Metal Ion Controlled Self-Assembly of a Chemically Reengineered Protein Drug Studied by Small-Angle X-ray Scattering

Precise control of the oligomeric state of proteins is of central importance for biological function and for the properties of biopharmaceutical drugs. Here, the self-assembly of 2,2'-bipyridine conjugated monomeric insulin analogues, induced through coordination to divalent metal ions, was studied. This protein drug system was designed to form non-native homo-oligomers through selective coordination of two divalent metal ions, Fe(II) and Zn(II), respectively. The insulin type chosen for this study is a variant designed for a reduced tendency toward native dimer formation at physiological concentrations. A small-angle X-ray scattering analysis of the bipyridine-modified insulin system confirmed an organization into a novel well-ordered structure based on insulin trimers, as induced by the addition of Fe(II). In contrast, unmodified monomeric insulin formed larger and more randomly structured assemblies upon addition of Fe(II). The addition of Zn(II), on the other hand, led to the formation of small quantities of insulin hexamers for both the bipyridine-modified and the unmodified monomeric insulin. Interestingly, the location of the bipyridine-modification significantly affects the tendency to hexamer formation as compared to the unmodified insulin. Our study shows how combining a structural study and chemical design can be used to obtain molecular understanding and control of the self-assembly of a protein drug. This knowledge may eventually be employed to develop an optimized in vivo drug release profile.     

Perfluoroalkyl chains direct novel self-assembly of insulin

The self-assembly of biopharmaceutical peptides into multimeric, nanoscale objects, as well as their disassembly to monomers, is central for their mode of action. Here, we describe a bioorthogonal strategy, using a non-native recognition principle, for control of protein self-assembly based on intermolecular fluorous interactions and demonstrate it for the small protein insulin. Perfluorinated alkyl chains of varying length were attached to desB30 human insulin by acylation of the ε-amine of the side-chain of LysB29. The insulin analogues were formulated with Zn(II) and phenol to form hexamers. The self-segregation of fluorous groups directed the insulin hexamers to self-assemble. The structures of the systems were investigated by circular dichroism (CD) spectroscopy and synchrotron small-angle X-ray scattering. Also, the binding affinity to the insulin receptor was measured. Interestingly, varying the length of the perfluoroalkyl chain provided three different scenarios for self-assembly; the short chains hardly affected the native hexameric structure, the medium-length chains induced fractal-like structures with the insulin hexamer as the fundamental building block, while the longest chains lead to the formation of structures with local cylindrical geometry. This hierarchical self-assembly system, which combines Zn(II) mediated hexamer formation with fluorous interactions, is a promising tool to control the formation of high molecular weight complexes of insulin and potentially other proteins.     

Local and global effects of metal binding within the small subunit of ribonucleotide reductase

Each beta-protomer of the small betabeta subunit of Escherichia coli ribonucleotide reductase (R2) contains a binuclear iron cluster with inequivalent binding sites: Fe(A) and Fe(B). In anaerobic Fe(II) titrations of apoprotein under standard buffer conditions, we show that the majority of the protein binds only one Fe(II) atom per betabeta subunit. Additional iron occupation can be achieved upon exposure to O2 or in high glycerol buffers. The differential binding affinity of the A- and B-sites allows us to produce heterobinuclear Mn(II)Fe(II) and novel Mn(III)Fe(III) clusters within a single beta-protomer of R2. The oxidized species are produced with H2O2 addition. We demonstrate that no significant exchange of metal occurs between the A- and B-sites, and thus the binding of the first metal is under kinetic control, as has been suggested previously. The binding of first Fe(II) atom to the active site in a beta-protomer (betaI) induces a global protein conformational change that inhibits access of metal to the active site in the other beta-protomer (betaII). The binding of the same Fe(II) atom also induces a local effect at the active site in betaI-protomer, which lowers the affinity for metal in the A-site. The mixed metal FeMn species are quantitatively characterized with electron paramagnetic resonance spectroscopy. The previously reported catalase activity of Mn2(II)R2 is shown not to be associated with Mn.     

