potentiometry. Binding characteristics of borate and germanate

The complexation equilibria for the boric acid– d-glucosaminic acid and d-glucosaminic acid–metal(II) ion binary systems, the boric acid– d-glucosaminic acid–metal(II) ion ternary system, and the corresponding binary and ternary systems with germanic acid instead of boric acid involving nickel(II), zinc(II), cadmium(II) and lead(II) as the metal(II) ion have been investigated in aqueous solution at 25°C and I=0.1 mol dm −3 (NaClO 4) by potentiometric measurements ( d-glucosaminic acid is 2-amino-2-deoxy- d-gluconic acid). The complexes have been characterized in aqueous solution by 13C NMR spectra and the p K a values. The 1:2 ester of borate and d-glucosaminate shows a selectivity for metal(II) ions different from the free d-glucosaminate ligand. It has been demonstrated that the coordination site of the 1:2 ester can distinguish the difference in metal ion size. Such selectivity for metal(II) ions in the boric acid system is discussed by comparison with the germanic acid system.

Synthesis, Spectroscopic, and Electrochemical Properties of Rare Earth Double-Deckers with Tetra(tert-butyl)-2,3-naphthalocyaninato Ligands

A series of rare earth(III) double-deckers RE[Nc(tBu)4]2 [RE = La, Ce, Pr, Nd, Eu, Gd, Tb, Y, Er; Nc(tBu)4 = dianion of tetra(tert-butyl)-2,3-naphthalocyanine] (1–9) have been prepared by treating RE(acac)3·n H2O (acac = acetylacetonate) with 6-tert-butylnaphthalonitrile in refluxing n-octanol in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). These novel sandwich-type complexes have been spectroscopically characterized. The electrochemical studies show that the first oxidation and the first reduction potentials increase with the size of the central metal ions with a relatively small separation (0.28–0.33 V), reflecting the narrow HOMO–LUMO gap.

IR and Raman assignments for zinc phthalocyanine from DFT calculations

Density functional theory (DFT) calculations have been used to predict the IR and Raman spectra for zinc phthalocyanine (ZnPc). There is very good agreement in the frequency data (RMS error of 10 cm−1 for IR and 12 cm−1 for Raman scattering, with half of the bands in both cases predicted to within 6 cm−1). The precision of the data makes it possible to provide reliable frequency assignments and hence to interpret vibrational changes in terms of structural modifications. The sensitivity of the three IR bands between 700 and 800 cm−1 to the phthalocyanine polymorphic form is explained by the assignment of two modes to out-of-plane vibrations and one mode to an in-plane vibration. The out-of-plane vibrations are expected to be more sensitive to the solid-state packing. The most intense band in the Raman scattering, predicted at 1517 cm−1 for ZnPc, has large displacements on the C–N–C bridge bonds and shifts significantly with metal ion size, making it a good marker for changes in the metal ion environment.

Structure of Cationized Arginine (Arg·M+, M = H, Li, Na, K, Rb, and Cs) in the Gas Phase: Further Evidence for Zwitterionic Arginine

The gas-phase structures of cationized arginine, Arg.M(+), M = Li, Na, K, Rb, and Cs, were studied both by hybrid method density functional theory calculations and experimentally using low-energy collisionally activated and thermal radiative dissociation. Calculations at the B3LYP/LACVP++** level of theory show that the salt-bridge structures in which the arginine is a zwitterion (protonated side chain, deprotonated C-terminus) become more stable than the charge-solvated structures with increasing metal ion size. The difference in energy between the most stable charge-solvated structure and salt-bridge structure of Arg.M(+) increases from -0.7 kcal/mol for Arg.Li(+) to +3.3 kcal/mol for Arg.Cs(+). The stabilities of the salt-bridge and charge-solvated structures reverse between M = Li and Na. These calculations are in good agreement with the results of dissociation experiments. The low-energy dissociation pathways depend on the cation size. Arginine complexed with small cations (Li and Na) loses H(2)O, while arginine complexed with larger cations (K, Rb, and Cs) loses NH(3). Loss of H(2)O must come from a charge-solvated ion, whereas the loss of NH(3) can come from the protonated side chain of a salt-bridge structure. The results of dissociation experiments using several cationized arginine derivatives are consistent with the existence of these two distinct structures. In particular, arginine methyl esters, which cannot form salt bridges, dissociate by loss of methanol, analogous to loss of H(2)O from Arg.M(+); no loss of NH(3) is observed. Although dissociation experiments probe gas-phase structure indirectly, the observed fragmentation pathways are in good agreement with the calculated lowest energy isomers. The combination of the results from experiment and theory provides strong evidence that the structure of arginine-alkali metal ion complexes in the gas phase changes from a charge-solvated structure to a salt-bridge structure as the size of the metal ion increases.

