Nanodiscs as a Modular Platform for Multimodal MR-Optical Imaging

Nanodiscs are monodisperse, self-assembled discoidal particles that consist of a lipid bilayer encircled by membrane scaffold proteins (MSP). Nanodiscs have been used to solubilize membrane proteins for structural and functional studies and deliver therapeutic phospholipids. Herein, we report on tetramethylrhodamine (TMR) tagged nanodiscs that solubilize lipophilic MR contrast agents for generation of multimodal nanoparticles for cellular imaging. We incorporate both multimeric and monomeric Gd(III)-based contrast agents into nanodiscs and show that particles containing the monomeric agent (ND2) label cells with high efficiency and generate significant image contrast at 7 T compared to nanodiscs containing the multimeric agent (ND1) and Prohance, a clinically approved contrast agent.

Modulation of amyloid-β aggregation by histidine-coordinating Cobalt(III) Schiff base complexes

Oligomers of the Aβ42 peptide are significant neurotoxins linked to Alzheimer's disease (AD). Histidine (His) residues present at the N terminus of Aβ42 are believed to influence toxicity by either serving as metal-ion binding sites (which promote oligomerization and oxidative damage) or facilitating synaptic binding. Transition metal complexes that bind to these residues and modulate Aβ toxicity have emerged as therapeutic candidates. Cobalt(III) Schiff base complexes (Co-sb) were evaluated for their ability to interact with Aβ peptides. HPLC-MS, NMR, fluorescence, and DFT studies demonstrated that Co-sb complexes could interact with the His residues in a truncated Aβ16 peptide representing the Aβ42 N terminus. Coordination of Co-sb complexes altered the structure of Aβ42 peptides and promoted the formation of large soluble oligomers. Interestingly, this structural perturbation of Aβ correlated to reduced synaptic binding to hippocampal neurons. These results demonstrate the promise of Co-sb complexes in anti-AD therapeutic approaches.

Axial ligand exchange of N-heterocyclic cobalt(III) Schiff base complexes: molecular structure and NMR solution dynamics

The kinetic and thermodynamic ligand exchange dynamics are important considerations in the rational design of metal-based therapeutics and therefore, require detailed investigation. Co(III) Schiff base complex derivatives of bis(acetylacetone)ethylenediimine [acacen] have been found to be potent enzyme and transcription factor inhibitors. These complexes undergo solution exchange of labile axial ligands. Upon dissociation, Co(III) irreversibly interacts with specific histidine residues of a protein, and consequently alters structure and causes inhibition. To guide the rational design of next generation agents, understanding the mechanism and dynamics of the ligand exchange process is essential. To investigate the lability, pH stability, and axial ligand exchange of these complexes in the absence of proteins, the pD- and temperature-dependent axial ligand substitution dynamics of a series of N-heterocyclic [Co(acacen)(X)(2)](+) complexes [where X = 2-methylimidazole (2MeIm), 4-methylimidazole (4MeIm), ammine (NH(3)), N-methylimidazole (NMeIm), and pyridine (Py)] were characterized by NMR spectroscopy. The pD stability was shown to be closely related to the nature of the axial ligand with the following trend toward aquation: 2MeIm > NH(3) ≫ 4MeIm > Py > Im > NMeIm. Reaction of each [Co(III)(acacen)(X)(2)](+) derivative with 4MeIm showed formation of a mixed ligand Co(III) intermediate via a dissociative ligand exchange mechanism. The stability of the mixed ligand adduct was directly correlated to the pD-dependent stability of the starting Co(III) Schiff base with respect to [Co(acacen)(4MeIm)(2)](+). Crystal structure analysis of the [Co(acacen)(X)(2)](+) derivatives confirmed the trends in stability observed by NMR spectroscopy. Bond distances between the Co(III) and the axial nitrogen atoms were longest in the 2MeIm derivative as a result of distortion in the planar tetradentate ligand, and this was directly correlated to axial ligand lability and propensity toward exchange.