Characterization of an O2 adduct of an active cobalt-substituted extradiol-cleaving catechol dioxygenase

The first example of an O(2) adduct of an active Co-substituted oxygenase has been observed in the extradiol ring cleavage of the electron-poor substrate 4-nitrocatechol (4NC) by Co(II)-homoprotocatechuate 2,3-dioxygenase (Co-HPCD). Upon O(2) binding to the high-spin Co(II) (S = (3)/(2)) enzyme-substrate complex, an S = (1)/(2) EPR signal exhibiting (59)Co hyperfine splitting (A = 24 G) typical of a low-spin Co(III)-superoxide complex was observed. Both the formation and decay of the new intermediate are very slow in comparison to the analogous steps for turnover of 4NC by native high-spin Fe(II)-HPCD, which is likely to remain high-spin upon O(2) binding. A similar but effectively stable S = (1)/(2) intermediate was formed by the inactive [H200N-Co-HPCD(4NC)] variant. The observations presented shed light on the key roles played by the substrate, the second-sphere His200 residue, and the spin state of the metal center in facilitating O(2) binding and activation.     

Electrochemical functions of metallosupramolecular nanomaterials

Self-assembly of metal ions and organic ligands results in the formation of extended or discrete metallosupramolecular structures. In case of neutral ditopic ligands such as bisterpyridines, extended metallosupramolecular coordination polyelectrolytes (MEPEs) are formed. Metal ion-induced self-assembly of 1,4-bis(2,2':6',2'-terpyridin-4'-yl)benzene with Fe(II) or Co(II) results in MEPEs with interesting electrochemical properties. These MEPEs reversibly change their color when oxidized or reduced. The heterometallic MEPE consisting of Fe(II) and Co(II) combines the properties of the individual MEPEs and therefore shows their different states: red-purple, blue, and transparent. On the other hand, complexation of cyclic phenylazomethines with metal ions results in discrete metallosupramolecular structures. We find that metal ion assembly to the organic module occurs in a stepwise fashion because of a difference in the basicity of the imine conformers, and the metal ion assembly can be controlled electrochemically. This example illustrates how metal ion binding can be controlled by the conformation of the receptor, an important step toward assembling organic ligands and metal ions in predictable ways.     

Mechanistic implications for the formation of the diiron cluster in ribonucleotide reductase provided by quantitative EPR spectroscopy

