Multivalent Coordination Research Articles

Triggering the nanophase separation of albumin through multivalent binding to glycogen for drug delivery in 2D and 3D multicellular constructs.

Engineered nanoparticles for the encapsulation of bioactive agents hold promise to improve disease diagnosis, prevention and therapy. To advance this field and enable clinical translation, the rational design of nanoparticles with controlled functionalities and a robust understanding of nanoparticle-cell interactions in the complex biological milieu are of paramount importance. Herein, a simple platform obtained through the nanocomplexation of glycogen nanoparticles and albumin is introduced for the delivery of chemotherapeutics in complex multicellular 2D and 3D systems. We found that the dendrimer-like structure of aminated glycogen nanoparticles is key to controlling the multivalent coordination and phase separation of albumin molecules to form stable glycogen-albumin nanocomplexes. The pH-responsive glycogen scaffold conferred the nanocomplexes the ability to undergo partial endosomal escape in tumour, stromal and immune cells while albumin enabled nanocomplexes to cross endothelial cells and carry therapeutic agents. Limited interactions of nanocomplexes with T cells, B cells and natural killer cells derived from human blood were observed. The nanocomplexes can accommodate chemotherapeutic drugs and release them in multicellular 2D and 3D constructs. The drugs loaded on the nanocomplexes retained their cytotoxic activity, which is comparable with the activity of the free drugs. Cancer cells were found to be more sensitive to the drugs in the presence of stromal and immune cells. Penetration and cytotoxicity of the drug-loaded nanocomplexes in tumour mimicking tissues were validated using a 3D multicellular-collagen construct in a perfusion bioreactor. The results highlight a simple and potentially scalable strategy for engineering nanocomplexes made entirely of biological macromolecules with potential use for drug delivery.

MOF × Biopolymer: Collaborative Combination of Metal-Organic Framework and Biopolymer for Advanced Anticancer Therapy.

Metal-organic framework (MOF) nanoparticles with high porosity and greater tunability have emerged as new drug delivery vehicles. However, premature drug release still remains a challenge in the MOF delivery system. Here, we report an enzyme-responsive, polymer-coated MOF gatekeeper system using hyaluronic acid (HA) and PCN-224 nanoMOF. The external surface of nanoMOF can be stably covered by HA through multivalent coordination bonding between the Zr cluster and carboxylic acid of HA, which acts as a gatekeeper. HA allows selective accumulation of drug carriers in CD44 overexpressed cancer cells and enzyme-responsive drug release in the cancer cell environment. In particular, inherent characteristics of PCN-224, which is used as a drug carrier, facilitates the transfer of the drug to cancer cells more stably and allows photodynamic therapy. This HA-PCN system enables a dual chemo and photodynamic therapy to enhance the cancer therapy effect.

Hyperbranched polylysine: Synthesis, mechanism and preparation for NIR-absorbing gold nanoparticles

The ring-opening polymerization (ROP) of N-(tetrafluoroboran ammonium)-l-lysine-N-carboxyanhydride (NH3BF4-Lys NCA) was initiated by triethylamine to prepare hyperbranched polylysine (HPlys) fluoroborate with narrow polydispersity at a low temperature. Both mass spectroscopy analysis and DFT computation evidenced that the resultant cyclic dimer mainly initiated ROP as a secondary species according to normal amine mechanism. The hyperbranched polylysine in aqueous solution existed in a unimolecular state at low concentrations, but transformed into spherical micellar aggregates at a high concentration of 5 mg/mL, as characterized by dynamic laser scattering and transmission electron microscopy. By utilizing multivalent coordination interactions between hyperbranched polylysine and gold, the plasmonic gold nanoparticles (HPlys@Au NPs) were successfully prepared in aqueous solution, which elevated the solution by 23.1 °C upon 5 min of NIR laser irradiation (808 nm, 2 W/cm2).

A pH-responsive self-healing hydrogel based on multivalent coordination of Ni2+ with polyhistidine-terminated PEG and IDA-modified oligochitosan.