Cobalt derivatives as promising therapeutic agents

Inorganic complexes are versatile platforms for the development of potent and selective pharmaceutical agents. Cobalt possesses a diverse array of properties that can be manipulated to yield promising drug candidates. Investigations into the mechanism of cobalt therapeutic agents can provide valuable insight into the physicochemical properties that can be harnessed for drug development. This review presents examples of bioactive cobalt complexes with special attention to their mechanisms of action. Specifically, cobalt complexes that elicit biological effects through protein inhibition, modification of drug activity, and bioreductive activation are discussed. Insights gained from these examples reveal features of cobalt that can be rationally tuned to produce therapeutics with high specificity and improved efficacy for the biomolecule or pathway of interest.

Synapse-binding subpopulations of Aβ oligomers sensitive to peptide assembly blockers and scFv antibodies

Amyloid β42 self-assembly is complex, with multiple pathways leading to large insoluble fibrils or soluble oligomers. Oligomers are now regarded as most germane to Alzheimer's pathogenesis. We have investigated the hypothesis that oligomer formation itself occurs through alternative pathways, with some leading to synapse-binding toxins. Immediately after adding synthetic peptide to buffer, solutions of Aβ42 were separated by a 50 kDa filter and fractions assessed by SDS-PAGE silver stain, Western blot, immunoprecipitation, and capacity for synaptic binding. Aβ42 rapidly assembled into aqueous-stable oligomers, with similar protein abundance in small (<50 kDa) and large (>50 kDa) oligomer fractions. Initially, both fractions were SDS-labile and resolved into tetramers, trimers, and monomers by SDS-PAGE. Upon continued incubation, the larger oligomers developed a small population of SDS-stable 10-16mers, and the smaller oligomers generated gel-impermeant complexes. The two fractions associated differently with neurons, with prominent synaptic binding limited to larger oligomers. Even within the family of larger oligomers, synaptic binding was associated with only a subset of these species, as a new scFv antibody (NUsc1) immunoprecipitated only a small portion of the oligomers while eliminating synaptic binding. Interestingly, low doses of the peptide KLVFFA blocked assembly of the 10-16mers, and this result was associated with loss of the smaller clusters of oligomers observed at synaptic sites. What distinguishes these smaller clusters from the unaffected larger clusters is not yet known. Results indicate that distinct species of Aβ oligomers are generated by alternative assembly pathways and that synapse-binding subpopulations of Aβ oligomers could be specifically targeted for Alzheimer's therapeutics.

Analytical methods for characterizing magnetic resonance probes

The efficiency of Gd(III) contrast agents in magnetic resonance image enhancement is governed by a set of tunable structural parameters. Understanding and measuring these parameters requires specific analytical techniques. This Feature describes strategies to optimize each of the critical Gd(III) relaxation parameters for molecular imaging applications and the methods employed for their evaluation.

Trinuclear ruthenium clusters as bivalent electrochemical probes for ligand-receptor binding interactions

Despite their popularity, electrochemical biosensors often suffer from low sensitivity. One possible approach to overcome low sensitivity in protein biosensors is to utilize multivalent ligand-receptor interactions. Controlling the spatial arrangement of ligands on surfaces is another crucial aspect of electrochemical biosensor design. We have synthesized and characterized five biotinylated trinuclear ruthenium clusters as potential new biosensor platforms: [Ru(3)O(OAc)(6)CO(4-BMP)(py)](0) (3), [Ru(3)O(OAc)(6)CO(4-BMP)(2)](0) (4), [Ru(3)O(OAc)(6)L(4-BMP)(py)](+) (8), [Ru(3)O(OAc)(6)L(4-BMP)(2)](+) (9), and [Ru(3)O(OAc)(6)L(py)(2)](+) (10) (OAc = acetate, 4-BMP = biotin aminomethylpyridine, py = pyridine, L = pyC16SH). HABA/avidin assays and isothermal titration calorimetry were used to evaluate the avidin binding properties of 3 and 4. The binding constants were found to range from (6.5-8.0) × 10(6) M(-1). Intermolecular protein binding of 4 in solution was determined by native gel electrophoresis. QM, MM, and MD calculations show the capability for the bivalent cluster, 4, to intramolecularly bind to avidin. Electrochemical measurements in solution of 3a and 4a show shifts in E(1/2) of -58 and -53 mV in the presence of avidin, respectively. Self-assembled monolayers formed with 8-10 were investigated as a model biosensor system. Diluent/cluster ratio and composition were found to have a significant effect on the ability of avidin to adequately bind to the cluster. Complexes 8 and 10 showed negligible changes in E(1/2), while complex 9 showed a shift in E(1/2) of -43 mV upon avidin addition. These results suggest that multivalent interactions can have a positive impact on the sensitivity of electrochemical protein biosensors.

