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

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.

Chemoselective cluster labelling of biomolecules

The Brookhaven STEM is capable of visualizing single heavy atoms at a beam dose of >103 el/Å2. Heteropolytungstate clusters, including W12PO403, have been found to incorporate several desirable properties as labels for biological specimens. They may be resolved at much lower beam doses due to their high concentrations of multiple heavy atoms and are directly visible labels. A lower beam dose also helps to preserve the biological structure of the specimens. Furthermore, they are extremely stable in the electron beam. Lastly, they are capable of being derivatized as chemoselective reagents for specific binding sites on biomolecules, as in the previously reported undecagold compound.

Two new classes of heavy atom labels, one specific for sulfhydryl and the other specific for both amino and sulfhydryl binding sites on proteins, have been synthesized by reactions analogous to those illustrated in Scheme 1.

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.

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.

Surface modification, functionalization and bioconjugation of colloidal inorganic nanoparticles

Inorganic colloidal nanoparticles are very small, nanoscale objects with inorganic cores that are dispersed in a solvent. Depending on the material they consist of, nanoparticles can possess a number of different properties such as high electron density and strong optical absorption (e.g. metal particles, in particular Au), photoluminescence in the form of fluorescence (semiconductor quantum dots, e.g. CdSe or CdTe) or phosphorescence (doped oxide materials, e.g. Y(2)O(3)), or magnetic moment (e.g. iron oxide or cobalt nanoparticles). Prerequisite for every possible application is the proper surface functionalization of such nanoparticles, which determines their interaction with the environment. These interactions ultimately affect the colloidal stability of the particles, and may yield to a controlled assembly or to the delivery of nanoparticles to a target, e.g. by appropriate functional molecules on the particle surface. This work aims to review different strategies of surface modification and functionalization of inorganic colloidal nanoparticles with a special focus on the material systems gold and semiconductor nanoparticles, such as CdSe/ZnS. However, the discussed strategies are often of general nature and apply in the same way to nanoparticles of other materials.

Nanogold as a specific marker for electron cryotomography

While electron cryotomography (ECT) provides "molecular" resolution, three-dimensional images of unique biological specimens, sample crowdedness, and/or resolution limitations can make it difficult to identify specific macromolecular components. Here we used a 1.4 nm Nanogold cluster specifically attached to the Fc fragment of IgG to monitor its interaction with the neonatal Fc receptor (FcRn), a membrane-bound receptor that transports IgG across cells in acidic intracellular vesicles. ECT was used to image complexes formed by Nanogold-labeled Fc bound to FcRn attached to the outer surface of synthetic liposomes. In the resulting three-dimensional reconstructions, 1.4 nm Nanogold particles were distributed predominantly along the interfaces where 2:1 FcRn-Fc complexes bridged adjacent lipid bilayers. These results demonstrate that the 1.4 nm Nanogold cluster is visible in tomograms of typically thick samples (approximately 250 nm) recorded with defocuses appropriate for large macromolecules and is thus an effective marker.

Evidence for an interaction between the SH3 domain and the N-terminal extension of the essential light chain in class II myosins

The function of the src-homology 3 (SH3) domain in class II myosins, a distinct beta-barrel structure, remains unknown. Here, we provide evidence, using electron cryomicroscopy, in conjunction with light-scattering, fluorescence and kinetic analyses, that the SH3 domain facilitates the binding of the N-terminal extension of the essential light chain isoform (ELC-1) to actin. The 41 residue extension contains four conserved lysine residues followed by a repeating sequence of seven Pro/Ala residues. It is widely believed that the highly charged region interacts with actin, while the Pro/Ala-rich sequence forms a rigid tether that bridges the approximately 9 nm distance between the myosin lever arm and the thin filament. In order to localize the N terminus of ELC in the actomyosin complex, an engineered Cys was reacted with undecagold-maleimide, and the labeled ELC was exchanged into myosin subfragment-1 (S1). Electron cryomicroscopy of S1-bound actin filaments, together with computer-based docking of the skeletal S1 crystal structure into 3D reconstructions, showed a well-defined peak for the gold cluster near the SH3 domain. Given that SH3 domains are known to bind proline-rich ligands, we suggest that the N-terminal extension of ELC interacts with actin and modulates myosin kinetics by binding to the SH3 domain during the ATPase cycle.