Metal coordination hydrogels have drawn intensive attention in controlled drug release due to their facile incorporation of stimuli and dynamic self-healing properties. However, many metal coordination hydrogels which are responsive to mild physiologically relevant stimuli are restricted by their poor stability in non-triggering environments. Here, we reported a new strategy for enhancing the neutral stability of a pH-responsive self-healing hydrogel based on multivalent metal coordination without loss of its responsiveness. The hydrogel was formed by multivalent coordination of Ni2+ with polyhistidine (PHis) and multiple iminodiacetic acid (IDA) ligands which were modified on the terminal groups of polyethylene glycol (PEG) and the side groups of oligochitosan (OChi), respectively. By incorporating multivalent coordination, the hydrogel maintains its self-healing character and weak acid responsiveness; it can also be used as an injectable hydrogel by injecting it into a neutral environment. Through varying the PHis : IDA ratio, the mechanical strength and dynamic relaxation of the hydrogel can be regulated conveniently. More importantly, the hydrogels with multivalent coordination can remain stable and integral in neutral buffer (pH 7.4); meanwhile, they dissolve quickly in a weak acid environment (pH 5.5), which is similar to some tissue disease conditions. In contrast, the control hydrogel without multivalent coordination disintegrated within 4 hours in the neutral non-triggering buffer. Because the hydrogel consists of large amounts of coordinative Ni2+ ions, it naturally possesses relatively high affinity to molecules containing polyhistidine motifs. Thus, pH-tuned controlled release of a model molecule, rhodamine-modified polyhistidine, was accomplished, with limited neutral leakage and quick release in weak acid. These results indicate potential applications for this multivalent coordination hydrogel in the controlled release field.

Polyhistidine-Based Metal Coordination Hydrogels with Physiologically Relevant pH Responsiveness and Enhanced Stability through a Novel Synthesis.

Utilizing the abnormal physiological conditions of disease tissues can result in a site-specific functionality with high control and efficiency of stimuli-responsive hydrogels. Here, a physiologically relevant pH-responsive and self-healing hydrogel is reported based on coordination between Ni2+ and four-arm poly(ethylene glycol)-b-polyhistidine (4PEG-PHis) that is synthesized by a novel and facile PHis preparation method using amino-terminalized four-arm PEG as the macroinitiator. Reversible PHisNi coordination bonds endow the hydrogel with multistimuli-triggered sol-gel transition (physiologically relevant pH, EDTA) and self-healing properties. It is also demonstrated that 4PEG-PHis could be used as an injectable hydrogel in phosphate buffer (pH 7.4), and excellent stability in neutral buffer via multivalent coordination is shown, thus indicating its potential applications in controlled drug release systems.

Electronic and Structural Effects of Inner Sphere Coordination of Chloride to a Homoleptic Copper(II) Diimine Complex

The reaction of CuCl2 with 2,9-dimethyl-1,10-phenanthroline (dmp) does not lead to the formation of [Cu(dmp)2](Cl)2 but instead to [Cu(dmp)2Cl]Cl, a 5-coordinated complex, in which one chloride is directly coordinated to the metal center. Attempts at removing the coordinated chloride by changing the counterion by metathesis were unsuccessful and resulted only in the exchange of the noncoordinated chloride, as confirmed from a crystal structure analysis. Complex [Cu(dmp)2Cl]PF6 exhibits a reversible cyclic voltammogram characterized by a significant peak splitting between the reductive and oxidative waves (0.85 and 0.60 V vs NHE, respectively), with a half-wave potential E1/2 = 0.73 V vs NHE. When reduced electrochemically, the complex does not convert into [Cu(dmp)2]+, as one may expect. Instead, [Cu(dmp)2]+ is isolated as a product when the reduction of [Cu(dmp)2Cl]PF6 is performed with l-ascorbic acid, as confirmed by electrochemistry, NMR spectroscopy, and diffractometry. [Cu(dmp)2]2+ complexes can be synthesized starting from Cu(II) salts with weakly and noncoordinating counterions, such as perchlorate. Growth of [Cu(dmp)2](ClO4)2 crystals in acetonitrile results in a 5-coordinated complex, [Cu(dmp)2(CH3CN)](ClO4)2, in which a solvent molecule is coordinated to the metal center. However, solvent coordination is associated with a dynamic decoordination-coordination behavior upon reduction and oxidation. Hence, the cyclic voltammogram of [Cu(dmp)2(CH3CN)]2+ is identical to the one of [Cu(dmp)2]+, if the measurements are performed in acetonitrile. The current results show that halide ions in precursors to Cu(II) metal-organic coordination compound synthesis, and most likely also other multivalent coordination centers, are not readily exchanged when exposed to presumed strongly binding and chelating ligand, and thus special care needs to be taken with respect to product characterization.