Probing the Chemical Stability of Mixed Ferrites: Implications for MR Contrast Agent Design

Nanomaterials with mixed composition, in particular magnetic spinel ferrites, are emerging as efficient contrast agents for magnetic resonance imaging (MRI). Many factors, including size, composition, atomic structure, and surface properties are crucial in the design of such nanoparticle-based probes due to their influence on the magnetic properties. Silica-coated iron oxide (IO-SiO(2)) and cobalt ferrite (CoIO-SiO(2)) nanoparticles were synthesized using standard high temperature thermal decomposition and base-catalyzed water-in-oil microemulsion techniques. Under neutral aqueous conditions, it was found that 50-75% of the cobalt content in the CoIO-SiO(2) nanoparticles leached out of the core structure. Leaching caused a 7.2-fold increase in longitudinal relaxivity and an increase in the saturation magnetization from ~48 emu/g core to ~65 emu/g core. X-ray absorption fine structure studies confirmed that the atomic structure of the ferrite core was altered following leaching, while TEM and DLS confirmed that the morphology and size of the nanoparticle remained unchanged. The CoIO-SiO(2) nanoparticles converted from a partially inverted spinel cation arrangement (unleached state) to an inverse spinel arrangement (leached state). The control IO-SiO(2) nanoparticles remained stable with no change in structure and negligible changes in magnetic behavior. This detailed analysis highlights how important understanding the properties of nanomaterials is in the development of reliable agents for diagnostic and therapeutic applications.

A modular system for the synthesis of multiplexed magnetic resonance probes

We have developed a modular architecture for preparing high-relaxivity multiplexed probes utilizing click chemistry. Our system incorporates azide bearing Gd(III) chelates and a trialkyne scaffold with a functional group for subsequent modification. In optimizing the relaxivity of this new complex, we undertook a study of the linker length between a chelate and the scaffold to determine its effect on relaxivity. The results show a strong dependence on flexibility between the individual chelates and the scaffold with decreasing linker length leading to significant increases in relaxivity. Nuclear magnetic resonance dispersion (NMRD) spectra were obtained to confirm a 10-fold increase in the rotational correlation time from 0.049 to 0.60 ns at 310 K. We have additionally obtained a crystal structure demonstrating that modification with an azide does not impact the coordination of the lanthanide. The resulting multinuclear center has a 500% increase in per Gd (or ionic) relaxivity at 1.41 T versus small molecule contrast agents and a 170% increase in relaxivity at 9.4 T.

Electrochemistry of redox-active self-assembled monolayers

Redox-active self-assembled monolayers (SAMs) provide an excellent platform for investigating electron transfer kinetics. Using a well-defined bridge, a redox center can be positioned at a fixed distance from the electrode and electron transfer kinetics probed using a variety of electrochemical techniques. Cyclic voltammetry, AC voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are most commonly used to determine the rate of electron transfer of redox-activated SAMs. A variety of redox species have been attached to SAMs, and include transition metal complexes (e.g., ferrocene, ruthenium pentaammine, osmium bisbipyridine, metal clusters) and organic molecules (e.g., galvinol, C(60)). SAMs offer an ideal environment to study the outer-sphere interactions of redox species. The composition and integrity of the monolayer and the electrode material influence the electron transfer kinetics and can be investigated using electrochemical methods. Theoretical models have been developed for investigating SAM structure. This review discusses methods and monolayer compositions for electrochemical measurements of redox-active SAMs.