The small subunit of Escherichia coli ribonucleotide reductase (R2) is a homodimeric (betabeta) protein, in which each beta-peptide contains a diiron cluster composed of two inequivalent iron sites. R2 is capable of reductively activating O(2) to produce a stable tyrosine radical (Y122*), which is essential for production of deoxyribonucleotides on the larger R1 subunit. In this work, the paramagnetic Mn(II) ion is used as a spectroscopic probe to characterize the assembly of the R2 site with EPR spectroscopy. Upon titration of Mn(II) into samples of apoR2, we have been able to quantitatively follow three species (aquaMn(II), mononuclear Mn(II)R2, and dinuclear Mn(2)(II)R2) and fit each to a sequential two binding site model. As previously observed for Fe(II) binding within apoR2, one of the sites has a greater binding affinity relative to the other, K(1) = (5.5 +/- 1.1) x 10(5) M(-)(1) and K(2) = (3.9 +/- 0.6) x 10(4) M(-)(1), which are assigned to the B and A sites, respectively. In multiple titrations, only one dinuclear Mn(2)(II)R2 site was created per homodimer of R2, indicating that only one of the two beta-peptides of R2 is capable of binding Mn(II) following addition of Mn(II) to apoR2. Under anaerobic conditions, addition of only 2 equiv of Fe(II) to R2 (Fe(2)(II)R2) completely prevented the formation of any bound MnR2 species. Upon reaction of this sample with O(2) in the presence of Mn(II), both Y122* and Mn(2)(II)R2 were produced in equal amounts. Previous stopped-flow absorption spectroscopy studies have indicated that apoR2 undergoes a protein conformational change upon binding of metal (Tong et al. J. Am. Chem. Soc. 1996, 118, 2107-2108). On the basis of these observations, we propose a model for R2 metal incorporation that invokes an allosteric interaction between the two beta-peptides of R2. Upon binding the first equiv of metal to a beta-peptide (beta(I)), the aforementioned protein conformational change prevents metal binding in the adjacent beta-peptide (beta(II)) approximately 25 A away. Furthermore, we show that metal incorporation into beta(II) occurs only during the O(2) activation chemistry of the beta(I)-peptide. This is the first direct evidence of an allosteric interaction between the two beta-peptides of R2. Furthermore, this model can explain the generally observed low Fe occupancy of R2. We also demonstrate that metal uptake and this newly observed allosteric effect are buffer dependent. Higher levels of glycerol cause loss of the allosteric effect. Reductive cycling of samples in the presence of Mn(II) produced a novel mixed metal Fe(III)Mn(III)R2 species within the active site of R2. The magnitude of the exchange coupling (J) determined for both the Mn(2)(II)R2 and Fe(III)Mn(III)R2 species was determined to be -1.8 +/- 0.3 and -18 +/- 3 cm(-)(1), respectively. Quantitative spectral simulations for the Fe(III)Mn(III)R2 and mononuclear Mn(II)R2 species are provided. This work represents the first instance where both X- and Q-band simulations of perpendicular and parallel mode spectra were used to quantitatively predict the concentration of a protein bound mononuclear Mn(II) species.     

Use of complexing reagents as additives to the eluent for optimization of separation selectivity in high-performance chelation ion chromatography

This paper details a study of the selectivity characteristics of high-performance chelation ion chromatography when separating a range of metal ions with a number of complexing eluents. It shows that exploitation of competitive metal complexation between ligands in the eluent and surface bonded chelating groups allows a wide range of control over the retention order and selectivity coefficients of groups of metal ions for specific applications. An indication of the metal separation characteristics found for simple non-complexing eluents on iminodiacetic acid (IDA) silica bonded substrates is given first, followed by an illustration of the selectivity changes that can be achieved by using complexing eluents. Using a novel approach, plots of logbeta(1) of the metal complexes of a chosen eluent ligand against the surface bonded IDA metal complexes were found to be useful indicators of which metals may show unusual selectivity changes during separation. Example chromatograms of metal separations are given for three selected complexing eluent reagents, namely, oxalic acid, picolinic acid, and chloride, either singly or in admixture. For special mention it was found that very specific retention control could be achieved for Cu(II) with picolinic acid, Fe(III) and Fe(II) speciation with oxalic acid, Pb with dipicolinic acid and Cd with chloride.     

Formation of Microcages from a Collagen Mimetic Peptide via Metal-Ligand Interactions

Here, the hierarchical assembly of a collagen mimetic peptide (CMP) displaying four bipyridine moieties is described. The CMP was capable of forming triple helices followed by self-assembly into disks and domes. Treatment of these disks and domes with metal ions such as Fe(II), Cu(II), Zn(II), Co(II), and Ru(III) triggered the formation of microcages, and micron-sized cup-like structures. Mechanistic studies suggest that the formation of the microcages proceeds from the disks and domes in a metal-dependent fashion. Fluorescently-labeled dextrans were encapsulated within the cages and displayed a time-dependent release using thermal conditions.     

