Synthesis of undecagold cluster molecules as biochemical labeling reagents. 1. Monoacyl and mono[N-(succinimidooxy)succinyl] undecagold clusters

Precision nanoengineering for functional self-assemblies across length scales

As nanotechnology continues to push the boundaries across disciplines, there is an increasing need for engineering nanomaterials with atomic-level precision for self-assembly across length scales, i.e., from the nanoscale to the macroscale. Although molecular self-assembly allows atomic precision, extending it beyond certain length scales presents a challenge. Therefore, the attention has turned to size and shape-controlled metal nanoparticles as building blocks for multifunctional colloidal self-assemblies. However, traditionally, metal nanoparticles suffer from polydispersity, uncontrolled aggregation, and inhomogeneous ligand distribution, resulting in heterogeneous end products. In this feature article, I will discuss how virus capsids provide clues for designing subunit-based, precise, efficient, and error-free self-assembly of colloidal molecules. The atomically precise nanoscale proteinic subunits of capsids display rigidity (conformational and structural) and patchy distribution of interacting sites. Recent experimental evidence suggests that atomically precise noble metal nanoclusters display an anisotropic distribution of ligands and patchy ligand bundles. This enables symmetry breaking, consequently offering a facile route for two-dimensional colloidal crystals, bilayers, and elastic monolayer membranes. Furthermore, inter-nanocluster interactions mediated via the ligand functional groups are versatile, offering routes for discrete supracolloidal capsids, composite cages, toroids, and macroscopic hierarchically porous frameworks. Therefore, engineered nanoparticles with atomically precise structures have the potential to overcome the limitations of molecular self-assembly and large colloidal particles. Self-assembly allows the emergence of new optical properties, mechanical strength, photothermal stability, catalytic efficiency, quantum yield, and biological properties. The self-assembled structures allow reproducible optoelectronic properties, mechanical performance, and accurate sensing. More importantly, the intrinsic properties of individual nanoclusters are retained across length scales. The atomically precise nanoparticles offer enormous potential for next-generation functional materials, optoelectronics, precision sensors, and photonic devices.

Heavy Atom Clusters as Specific Labels

An important milestone in electron microscopy was the first visualization of single atoms in 1970 with the STEM designed by Albert Crewe. This achievement inspired thoughts that single heavy atoms could be used as super high resolution labels of biological structures by, for example, covalently reacting a heavy atom reagent at the active site of an enzyme. Further investigation of heavy atoms on thin carbon films revealed that they hopped about and that this was not solely thermal motion, but beam induced, since cooling the specimen had little effect. Attempts were made to try various heavy atom compounds but alas, these all behaved similarly, with about 10% of the atoms moving 3-10 Å on successive scans. A gallant effort by M. Beer to sequence DNA using heavy atom base specific labels befell similar problems where the motion of the label prevented high resolution coordinates from being measured.

Biocompatible gold nanoclusters: synthetic strategies and biomedical prospects

The latest advances concerning ultra-small gold nanoparticles (≤2 nm) commonly known as gold nanoclusters (AuNCs) are reviewed and discussed in the context of biological and biomedical applications (labeling, delivery, imaging and therapy). A great diversity of synthetic methods has been developed and optimized aiming to improve the chemical structures and physicochemical properties of the resulting AuNCs. The main synthetic approaches were surveyed with emphasis on methods leading to water-soluble AuNCs since aqueous solutions are the preferred media for biological applications. The most representative and recent experimental results are discussed in relationship to their potential for biomedical applications.

Cooperative Gold Nanoparticle Stabilization by Acetylenic Phosphaalkenes

Acetylenic phosphaalkenes (APAs) are used as a novel type of ligands for the stabilization of gold nanoparticles (AuNP). As demonstrated by a variety of experimental and analytical methods, both structural features of the APA, that is, the P=C as well as the C≡C units are essential for NP stabilization. The presence of intact APAs on the AuNP is demonstrated by surface-enhanced Raman spectroscopy (SERS), and first principle calculations indicate that bonding occurs most likely at defect sites on the Au surface. AuNP-bound APAs are in chemical equilibrium with free APAs in solution, leading to a dynamic behavior that can be explored for facile place-exchange reactions with other types of anchor groups such as thiols or more weakly binding phosphine ligands.