Kinetic dispersion in redox-active dithiocarbamate monolayers

Dithiocarbamates (dtcs) have been implicated as important gold-binding groups in molecular electronics. Dtcs have two alkane branches connected at a single anchoring point that has a bidentate resonance structure. Forming readily in situ by the combination of secondary amines and CS(2), dtcs adsorb quickly onto gold surfaces. Electroactive self-assembled monolayers (eSAMs) were prepared by the coadsorption of ferrocene dialkyldithiocarbamates (Fc dtcs) with diluent dtcs on gold electrodes. Short and long alkane chains were used (11 and 16 methylene groups, respectively), and a polar ester group was incorporated. Cyclic voltammetry (CV) shows that the electrochemistry is quasi-reversible. At high surface coverage, the peak separations and full widths at half-maximum for Fc dtcs deviate from theoretical values and are analogous to that of ferrocene alkane thiols on gold at high surface coverage. Importantly, these features do not change at low Fc dtc surface coverage as observed for ferrocene alkane thiols. Ferrocene dtcs were used to label monolayer defect sites and to demonstrate the exchange of surface-bound dtcs with solution dtcs. Finally, the rate of electron transfer was analyzed using Tafel plots and ac voltammetric methods. The results for both techniques are consistent with a kinetically disperse population of redox sites. The length of the diluent alkane chain appears to have an effect on the distribution of electron-transfer rates, likely because of the eSAM structure. This work indicates that structurally, Fc dtc eSAMs are fundamentally different from alkane thiol SAMs on gold.

Ultrasmall, Water-Soluble Magnetite Nanoparticles with High Relaxivity for Magnetic Resonance Imaging

Ultrasmall (3, 4, 5, and 6 nm), water-soluble Fe3O4 magnetic nanoparticles were synthesized in diethylene glycol (DEG) via a facile one-pot reaction. Hydrodynamic size and relaxation time measurements did not show particle aggregation when Fe3O4 nanoparticles were dispersed in phosphate buffered saline, fetal bovine serum, or calf bovine serum for 1 week. Furthermore, the new Fe3O4 nanoparticles tolerated high salt concentrations (≤1 M NaCl) and a wide pH range from 5 to 11. Surface modification of the nanoparticles with poly(ethylene glycol) bis(carboxymethyl) ether (HOOC-PEG-COOH, 600 g/mol) was accomplished through a ligand-exchange reaction. The effects of PEG modification on magnetization and relaxivity of the Fe3O4 nanoparticles were investigated, and the results indicate that the increase in transverse relaxivity after PEG modification may be due to the increased volume of slowly diffusing water surrounding each nanoparticle. In vitro experiments showed that the DEG- and PEG-coated Fe3O4 nanoparticles have little effect on NIH/3T3 cell viability.

Highly dispersible, superparamagnetic magnetite nanoflowers for magnetic resonance imaging

A one-pot reaction process was developed to synthesize highly dispersible, superparamagnetic Fe(3)O(4) nanoflowers; the potential of these nanoflowers as MRI contrast agents was investigated.

Electroactive self-assembled monolayers on gold via bipodal dithiazepane anchoring groups

Novel dithiazepane-functionalized ferrocenyl-phenylethynyl oligomers 1 and 2 have been synthesized. Self-assembled monolayers (SAMs) of these ferrocene derivatives have been studied by X-ray photoelectron spectroscopy, ellipsometry, and cyclic voltammetry. It has been shown by XPS that monolayers of the dithiazepane-anchored molecules on gold electrodes contain gold-thiolate species. Cyclic voltammetry of the SAMs were characteristic of stable electroactive monolayers even for single-component SAMs of 1 and 2, with the more ideal responses recorded for the two-component SAMs diluted with undecanethiol. The small variation in peak splittings at progressively higher scan rates in these SAMs makes dithiazepane-bridged redox species promising candidates for further studies on molecular wires with bipodal anchoring.

Synthesis and Electrochemical Characterization of a Transition-Metal-Modified Ligand−Receptor Pair

The energetics of weak interactions (van der Waals forces, hydrogen bonding) are difficult to quantify in biological ligand-receptor pairs. Insight into the biochemical role these forces play is critical to an understanding of signal transduction events and the drug discovery process. Ruthenium pentaammine and iron tetracyano complexes modified with either biotin or desthiobiotin have been synthesized and characterized. These modified biological ligands bind to the protein avidin in a manner similar to that of native biotin. Experiments using redox mediators show that the avidin-bound complexes are electrochemically accessible.