Facile syntheses of monodisperse ultrasmall Au clusters

During our effort to synthesize the tetrahedral Au20 cluster, we found a facile synthetic route to prepare monodisperse suspensions of ultrasmall Au clusters AuN (N < 12) using diphosphine ligands. In our monophasic and single-pot synthesis, a Au precursor ClAu(I)PPh3 (Ph = phenyl) and a bidentate phosphine ligand P(Ph)2(CH2)(M)P(Ph)2 are dissolved in an organic solvent. Au(I) is reduced slowly by a borane-tert-butylamine complex to form Au clusters coordinated by the diphosphine ligand. The Au clusters are characterized by both high-resolution mass spectrometry and UV-vis absorption spectroscopy. We found that the mean cluster size obtained depends on the chain length M of the ligand. In particular, a single monodispersed Au11 cluster is obtained with the P(Ph)2(CH2)3P(Ph)2 ligand, whereas P(Ph)2(CH2)(M)P(Ph)2 ligands with M = 5 and 6 yield Au10 and Au8 clusters. The simplicity of our synthetic method makes it suitable for large-scale production of nearly monodisperse ultrasmall Au clusters. It is suggested that diphosphines provide a set of flexible ligands to allow size-controlled synthesis of Au nanoparticles.

Stabilization of gold nanoparticles by hydrophobically-modified polycations

Surface-modified gold nanoparticles have pronounced benefits in the biomedical field due to their significant interaction with delivery materials. In the present study we used hydrophobically-modified polycations (i.e., N-acylated chitosan) to stabilize gold nanoparticles. Aliphatic hydrophobic groups, having carbon chains of different lengths, were first grafted onto the backbone of chitosan by N-acylation with fatty-acid chlorides in order to increase its hydrophobicity. Gold nanoparticles stabilized with native chitosan and N-acylated chitosan were prepared by the graft-onto approach. Chemical modification and its quantification were studied by Fourier-transform infrared (FT-IR) spectroscopy. Further, the stabilized gold nanoparticles were characterized by different physico-chemical techniques such as UV-Vis, FT-IR, TEM, TGA and DLS. Spectral studies of gold nanoparticles show the backbone and the side chain functional groups of chitosan were not cleaved during the conjugation process. TEM observations revealed that the modified chitosan gold nanoparticles were well dispersed and spherical in shape with average size around 10-12 nm in triply-distilled water at pH 7.4, whereas the native chitosan gold nanoparticles appeared as clusters with 9.9 nm as average diameter and were dispersed only in dilute HCl. The size of modified chitosan gold nanoparticles varied depending on the length of grafting molecules.

Chiroptical Activity of BINAP-Stabilized Undecagold Clusters

Undecagold cluster compounds [Au11(BINAP)4X2]+ (X = Cl and Br) were synthesized by chemical reduction of the corresponding precursor complexes, Au2X2(BINAP), where BINAP represents the bidentate phosphine ligand 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl. The circular dichroism spectra of Au11 stabilized by the enantiomers [Au11(R-BINAP)4X2]+ and [Au11(S-BINAP)4X2]+ exhibited intense and mirror-image Cotton effect, whereas those of Au11(3+) clusters stabilized by achiral monodentate phosphine ligands did not. The origin of the chiroptical activity of [Au11(BINAP)4X2]+ is discussed in the context of the structural deformation of the Au11(3+) core.

Gold Nanocluster Formation using Metallothionein: Mass Spectrometry and Electron Microscopy

Clonable contrasting agents for light microscopy, such as green fluorescent protein, have revolutionized biology, but few such agents have been developed for transmission electron microscopy (TEM). As an attempt to develop a novel clonable contrasting agent for TEM, we have evaluated metallothionein, a small metal-binding protein, reacted with aurothiomalate, an anti-arthritic gold compound. Electro spray ionization and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry measurements show a distribution of gold atoms bound to individual metallothionein molecules. Unlike previous reports, these data show gold binding occurred as the addition of single atoms without retention of additional ligands. Moreover, under certain conditions, MALDI spectra show gold binding ratios of greater than 1:1 with the cysteine residues of metallothionein. Together, this may hint at a gold-binding mechanism similar to gold nanocluster formation. Finally, metallothionein-gold complexes visualized in the TEM show a range of sizes similar to those used as current TEM labels, and show the potential of the protein as a clonable TEM label in which the gold cluster is grown on the label, thereby circumventing the problems associated with attaching gold clusters.