Sub-nanometre sized metal clusters: from synthetic challenges to the unique property discoveries

Sub-nanometre sized metal clusters, with dimensions between metal atoms and nanoparticles, have attracted more and more attention due to their unique electronic structures and the subsequent unusual physical and chemical properties. However, the tiny size of the metal clusters brings the difficulty of their synthesis compared to the easier preparation of large nanoparticles. Up to now various synthetic techniques and routes have been successfully applied to the preparation of sub-nanometre clusters. Among the metals, gold clusters, especially the alkanethiolate monolayer protected clusters (MPCs), have been extensively investigated during the past decades. In recent years, silver and copper nanoclusters have also attracted enormous interest mainly due to their excellent photoluminescent properties. Meanwhile, more structural characteristics, particular optical, catalytic, electronic and magnetic properties and the related technical applications of the metal nanoclusters have been discovered in recent years. In this critical review, recent advances in sub-nanometre sized metal clusters (Au, Ag, Cu, etc.) including the synthetic techniques, structural characterizations, novel physical, chemical and optical properties and their potential applications are discussed in detail. We finally give a brief outlook on the future development of metal nanoclusters from the viewpoint of controlled synthesis and their potential applications.

Electron microscopic visualization of the filament binding mode of actin-binding proteins

A large number of actin-binding proteins (ABPs) regulate various kinds of cellular events in which the superstructure of the actin cytoskeleton is dynamically changed. Thus, to understand the actin dynamics in the cell, the mechanisms of actin regulation by ABPs must be elucidated. Moreover, it is particularly important to identify the side, barbed-end or pointed-end ABP binding sites on the actin filament. However, a simple, reliable method to determine the ABP binding sites on the actin filament is missing. Here, a novel electron microscopic method for determining the ABP binding sites is presented. This approach uses a gold nanoparticle that recognizes a histidine tag on an ABP and an image analysis procedure that can determine the polarity of the actin filament. This method will facilitate future study of ABPs.

Preparation and high-resolution microscopy of gold cluster labeled nucleic acid conjugates and nanodevices

Nanogold and undecagold are covalently linked gold cluster labels which enable the identification and localization of biological components with molecular precision and resolution. They can be prepared with different reactivities, which means they can be conjugated to a wide variety of molecules, including nucleic acids, at specific, unique sites. The location of these sites can be synthetically programmed in order to preserve the binding affinity of the conjugate and impart novel characteristics and useful functionality. Methods for the conjugation of undecagold and Nanogold to DNA and RNA are discussed, and applications of labeled conjugates to the high-resolution microscopic identification of binding sites and characterization of biological macromolecular assemblies are described. In addition to providing insights into their molecular structure and function, high-resolution microscopic methods also show how Nanogold and undecagold conjugates can be synthetically assembled, or self-assemble, into supramolecular materials to which the gold cluster labels impart useful functionality.

One-step synthesis of phosphine-stabilized gold nanoparticles using the mild reducing agent 9-BBN

A simple method to synthesize phosphine-stabilized gold nanoparticles (AuNPs) of narrow size dispersion using the mild reducing agent 9-borabicyclo[3.3.1]nonane (9-BBN) is described. The methodology produces particles 1.2-2.8 nm in size depending on the reaction conditions and the phosphine ligand used. The phosphine-stabilized AuNPs exhibit size dependent localized surface plasmon resonance (LSPR) behavior as measured by UV-visible spectroscopy. (31)P NMR spectroscopy analysis of triphenylphosphine-AuNPs (TPP-AuNPs) shows a peak shift to 63.0 ppm compared to pure TPP at -5.4 ppm which is attributed to adsorption of TPP on the AuNP surface. Synthesis of trioctylphosphine-stabilized AuNPs demonstrates the versatility of the 9-BBN-based method. We present initial investigations of using TPP-AuNPs as precursor materials for nanoparticles functionalized with other ligands through ligand exchange reactions with dodecanethiol (DDT) and 11-mercaptoundecanoic acid (MUA).