Organelle-Specific Triggered Release of Immunostimulatory Oligonucleotides from Intrinsically Coordinated DNA-Metal-Organic Frameworks with Soluble Exoskeleton.

DNA has proven of high utility to modulate the surface functionality of metal-organic frameworks (MOFs) for various biomedical applications. Nevertheless, current methods for preparing DNA-MOF nanoparticles rely on either inefficient covalent conjugation or specific modification of oligonucleotides. In this work, we report that unmodified oligonucleotides can be loaded on MOFs with high density (∼2500 strands/particle) via intrinsic, multivalent coordination between DNA backbone phosphate and unsaturated zirconium sites on MOFs. More significantly, surface-bound DNA can be efficiently released in either bulk solution or specific organelles in live cells when free phosphate ions are present. As a proof-of-concept for using this novel type of DNA-MOFs in immunotherapy, we prepared a construct of immunostimulatory DNA-MOFs (isMOFs) by intrinsically coordinating cytosine-phosphate-guanosine (CpG) oligonucleotides on biocompatible zirconium MOF nanoparticles, which was further armed by a protection shell of calcium phosphate (CaP) exoskeleton. We demonstrated that isMOFs exhibited high cellular uptake, organelle specificity, and spatiotemporal control of Toll-like receptors (TLR)-triggered immune responses. When isMOF reached endolysosomes via microtubule-mediated trafficking, the CaP exoskeleton dissolved in the acidic environment and in situ generated free phosphate ions. As a result, CpG was released from isMOFs and stimulated potent immunostimulation in living macrophage cells. Compared with naked CpG-MOF, isMOFs exhibited 83-fold up-regulation in stimulated secretion of cytokines. We thus expect this isMOF design with soluble CaP exoskeleton and an embedded sequential "protect-release" program provides a highly generic approach for intracellular delivery of therapeutic nucleic acids.

ChemInform Abstract: Structural‐Engineering Rationales of Gold Nanoparticles for Cancer Theranostics

AbstractReview: 206 refs.

Structural-Engineering Rationales of Gold Nanoparticles for Cancer Theranostics.

Personalized theranostics of cancer is increasingly desired, and can be realized by virtue of multifunctional nanomaterials with possible high performances. Gold nanoparticles (GNPs) are a type of especially promising candidate for cancer theranostics, because their synthesis and modification are facile, their structures and physicochemical properties are flexibly controlled, and they are also biocompatible. Especially, the localized surface plasmon resonance and multivalent coordination effects on the surface endow them with NIR light-triggered photothermal imaging and therapy, controlled drug release, and targeted drug delivery. Although the structure, properties, and theranostic application of GNPs are considerably plentiful, no expert review systematically explains the relationships among their structure, property. and application and induces the engineering rationales of GNPs for cancer theranostics. Hence, there are no clear rules to guide the facile construction of optimal GNP structures aiming at a specific theranostic application. A series of structural-engineering rationales of GNPs for cancer theranostics is proposed through digging out the deep relationships between the structure and properties of GNPs. These rationales will be inspiring for guiding the engineering of specific and advanced GNPs for personalized cancer theranostics.