Interaction of the G′ Domain of Elongation Factor G and the C-Terminal Domain of Ribosomal Protein L7/L12 during Translocation as Revealed by Cryo-EM

During tRNA translocation on the ribosome, an arc-like connection (ALC) is formed between the G' domain of elongation factor G (EF-G) and the L7/L12-stalk base of the large ribosomal subunit in the GDP state. To delineate the boundary of EF-G within the ALC, we tagged an amino acid residue near the tip of the G' domain of EF-G with undecagold, which was then visualized with three-dimensional cryo-electron microscopy (cryo-EM). Two distinct positions for the undecagold, observed in the GTP-state and GDP-state cryo-EM maps of the ribosome bound EF-G, allowed us to determine the movement of the labeled amino acid. Molecular analyses of the cryo-EM maps show: (1) that three structural components, the N-terminal domain of ribosomal protein L11, the C-terminal domain of ribosomal protein L7/L12, and the G' domain of EF-G, participate in formation of the ALC; and (2) that both EF-G and the ribosomal protein L7/L12 undergo large conformational changes to form the ALC.

Identification of the beta1-integrin binding site on alpha-actinin by cryoelectron microscopy

Cell-matrix adhesions in migrating cells are usually mediated by integrins, alpha-beta heterodimeric transmembrane proteins that link extracellular matrix molecules such as fibronectin to the cytoskeleton. We have synthesized the cytoplasmic domain of the beta1-integrin (residues H738-K778) with a histidine tag at its N-terminus. The binding of this peptide to a lipid monolayer containing a chelated-nickel group (dimyristoylphosphatidyl choline-suberimide-nitriloacetic acid:nickel salt) mimics the native environment at the cytoplasmic leaflet of the plasma membrane. A Nanogold particle was covalently linked to cysteines introduced at the C-terminus and after residue T757 on the integrin peptide, and co-crystallized with chicken smooth muscle alpha-actinin. The 2-D arrays of the beta1-integrin-alpha-actinin complex were examined by cryoelectron microscopy, with and without the gold label. Averaged projections were calculated for each specimen along with a difference map to determine the relative position of the gold-labeled beta1-integrin peptide. The difference maps indicate that the beta1-integrin cytoplasmic domain binds alpha-actinin between the first and second, 3-helix motifs in the central rod domain.

MAP2 and tau bind longitudinally along the outer ridges of microtubule protofilaments

MAP2 and tau exhibit microtubule-stabilizing activities that are implicated in the development and maintenance of neuronal axons and dendrites. The proteins share a homologous COOH-terminal domain, composed of three or four microtubule binding repeats separated by inter-repeats (IRs). To investigate how MAP2 and tau stabilize microtubules, we calculated 3D maps of microtubules fully decorated with MAP2c or tau using cryo-EM and helical image analysis. Comparing these maps with an undecorated microtubule map revealed additional densities along protofilament ridges on the microtubule exterior, indicating that MAP2c and tau form an ordered structure when they bind microtubules. Localization of undecagold attached to the second IR of MAP2c showed that IRs also lie along the ridges, not between protofilaments. The densities attributable to the microtubule-associated proteins lie in close proximity to helices 11 and 12 and the COOH terminus of tubulin. Our data further suggest that the evolutionarily maintained differences observed in the repeat domain may be important for the specific targeting of different repeats to either alpha or beta tubulin. These results provide strong evidence suggesting that MAP2c and tau stabilize microtubules by binding along individual protofilaments, possibly by bridging the tubulin interfaces.

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.

Undecagold cluster labeling of proteins at reactive cysteine residues

Undecagold cluster labeling of reactive cysteine residues in numerous proteins has allowed the labeled sites to be identified by electron microscopy, providing high-resolution information on the location and orientation of subunits in oligomeric enzymes, virus capsids, crystalline sheets of membrane proteins, and muscle thin filaments. The range of applications of undecagold cluster labeling has been greatly extended by the availability of site-directed mutagenesis to introduce cysteine residues at sites of interest. In this paper I discuss factors that can influence the extent and specificity of labeling, methods for biochemical analysis of undecagold-labeled proteins, and the effects of undecagold cluster labeling on biological activity.