Thiol-functionalized undecagold clusters by ligand exchange: synthesis, mechanism, and properties

Ligand exchange of phosphine-stabilized undecagold precursor particles, Au11(PPh3)8Cl3, with omega-functionalized thiols provides a convenient and general approach for the rapid preparation of large families of thiol-stabilized, subnanometer (dCORE approximately 0.8 nm) particles. The approach permits rapid incorporation of specific functionality into the stabilizing ligand shell, is tolerant of a wide range of functional groups, and provides convenient access to new materials inaccessible by other methods. Mechanistic studies and trapping experiments give insight into the progression of the ligand exchange, providing evidence that the core size of the phosphine-stabilized undecagold precursor particles is preserved during ligand exchange. The optical properties of the thiol-stabilized nanoparticles depend strongly on the composition of the ligand shell, and a series of studies suggests that this dependence is a result of the ligand shell's influence on the electronic structure of the particle core, as opposed to a structural change within the nanoparticle core.

Remote electronic control of DNA hybridization through inductive coupling to an attached metal nanocrystal antenna

Increasingly detailed structural and dynamic studies are highlighting the precision with which biomolecules execute often complex tasks at the molecular scale. The efficiency and versatility of these processes have inspired many attempts to mimic or harness them. To date, biomolecules have been used to perform computational operations and actuation, to construct artificial transcriptional loops that behave like simple circuit elements and to direct the assembly of nanocrystals. Further development of these approaches requires new tools for the physical and chemical manipulation of biological systems. Biomolecular activity has been triggered optically through the use of chromophores, but direct electronic control over biomolecular 'machinery' in a specific and fully reversible manner has not yet been achieved. Here we demonstrate remote electronic control over the hybridization behaviour of DNA molecules, by inductive coupling of a radio-frequency magnetic field to a metal nanocrystal covalently linked to DNA. Inductive coupling to the nanocrystal increases the local temperature of the bound DNA, thereby inducing denaturation while leaving surrounding molecules relatively unaffected. Moreover, because dissolved biomolecules dissipate heat in less than 50 picoseconds (ref. 16), the switching is fully reversible. Inductive heating of macroscopic samples is widely used, but the present approach should allow extension of this concept to the control of hybridization and thus of a broad range of biological functions on the molecular scale.

New frontiers in gold labeling

Recent advances in gold technology have led to probes with improved properties and performance for cell biologists: higher labeling density, better sensitivity, and greater penetration into tissues. Gold clusters, such as the 1.4-nm Nanogold, are gold compounds that can be covalently linked to Fab' antibody fragments, making small and stable probes. Silver enhancement then makes these small gold particles easily visible by EM, LM, and directly by eye. Another advance is the combination of fluorescent and gold probes for correlative microscopy. Chemical crosslinking of gold particles to many biologically active molecules has made possible many novel probes, such as gold-lipids, gold-Ni-NTA, and gold-ATP.

Metallosomes

Structures and ordered arrays containing organometallic particles have potential application in nanofabrication, smaller computer components, optical devices, sensors, and membrane probes and as detection agents. Here, we describe construction of gold clusters covalently attached to lipids and their use in forming typical lipid structures: micelles, liposomes ("metallosomes"), and sheets on an air-water interface. Two sizes of gold clusters were used, undecagold, with an 11-gold atom core 0.8 nm in diameter, and the larger Nanogold, with a 1.4-nm gold core. The morphology of the structures formed was determined by electron microscopy at a resolution at which single gold-lipid molecules were visualized. Further modification by additional catalytic metal deposition enhanced detectability. The approach is flexible and permits a wide variety of metal particle structures to be created using known lipid structures as templates. Additionally, these gold-lipids may serve as useful membrane labels.