Syntheses of the 47 Electron Clusters [(Cp*Fe)3(μ3-X)2] (X = S, Se) and the First Fe/Sn/Se Heterocubane Cluster [(Cp*Fe)3(SnCl3)(μ3-Se)4]·DME by the Use of Chalcogenostannate Salts

By reacting 1-aminoethylammonium (H2NCH2CH2NH3+ = enH+) salts of [Sn2E6]4- anions (E = S, Se), [enH]4[Sn2S6] (1) and [enH]4[Sn2Se6] x en (2), with FeCl2/LiCp, three novel (partly) oxidized, Cp* ligated iron chalcogenide clusters were synthesized. Two of them, [(CpFe)3(mu3-S)2] (3) and [(Cp*Fe)3(mu3-Se)2] (4), contain formally 47 valence electrons. [(Cp*Fe)3(SnCl3)(mu3-Se)4] x DME (5) represents the first known mixed metal Fe/Sn/Se heterocubane type cluster. Compounds 3-5 were structurally characterized by single-crystal X-ray diffraction, and the odd valence electron number of the [Fe3E2] clusters (E = S, Se) was confirmed by density functional (DFT) investigations, mass spectrometry, cyclic voltammetry and a susceptibility measurement of 3.

New class of ruthenium sulfide clusters: Ru(4)S(6)(PPh(3))(4), Ru(5)S(6)(PPh(3))(5), and Ru(6)S(8)(PPh(3))(6)

Reaction of RuCl(2)(PPh(3))(3) with S(2)(-) sources yields a family of phosphine-containing Ru-S clusters which have been characterized crystallographically and by MALDI-MS. Ru(4)S(6)(PPh(3))(4) (Ru-Ru(av) = 2.94 A) has idealized T(d)() symmetry whereas Ru(6)S(8)(PPh(3))(6) (Ru-Ru(av) = 2.82 A) adopts the idealized O(h)() symmetry characteristic of Chevrel clusters. Ru(5)S(6)(PPh(3))(5) is formally derived by the addition of Ru(PPh(3)) to one face of Ru(4)S(6)(PPh(3))(4). In terms of its M-S connectivity, the Ru(5)S(6) cluster resembles a fragment of the FeMo cluster in nitrogenase.

Syntheses of Ru-S clusters with kinetically labile ligands via the photolysis of [(cymene)3RuS2](PF6)2

Three ruthenium sulfide clusters with labile CH3CN ligands have been photochemically synthesized. Irradiation of [(cymene)3Ru3S2](PF6)2 ([1](PF6)2) in CH3CN gives [(cymene)2(CH3CN)3Ru3S2](PF6)2 ([2](PF6)2), which has been characterized by 1H NMR spectroscopy, ESI mass spectrometry, and chemical reactivity. Treatment of [2](PF6)2 with PPh3 gives [(cymene)2(CH3CN)2(PPh3)Ru3S2](PF6)2 ([3](PF6)2) and [(cymene)2(CH3CN)(PPh3)2Ru3S2](PF6)2 ([4](PF6)2), while treatment with 1,4,7-trithiacyclononane (9S3) gives [(cymene)2(9S3)Ru3S2](PF6)2 ([5](PF6)2). A crystallographic study demonstrated that the Ru3 core in [3](PF6)2, [4](PF6)2, and [5](PF6)2 is distorted with a pair of elongated Ru-Ru bonds. Cyclic voltammetry shows that [3](PF6)2 and [4](PF6)2 undergo two closely spaced reversible one-electron reductions whereas [5](PF6)2 undergoes one irreversible one-electron reduction and one reversible one-electron reduction. Prolonged irradiation of [1](PF6)2 in CH3CN causes decomposition, resulting in the pentanuclear cluster [(cymene)4Ru5S4](PF6)2 ([6](PF6)2).