Synthesis of an Alternated Heterobimetallic Supramolecular Polymer Based on Ru(II) and Fe(II)

A heterobimetallic supramolecular polymer (polyRuFe) with alternately complexed Ru(II) and Fe(II) is prepared following a stepwise synthetic route through harnessing first the strongly binding metal ion Ru(II) and then the weakly binding metal ion Fe(II). A high yield of product is achieved in each step. The heterometal ions are incorporated into the polymer chain in identical coordination environments formed by two 2,2′:6′,2″-terpyridine moieties. Characterization is accomplished by NMR spectroscopy, MALDI–TOF mass spectrometry, UV–Vis spectroscopy, and cyclic voltammetry. PolyRuFe shows a wide optical window (λ = 311–577 nm) and a broad distinct reversible redox nature of two types, originated from the coupling of the two heterometallic segments into the polymer chain. Such characteristics of polyRuFe suggest its potential for various electrochemical and electro-optical applications.     

Kinetic analysis of a photosensitive chelator and its complex with Zn(II)

The reversible sequestration and release of metal ions is an important objective in biological and environmental research. Unfortunately, although there have been dramatic examples of metal ion activity control, there are very few quantitative investigations of stoichiometry, equilibria and kinetics. A significant contributor to this lack of quantitative work is the complexity of many photochromic systems. Therefore, we have attempted to create a simple, reversible photochromic metal-ion chelator that can be analyzed quantitatively. The chelator should have certain other attributes as well, namely, that it binds to divalent metal ions (because of their extreme biological importance) and that it binds metal ions in the dark so that light is used to release metal ions rather than sequester them. The photochromic chelator (1) binds to divalent metal ions [Zn(II), Cu(II), Pb(II), Hg(II), Fe(II), Co(II) and Cd(II); other metal ions have not yet been tested] in the dark with a significant binding strength. In both methanol (by spectrophotometry) and methanol-water (by voltammetry), the stoichiometry of the 1-Zn(II) complex is 2:1. The binding constant (K1K2) is on the order of 10(12)-10(14) M(-2) in methanol and 5.0 x 10(8) M(-2) in 50% aqueous methanol. The chelator 1 is photolabile, yielding 2 with a quantum efficiency of 0.91. In a solution containing excess Zn(II), so that over 99% of the ligand exists as the monodentate complex, photolysis produces 2 with a quantum efficiency of 0.15. A kinetic analysis leads to the conclusion that the complex itself is photolabile.     

Self-assembly of supramolecular architectures and polymers by orthogonal metal complexation and hydrogen-bonding motifs

A modular construction kit with two orthogonal noncovalent binding sites for self-assembly of supramolecular architectures is presented. The heteroditopic building blocks contain a terpyridine (tpy) unit for coordination of metal ions and a Hamilton receptor for multiple H-bonding of cyanuric acid derivatives. The association constants of ligand binding of M(II) complexes (M=Ru, Zn, Fe, and Pt) with a dendritic end cap were determined to be in the range of 10(2) and 10(4) L mol(-1) in chloroform. The capabilities for binding of metal ions were investigated by (1)H NMR and UV/Vis spectroscopy. The Fe complexes are most appropriate for the generation of discrete and high-ordered architectures due to their strong tendency to form FeL(2) complexes. Superstructures are readily formed in a one-pot procedure at room temperature. No mutual interactions between the orthogonal binding motifs were observed, and this demonstrates the highly specific nature of each binding process. Decomplexation experiments were carried out to examine the reversibility of Fe-tpy coordination. Substitution of the terminal end cap with a homoditopic bis-cyanurate linkage leads to formation of an iron-containing supramolecular strand. Formation of coordination polymers was confirmed by viscosity measurements. The supramolecular polymer strands can be reversibly cleaved by addition of a terminating cyanuric acid building block, and this proves the dynamic nature of this noncovalent polymerization process.     

Characterization of multimetallic grid-type complexes by electrospray mass spectrometry

The self-assembly of the terdentate ligands 1a-h, based on terpyridine-like binding sites, with octahedrally coordinated metal ions, such as Fe(II), Co(II), Cu(II), Zn(II), Cd(II), Hg(II) and Pb(II), leads to the formation of the supramolecular grid-type complexes 2a-c(M(II)), 3d-g(M(II)) and 4h(M(II)). The structures and compositions of these coordination complexes in solution were deduced from electrospray mass spectrometry (ESMS) measurements. The results agree with the data available from x-ray radiocrystallography in the solid state and/or NMR spectroscopy in solution. ESMS may be applied in cases where other methods are difficult to use or inconclusive. This study stresses the power of ESMS in supramolecular chemistry.     