Complexes of 1-thia-4,7,10-triazacyclododecane ([12]aneN 3S) : Synthesis, structure and stability constants

The 12-membered macrocyclic ligand 1-thia-4,7,10-triazacyclododecane ([12]aneN 3S) has been synthesised, although upon crystallization from acetonitrile a product in which carbon dioxide had added to one secondary amine in the macrocyclic ring (H[12]aneN 3S–CO 2·H 2O) was isolated and subsequently characterised by X-ray crystallography. The protonation constants for [12]aneN 3S and stability constants with Zn(II), Pb(II), Cd(II) and Cu(II) have been determined either potentiometrically or spectrophotometrically in aqueous solution, and compared with those measured or reported for the ligands 1-oxa-4,7,10-triazacyclododecane ([12]aneN 3O) and 1,4,7,10-tetraazacyclododecane ([12]aneN 4). The magnitudes of the stability constants are consistent with trends observed previously for macrocyclic ligands as secondary amine donors are replaced with oxygen and thioether donors although the stability constant for the [Hg([12]aneN 4)] 2+ complex has been estimated from an NMR experiment to be at least three orders of magnitude larger than reported previously. Zinc(II), mercury(II), lead(II), copper(II) and nickel(II) complexes of [12]aneN 3S have been isolated and characterised by X-ray crystallography. In the case of copper(II), two complexes [Cu([12]aneN 3S)(H 2O)](ClO 4) 2 and [Cu 2([12]aneN 3S) 2(OH) 2](ClO 4) 2 were isolated, depending on the conditions employed. Molecular mechanics calculations have been employed to investigate the relative metal ion size preferences of the [3333], asym-[2424] and sym-[2424] conformation isomers. The calculations predict that the asym-[2424] conformer is most stable for M–N bond lengths in the range 2.00–2.25 Å whilst for the larger metal ions the [3333] conformer is dominant. The disorder seen in the structure of the [Zn([12]aneN 3S)(NO 3)] + complex is also explained by the calculations.

Tuning of Magnetic Anisotropy in Hexairon(III) Rings by Host-Guest Interactions: An Investigation by High-Field Torque Magnetometry

Full chemical control of magnetic anisotropy in hexairon(III) rings can be achieved by varying the size of the guest alkali metal ion. Dramatically different anisotropies characterize the Li(I) and Na(I) complexes of [Fe(6)(OMe)(12)(L)(6)] (L=1,3-propanedione derivatives; a schematic representation of the Li(I) complex is shown), as revealed by high-field torque magnetometry-Iron: (g), oxygen: o, carbon: o, Li(+): plus sign in circle.

Bent Tridentate Receptors in Calamitic Mesophases with Predetermined Photophysical Properties: New Luminescent Lanthanide-Containing Materials

A new synthetic strategy has been developed to introduce bent and rigid tridentate 2,6-bis(benzimidazol-2‘-yl)pyridine cores into rodlike ligands L11-17. The crystal structure of the nonmesogenic ligand L13 (C39H37N5O4, triclinic, P, Z = 2) shows the expected trans−trans conformation of the tridentate binding unit, which provides a linear arrangement of the semirigid aromatic sidearms. The crystal structure of the related mesogenic ligand L16 (C61H81N5O4, triclinic, P, Z = 2) demonstrates the fully extended conformation adopted by the lipophilic side chains, leading to a slightly helically twisted I-shaped molecule. A rich and varied mesomorphism results which can be combined with the simultaneous tuning of electronic and photophysical properties via a judicious choice of the spacers between the rigid central core and the semirigid lipophilic sidearms. Ligands L13,14 react with Ln(NO3)3·xH2O to give quantitatively and selectively the neutral 1:1 complexes [Ln(Li)(NO3)3] (Ln = La to Lu), which are stable in the solid state at room temperature but partially dissociate in acetonitrile to give the cationic species [Ln(Li)(NO3)2]+. The crystal structure of [Lu(L13)(NO3)3]·3CH3CN (30, LuC45H46N11O13, monoclinic, C2/c, Z = 8) reveals a U-shaped arrangement of the ligand strand arising from the cis−cis conformation of the coordinated tridentate binding unit. This drastic geometric change strongly affects the thermal behavior and the photophysical and electronic properties of the lipophilic complexes [Ln(L14)(NO3)3]. Particular attention has been focused on structure−properties relationships, which can be modulated by the size of the lanthanide metal ions.