Improved immuno double labelling for cell and structural biology

Colloidal gold is the most widely used electron dense marker in biological electron microscopy. The development of procedures for making gold particles of very defined sizes has made double or even multiple labelling possible using gold of two or more different sizes. Lately a new type of electron dense marker has been developed consisting of ligand-stabilized metal atom clusters rather than colloidal particles. The differences between these two types of markers are highlighted and the advantages of using metal atom clusters for immuno labelling of certain biological specimens are discussed. Possible methods of distinguishing two such cluster labels in double labelling experiments are reviewed.

Undecagold cluster modified tRNA(Phe) from Escherichia coli and its activity in the protein elongation cycle

An undecagold cluster (Au11) of molecular mass 6200Da was attached to the 3-(3-amino-3-carboxypropyl)uridine at position 47 of tRNA(Phe) from Escherichia coli. This modified tRNA can be enzymically aminoacylated with phenylalanine in the reaction catalyzed by phenylalanyl-tRNA synthetase. Au11-labeled Phe-tRNA(Phe) forms a ternary complex with the elongation factor Tu.GTP and is active in poly(U)-dependent poly(phe) synthesis. The Au11 modification does not hinder the specific binding of tRNA to distinct ribosomal binding sites or the precise positioning of the aminoacyl and peptidyl residues in the peptidyltransferase center, and does not impair the translocation. The modified tRNA is suitable for the identification of ribosomal binding sites by scanning transmission electron microscopy and for crystallographic studies of the 70S ribosome at different states of the protein-elongation cycle.

Gold cluster-labelled antibodies

The Fab' fragments of antibodies can be combined with eleven gold-atom clusters to produce the smallest gold-conjugated antibody probes yet developed.

A small gold-conjugated antibody label: improved resolution for electron microscopy

A general method has been developed to make the smallest gold-conjugated antibody label yet developed for electron microscopy. It should have wide application in domainal mapping of single molecules or in pinpointing specific molecules, sites, or sequences in supramolecular complexes. It permits electron microscopic visualization of single antigen-binding antibody fragments (Fab') by covalently linking an 11-atom gold cluster to a specific residue on each Fab' such that the antigenic specificity and capacity are preserved. The distance of the gold cluster from the antigen is a maximum of 5.0 nanometers when the undecagold-Fab' probe is used as an immunolabel.

Visibility and stability of a 12-tungsten atom complex in the scanning transmission electron microscope

A complex consisting of 12 tungsten atoms has been studied in terms of signal-to-noise (S/N) and dose response in the scanning transmission electron microscope (STEM), to evaluate its suitability for use as a approximately 1 nm resolution biological label. Molecular weight of the complex was measured as a function of radius of integration, and results were in agreement with the calculated formula weight. S/N was highest at the lowest radius of integration (0.25 nm), and decreased monotonically with increasing radius. The complex was clearly visible at a dose of 4 X 10(3) e/nm2, and exhibited negligible mass loss (approximately 8%) after an accumulated dose of 1.28 X 10(5) e/nm2. Beam-induced motion was small, 0.46 nm rms after 4 X 10(4) e/nm2. Some intensity fluctuations were observed between successive scans of the same clusters, for which a diffraction-based explanation is advanced. Upon suitable functionalization, the tungsten complex is expected to complement the undecagold cluster already in use for site-specific labeling.

Covalent labelling of histones with aurothiomalate. Gold-labelled H2A-H2B dimers assemble to octamers, which form crystalline helical tubes

Histone octamers were covalently labelled with aurothiomalate at amino groups by the method of carbodiimide activation. The labelling procedure was demonstrated to result in the specific covalent coupling through a single bond of the heavy metal atom label to protein amino groups. Such octamers were dissociated to yield soluble H2A-H2B dimers containing three gold atoms per dimer. The dimers were reconstituted with native H3-H4 tetramers to form labelled octamers, which were crystallized to form helical tubes. This strongly suggests that this procedure resulted in minimal changes of protein conformation.

Undecagold clusters for site-specific labeling of biological macromolecules: simplified preparation and model applications

We report simple and rapid procedures for the synthesis of a variety of stable, water-soluble undecagold cluster, and model applications of a thiol-reactive gold cluster for the specific labeling of cysteine residues in proteins.