A polyelectrolyte bearing metal ion receptors and electrostatic functionality for layer-by-layer self-assembly

A polyelectrolyte (BiPE) containing bipyridine ligands as metal ion receptors and quaternary ammonium groups is described, which can be assembled via electrostatic interactions or metal ion coordination. Electrostatic layer-by-layer self-assembly of BiPE with sodium poly(styrene sulfonate) (PSS) as oppositely charged component results in striated multilayers. The BiPE/PSS multilayers can reversibly bind and release transition metal ions including Fe(II), Ni(II), and Zn(II). Formation of 2-D arrays of metallo-units is achieved by μ-contact stamping transition metal salts onto the BiPE/PSS interface. Also, multilayers of BiPE are readily assembled through metal ion coordination. Due to the reversible nature of metal ion coordination, exposure of the multilayers to EDTA causes instant disassembly of the layer, a property needed to implement stimulus triggered release functions. The importance of metal ion coordination for multilayer formation is demonstrated by force-distance curves measured with AFM.     

DNA affinity cleaving : Sequence specific cleavage of DNA by Distamycin-EDTA - Fe(II) and EDTA-distamycin Fe(II)

The attachment of EDTA· Fe(II) to distamycin changes the sequence specific DNA binding antibiotic into a sequence specific DNA cleaving molecule. We report the synthesis of EDTA-distamycin (ED) which has the metal chelator, EDTA, tethered to the carboxy terminus of the N-methylpyrrole tripeptide moiety of the antiobiotic, distamycin. EDTA-distamycin- Fe(II) (ED)·FeII at 10^-6M concentration efficiently cleaves pBR322 DNA (10^-5M in base pairs) in the presence of oxygen and dithiothreitol (DTT). Using Maxam-Gilbert sequencing gel analyses, we find that ED· Fe(II) affords DNA cleavage patterns of unequal intensity covering two to four contiguous base pairs adjacent to a five base pair site consisting of adenines (A) and thymines (T). The multiple cleavages at each site might be evidence for a diffusible oxidizing species, perhaps hydroxyl radical. The unequal intensity of cleavage on each side of the A + T site permit assignment of major and minor orientations of the tripeptide binding unit. A comparison of the cleavage specificity of ED· Fe(II) with distamycin-EDTA· Fe(II), (DE· Fe(II)) which has EDTA · Fe(II) attached to the amino terminus of the N-methylpyrrole tripeptide, shows DNA cleavage patterns at the same sites but with intensities of opposite polarity. Maxam-Gilbert sequencing el analysis of the DNA cleavage patterns by ED Fe(II) and DE Fe(II) on both DNA strands of a 381 se pair restriction fragment reveals asymmetric DNA cleavage patterns. Cleavage is shifted to the 3' de of each DNA strand. A model consistent with this cleavage pattern indicates one preferred binding te for ED Fe(II) and DE Fe(II) is 3'-TTTAA-5' with the “amino end” of the tripeptide oriented to e 3' end of the thymine rich strand. p]This “DNA affinity cleavage” method which consists of attaching cleaving functions to DNA binding molecules followed by DNA cleavage pattern analyses using Maxam-Gilbert sequencing gels may be a useful direct method for determining the binding site and orientation of small molecules on native DNA.     