Supramolecular design of coordination bonding architecture on zein nanoparticles for pH-responsive anticancer drug delivery

A pH-responsive system by constructing a designable coordination bonding-based metal-tannic acid (TA) architecture on zein nanoparticles (NPs) has been investigated. Film formation was initiated by the adsorption of the polyphenol and directed by pH-dependent, multivalent coordination bonding. The prepared metal-TA coated zein NPs (zein-TA/metal NPs) demonstrated good stability to maintain particle size in cell culture medium at 37°C. The microstructure of the NPs was revealed by transmission electron microscopy (TEM). To confirm the surface chemical information of the NPs, XPS analysis was performed. Furthermore, in vitro viability studies revealed that the zein-TA/metal NPs showed no significant cytotoxicity against HepG2 cells for 24h. Because of the pH-responsive coordination bonding between TA and metal ions, the functional property of the metal-TA films was tailored for drug delivery. Biocompatible AuNPs were produced using zein-TA/metal NPs as reducing and stabilizing agents which were promising in the photothermal therapy of cancers and other diseases.

Chemisorbed Monolayers of Corannulene Penta-Thioethers on Gold

Penta(tert-butylthio)corannulene and penta(4-dimethylaminophenylthio)corannulene form highly stable monolayers on gold surfaces, as indicated by X-ray photoelectron spectroscopy (XPS). Formation of these homogeneous monolayers involves multivalent coordination of the five sulfur atoms to gold with the peripheral alkyl or aryl substituents pointing away from the surface. No dissociation of C-S bonds upon binding could be observed at room temperature. Yet, the XPS experiments reveal strong chemical bonding between the thioether groups and gold. Temperature-dependent XPS study shows that the thermal stability of the monolayers is higher than the typical stability of self-assembled monolayers (SAMs) of thiolates on gold.

Colorimetric sensing of cations and anions by clicked polystyrenes bearing side chain donor–acceptor chromophores

Donor–acceptor chromophores were introduced into the side chains of a polystyrene derivative through the efficient click postfunctional addition reaction between dialkylaniline-activated alkynes and small acceptors, such as tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The resulting polystyrenes were fully characterized by GPC, 1H NMR, IR spectroscopy, and elemental analysis. After the postfunctionalization, the polymers were highly colored due to the formed donor–acceptor chromophores in the side chains and these chromophores showed a colorimetric sensing behavior for some metal ions and anions. Some hard-to-borderline metal ions were always recognized by the dialkylanilino nitrogen atom, resulting in a decrease in the charge-transfer (CT) band intensity of the donor–acceptor chromophores. However, the recognition site of the soft Ag+ ion was the cyano nitrogen atoms due to readily formed multivalent coordination, leading to a bathochromic shift in the CT bands for both TCNE- and TCNQ-adducted polymers. This remarkable dual colorimetric detection of two different types of metal ions was achieved by the elegant design of polymeric chemosensors. On the other hand, it was found that the dicyanoethene moieties of the side chain chromophores serve as a chemodosimeter for some anions. For example, the CN− ion underwent a nucleophilic addition reaction to the electron-deficient ethene moieties, resulting in a dramatic decrease in the CT bands. All types of sensing behavior of both cations and anions were visually recognized. The use of small molecular model compounds enabled the estimation of the kinetic parameters and the product structures.

Emergence of Colorimetric Chemosensor Ability of Metal Ions in TCNQ Polyester by Postfunctionalization

Postfunctionalization of the 7,7,8,8-tetracyano-1,4-quinodimethane (TCNQ) polyester by a [2+2] cycloaddition-cycloreversion process with an aniline-activated alkyne provided the colorimetric chemosensor ability of metal ions. The precursor TCNQ polyester did not show any spectral changes upon the addition of Fe3+ and Ag+ ions, whereas the postfunctionalized polyester displayed well-defined color changes due to the recognition of these metal ions. For example, the Fe3+ ion was recognized by the anilino nitrogen atoms, resulting in the disappearance of the charge-transfer (CT) band. In contrast, the Ag+ ion was recognized by the cyano nitrogen atoms due to the effective multivalent coordination, leading to the bathochromic shift in the CT band. The control experiments using the monomeric model compound suggested that the visual distinction between these two metal ions was achieved by the polymer effect.