Electrochromic multilayer films of tunable color by combination of copper or iron complex and monolacunary dawson-type polyoxometalate

Electrochromic multilayer films consisting of polyoxometalate (POM) cluster alpha-K(10)[P(2)W(17)O(61)].17H(2)O (P(2)W(17)), copper(II) complex [Cu(II)(phen)(2)](NO(3))(2) (phen = 1,10-phenithroline), and iron complex [Fe(II)(phen)(3)](ClO(4))(2) were fabricated on silicon, quartz and ITO substrates by layer-by-layer self-assembly method. The multilayer films, PSS/Cu(II)(phen)(2)/[(P(2)W(17)/Cu(II)(phen)(2))](n) and PSS/Fe(II)(phen)(3)/[(P(2)W(17)/Fe(II) (phen)(3))](n) were characterized by UV-vis spectra, X-ray photoelectron spectra, cyclic voltammetry (CV), chronoamperometric (CA) and in-situ spectral electrochemical measurements. The interesting feature of the electrochromic film is its adjustable color by reduction of both transition metal complex and polyoxometalate at different potentials. The multilayer films also exhibit high optical contrast, suitable response time and low operation potential due to the presence of mono-lacunary-substituted polyoxometalate and transition metal complex. This is the first example that the color of electrochromic film can be adjustable, which gives valuable information for exploring new electrochromic materials with tunable colors.     

Fluorescence studies of the kinetics of binding and removal of metal ions in proteins

Fluorescence-quenching studies involving native protein fluorescence are used to monitor the rates of binding and removal of Hg(II), Cu(II), Ag(I), methylmercury(I), and p-chloro-mercuribenzoate in various protein systems (ovalbumin, bovine serum albumin, myoglobin, lysozyme, and insulin). In some cases, the fluorescence quenching as a function of time can be used to evaluate the rate constants for the binding of a particular metal ion to a protein. In many cases, multiple binding sites with different rate constants can be differentiated. The restoration of fluorescence vs. time on addition of various chelating agents (BAL, EDTA, cysteine and penicilamine) to the metal/protein system can be used to monitor metal ion removal. Multiple binding sites also can be differentiated kinetically in the removal experiments. In some cases, the appearance of multiple steps in the binding or removal or a metal or ion could be explained by small conformational changes. The rates of removal can help in estimating the effectiveness of various reagents as models for drugs in the treatment of heavy-metal poisoning.     

Formation of 1 D and 3 D coordination polymers in the solid state induced by mechanochemical and annealing treatments: bis(3-cyano-pentane-2,4-dionato) metal complexes

Bis(3-cyano-pentane-2,4-dionato) (CNacac) metal complex, [M(CNacac)(2)], which acts as both a metal-ion-like and a ligand-like building unit, forms supramolecular structures by self-assembly. Co-grinding of the metal acetates of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) with CNacacH formed a CNacac complex in all cases: mononuclear complex was formed in the cases of Mn(II), Cu(II) and Zn(II), whereas polymeric ones were formed in the cases of Fe(II), Co(II) and Ni(II). Subsequent annealing converted the mononuclear complexes of Mn(II), Cu(II) and Zn(II) to their corresponding polymers as a result of dehydration of the mononuclear complexes. The resultant Mn(II), Fe(II), Co(II), Ni(II) and Zn(II) polymeric complexes had a common 3 D structure with high thermal stability. In the case of Cu(II), a 1 D polymer was obtained. The Mn(II), Cu(II) and Zn(II) polymeric complexes returned to their original mononuclear complexes on exposure to water vapour but they reverted to the polymeric complexes by re-annealing. Co-grinding of metal chlorides with CNacacH and annealing of the mononuclear CNacac complexes prepared from solution reactions were also examined for comparison. [Mn(CNacac)(2)(H(2)O)(2)], [M(CNacac)(2)(H(2)O)] (M=Cu(II) and Zn(II)) and [M(CNacac)(2)](infinity) (M=Mn(II), Fe(II) and Zn(II)) are new compounds, which clearly indicated the power of the combined mechanochemical/annealing method for the synthesis of varied metal coordination complexes.     