A Novel Polymeric Chemosensor: Dual Colorimetric Detection of Metal Ions Through Click Synthesis

A highly colored polystyrene derivative bearing side chain chromophores composed of dialkylanilino donor and cyano-based acceptor groups, prepared by atom-economic click postfunctionalization, displays the dual colorimetric detection behavior of several metal ions based on the specific interactions with different nitrogen atoms. Hard to borderline metal ions, such as Fe(3+) , Fe(2+) , and Sn(2+) , are always recognized by the dialkylanilino nitrogen atom, resulting in a decrease in the charge-transfer (CT) band intensity of the donor-acceptor chromophores. On the other hand, the recognition site of a soft metal ion of Ag(+) is the cyano nitrogen atom due to the readily formed multivalent coordination, which produces a bathochromic shift of the CT band.

Binuclear NiII‐DpaTyr Complex as a High Affinity Probe for an Oligo‐Aspartate Tag Tethered to Proteins

A complementary recognition pair of a short-peptide tag and a small molecular probe is a versatile molecular tool for protein detection, handling, and purification, and so forth. In this manuscript, we report that the binuclear Ni(II)-DpaTyr (DpaTyr=bis((dipicolylamino)methyl)tyrosine) complex serves as a strong binding probe for an oligo-aspartate tag tethered to a protein. Among various binuclear metal complexes of M-DpaTyr (M=Zn(II), Ni(II), Mn(II), Cu(II), Cd(II), Co(III), and Fe(III)), we have found that Ni(II)-DpaTyr (1-2Ni(II)) displays a strong-binding affinity (apparent binding constant: K(app) approximately 10(5) M(-1)) for an oligo-aspartate peptide under neutral aqueous conditions (50 mM HEPES, 100 mM NaCl, pH 7.2). Detailed isothermal-titration calorimetry (ITC) studies reveal that the tri-aspartate D3-tag (DDD) is an optimal sequence recognized by 1-2Ni(II) in a 1:1 binding stoichiometry. On the other hand, other metal complexes of DpaTyr, except for Ni(II)- and Zn(II)-DpaTyr, show a negligible binding affinity for the oligo-aspartate peptide. The binding affinity was greatly enhanced in the pair between the dimer of Ni(II)-DpaTyr and the repeated D3 tag peptide (D3x2), such as DDDXXDDD, on the basis of the multivalent coordination interaction between them. Most notably, a remarkably high-binding affinity (K(app)=2x10(9) M(-1)) was achieved between the Ni(II)-DpaTyr dimer 4-4Ni(II) and the D3x2 tag peptide (DDDNGDDD). This affinity is approximately 100-fold stronger than that observed in the binding pair of the Zn(II)-DpaTyr (4-4Zn(II)) and the D4x2 tag (DDDDGDDDD), a useful tag-probe pair previously reported by us. The recognition pair of the Ni(II)-DpaTyr probe and the D3x2 tag can also work effectively on a protein surface, that is, 4-4Ni(II) is strongly bound to the FKBP12 protein tethered with the D3x2 tag (DDDNGDDD) with a large K(app) value of 5x10(8) M(-1). Taking advantage of the strong-binding affinity, this pair was successfully applied to the selective inactivation of the tag-fused beta-galactosidase by using the chromophore-assisted light inactivation (CALI) technique under crude conditions, such as cell lysate.

Colorimetric Enzymatic Activity Assay Based on Noncrosslinking Aggregation of Gold Nanoparticles Induced by Adsorption of Substrate Peptides

The mechanisms of colorimetric assays based on aggregation of gold nanoparticles (GNPs) have been separated into two categories, crosslinking, and noncrosslinking aggregation. The noncrosslinking aggregation has recently been emerging as a simple and rapid mechanism and has been applied to enzymatic activity assays and DNA detection. We report here the detailed study of an enzymatic activity assay for protein kinases based on noncrosslinking aggregation. The principle of the assay is to detect kinase activity by utilizing the difference of coagulating ability of a cationic substrate peptide and its phosphorylated form toward GNPs with anionic surface charge. The critical coagulation concentrations (CCCs) of the peptides were about 10(3) times lower than those of the metal cations with the same cationic charges. The multivalent coordination bonds of the functional groups of the peptides with the GNP surface will strongly support the adsorption of the peptide on the GNP surface. The effect of the GNP size (10, 20, 40, 60 nm) on the dynamic range of OD before and after aggregation was studied. The dynamic range became a maximum for 20 nm GNP among those studied. The difference of CCC between the phosphorylated and nonphosphorylated peptides was governed by (1) the ratio between the peptide concentration and the surface area concentration of GNP and (2) the net charge of the peptides. When the assay system was applied to the activity assessment of protein kinase A, the dynamic range of OD was largest for 20 nm GNPs. However, when the peptide concentration was lowered, the largest 60 nm GNP was advantageous because of its smaller specific surface area.