Preparation and characterization of the native iron(II)-containing DNA repair AlkB protein directly from Escherichia coli

The Escherichia coli AlkB protein was recently found to repair cytotoxic DNA lesions 1-methyladenine and 3-methylcytosine by using a novel iron-catalyzed oxidative demethylation mechanism. This protein belongs to a family of 2-ketoglutarate-Fe(II)-dependent dioxygenase proteins that utilize iron and 2-ketoglutarate to activate dioxygen for oxidation reactions. We report here the overexpression and isolation of the native Fe(II)-AlkB with a bound cofactor, 2-ketoglutarate, directly from E. coli. UV-vis measurements showed an absorption peak at 560 nm, which is characteristic of a bidentate 2-ketoglutarate bound to an iron(II) ion. Addition of excess amounts of single-stranded DNA to this isolated Fe(II)-AlkB protein caused a 9 nm shift of the 560 nm band to a higher energy, indicating a DNA-binding-induced geometry change of the active site. X-ray absorption spectra of the active site iron(II) in AlkB suggest a five-coordinate iron(II) center in the protein itself and a centrosymmetric six-coordinate iron(II) site upon addition of single-stranded DNA. This geometry change may play important roles in the DNA damage-searching and damage-repair functions of AlkB. These results provide direct evidence for DNA binding to AlkB which modulates the active site iron(II) geometry. The isolation of the native Fe(II)-AlkB also allows for further investigation of the iron(II) center and detailed mechanistic studies of the dioxygen-activation and damage-repair reactions performed by AlkB.     

Switchable supramolecular polymers from the self-assembly of a small monomer with two orthogonal binding interactions

The low molecular weight heteroditopic monomer 1 forms supramolecular polymers in polar solution as shown, for example, by infrared laser-based dynamic light scattering (DLS), small-angle neutron scattering (SANS), electron microscopy (TEM, cryo-TEM), and viscosity measurements. Self-assembly of 1 is based on two orthogonal binding interactions, the formation of a Fe(II)-terpyridine 1:2 metal-ligand complex and the dimerization of a self-complementary guanidiniocarbonyl pyrrole carboxylate zwitterion. Both binding interactions have a sufficient stability in polar (DMSO) and even aqueous solutions to ensure formation of linear polymers of considerable length (up to 100 nm). The supramolecular polymerization follows a ring-chain mechanism causing a significant increase in the viscosity of the solutions at millimolar concentrations and above. The linear polymers then further aggregate in solution into larger globular aggregates with a densely packed core and a loose shell. Both binding interactions can be furthermore switched on and off either by adding a competing ligand to remove the metal ion and subsequent readdition of Fe(II) or by reversible protonation and deprotonation of the zwitterion upon addition of acid or base. The self-assembly of 1 can therefore be switched back and forth between four different states, the monomer, a metal-complexed dimer or an ion paired dimer, and finally the polymer.     

Understanding the mechanism of gelation and stimuli-responsive nature of a class of metallo-supramolecular gels

Utilizing metal-ligand binding as the driving force for self-assembly of a ditopic ligand, which consists of a 2,6-bis-(1'-methylbenzimidazolyl)-4-oxypyridine moiety attached to either end of a penta(ethylene glycol) core, in the presence of a transition metal ion (Zn(II)) and a lanthanide metal ion (La(III)), we have achieved formation of stimuli-responsive metallo-supramolecular gels. We describe herein a series of experimental studies, including optical and confocal microscopy, dynamic light scattering, wide-angle X-ray diffraction, and rheology, to explore the properties of such gels, as well as the nature of the gelation mechanism. Morphological and X-ray diffraction observations suggest gelation occurs via the flocculation of semicrystalline colloidal particles, which results in the gels exhibiting pronounced yielding and thixotropic behavior. Application of mechanical stress results in a decrease in the particle size, which is accompanied by an increase in gel strength after removal of the stress. Moreover, studies show that the presence of lanthanide(III) perchlorate increases the mechano-responsiveness of the gels, as a consequence of reduced crystallinity of the colloidal particles, presumably due to the different coordination ability of lanthanide(III) and zinc(II), which changes the nature of the self-assembly in these materials.