Non‐Ionic Water‐Soluble Crown‐Ether‐Substituted Polyfluorene as Fluorescent Probe for Lead Ion Assays

AbstractA new non‐ionic water‐soluble polyfluorene that contains two benzo‐18‐crown‐6 side chains per repeat unit (PFDC) is synthesized and characterized. Upon addition of Pb2+ ions to a solution of PFDC, the PFDC fluorescence is strongly quenched. The minor interference from other metal ions, especial from Cu2+ and Hg2+ions, clearly shows that the PFDC can be used to detect Pb2+ ions with high selectivity. The fluorescence quenching of PFDC in solution originates from the multivalent coordination of the crown‐ether moieties to Pb2+ ions followed by precipitation. In comparison to ionic conjugated polymers, the pH of the medium shows less effect on the binding of non‐ionic PFDC to Pb2+ ions.magnified image

PKR and eIF2α: Integration of Kinase Dimerization, Activation, and Substrate Docking

The antiviral RNA-dependent protein kinase, PKR, binds to viral double-stranded RNA in the cell and halts protein synthesis by phosphorylating the alpha subunit of the translation initiation factor eIF2. In this issue of Cell, two complementary papers Dar et al. (2005) and Dey et al. (2005) address the interaction between PKR and eIF2alpha. The structures of eIF2alpha bound to PKR reveal that PKR forms a dimer, the interface of which is essential for kinase activation, and demonstrate how this protein substrate docks to its kinase. The structures, coupled with mutagenesis analysis, also demonstrate how phosphorylation of the activation loop can allosterically couple two distal regions, the dimerization and substrate recognition interfaces.

Staggering of Subunits in NMDAR Channels

Functional N-methyl- d-aspartate receptors (NMDARs) are heteromultimers formed by NR1 and NR2 subunits. The M3 segment, as contributed by NR1, forms the core of the extracellular vestibule, including binding sites for channel blockers, and represents a critical molecular link between ligand binding and channel opening. Taking advantage of the substituted cysteine accessibility method along with channel block and multivalent coordination, we studied the contribution of the M3 segment in NR2C to the extracellular vestibule. We find that the M3 segment in NR2C, like that in NR1, contributes to the core of the extracellular vestibule. However, the M3 segments from the two subunits are staggered relative to each other in the vertical axis of the channel. Compared to NR1, homologous positions in NR2C, including those in the highly conserved SYTANLAAF motif, are located about four amino acids more externally. The staggering of subunits may represent a key structural feature underlying the distinct functional properties of NMDARs.

Structure and mechanism of CTP:phosphocholine cytidylyltransferase (LicC) from Streptococcus pneumoniae.

Pneumococcal LicC is a member of the nucleoside triphosphate transferase superfamily and catalyzes the transfer of a cytidine monophosphate from CTP to phosphocholine to form CDP-choline. The structures of apo-LicC and the LicC-CDP-choline-Mg(2+) ternary complex were determined, and the comparison of these structures reveals a significant conformational change driven by the multivalent coordination of Mg(2+). The key event is breaking the Glu(216)-Arg(129) salt bridge, which triggers the coalescence of four individual beta-strands into two extended beta-sheets. These movements reorient the side chains of Trp(136) and Tyr(190) for the optimal binding and alignment of the phosphocholine moiety. Consistent with these conformational changes, LicC operates via a compulsory ordered kinetic mechanism. The structures explain the substrate specificity of LicC for CTP and phosphocholine and implicate a direct role for Mg(2+) in aligning phosphocholine for in-line nucleophilic attack and stabilizing the negative charge that develops in the pentacoordinate transition state. These results provide a structural basis for assigning a specific role for magnesium in the catalytic mechanism of pneumococcal LicC.