Gold-tagged RNA-A probe for macromolecular assemblies

Ribonucleic acids (RNAs) play a key role in many fundamental life processes. These polymers are often found complexed with proteins in extremely large particles whose molecular mass may reach several millions of daltons (e.g., ribosomes, spliceosomes, and viruses). Structural studies of such RNA-protein complexes should help elucidate their mode of action. For the structural analyses of many macromolecular assemblies, electron microscopy (EM) has served an instrumental role. However, localization by EM of RNA within biological complexes is not yet a straightforward undertaking. Here we describe a methodology for the covalent tagging of RNA molecules with gold clusters, thereby enabling their direct visualization by microscopical methods. Our strategy involves transcription in vitro of RNAs that carry free thiol groups, using ribonucleoside triphosphate analogs containing a substituent with a terminal thiol group on their heterocyclic ring. This synthesis is followed by coupling of gold clusters to the thiolated transcript through a maleimido group. Visualization of such gold-tagged RNAs by transmission electron microscopy showed spots of gold clusters, with a diameter of 1-2 nm, arranged at nearly regular distances on an imaginary curve that presumably corresponds to the RNA chain. This assignment was corroborated by atomic force microscopy that exhibited images of RNA chains in which knob-like structures, whose height corresponds to the diameter of the gold clusters, were clearly seen. This study demonstrates the potential use of nucleic acids that are covalently labeled with gold clusters for the structural characterization of protein-RNA complexes.

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.

Cytochrome c oxidase: structural studies by electron microscopy of two-dimensional crystals

Cytochrome c oxidase is a complex integral membrane protein consisting of 13 different polypeptide chains and four metal centers having a total molecular weight of approximately 200,000 daltons. It can be isolated in two 2-dimensional crystalline forms differing in aggregation state of the enzyme. One crystal form consists of cytochrome oxidase dimers (approximately 400,000 daltons) embedded unidirectionally in the lipid bilayer of a collapsed vesicle while the other form consists of crystalline sheets of cytochrome oxidase monomers. Both crystal forms have been studied by electron microscopy during the past two decades, and this paper summarizes the results of early structural studies as well as more recent results applying techniques of cryoelectron microscopy and digital image processing. The structure of frozen-hydrated cytochrome oxidase dimers at 20 A resolution is discussed as well as the packing of monomers within dimers and the site of cytochrome c binding.

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.

Cryo-electron microscopy of vitrified muscle samples

A great deal of information on the 3-dimensional structure of the protein assemblies involved in muscle contraction has been obtained using conventional transmission electron microscopy. In recent years, developments in cryo-electron microscopy have facilitated work with fully hydrated, non-chemically fixed specimens. It is shown how this technique can be used to visualize muscle sarcomere filaments in quasi-native conditions, to access hitherto inaccessible states of the crossbridge cycle, and to obtain new high resolution structural information on their 3-dimensional protein structure. A short introduction to the crossbridge cycle and its biochemically accessible states illustrates the problems amenable to studies using the electron microscope, as well as the possibilities offered by cryo-microscopy on vitrified samples. Work on vitrified cryo-sections and myosin filament suspensions demonstrates the accessibility of crossbridge states and gives implications on the gross structural features of myosin filaments. Recent studies on actin filaments and myosin (S1) decorated actin filaments provide the first high resolution data on vitrified samples. The use of photolabile nucleotide precursors allows the trapping of short lived states in the millisecond time range, thereby visualizing intermediate states of the crossbridge cycle.

Molecular structure of F-actin and location of surface binding sites

Comparisons of three-dimensional maps of vertebrate muscle thin filaments obtained by cryo-electron microscopy and image analysis, reveal the molecular structure of F-actin, the location of the C terminus of the monomer and the positions of the binding sites of tropomyosin, the myosin head and the N-terminal portion of the myosin A1 light chain on the filament. These data provide strong constraints for evaluating models built from the atomic structure of the monomer and the subsequent identification of molecular contacts.

Radioactive gold cluster immunoconjugates: potential agents for cancer therapy

An 11 gold atom (undecagold) cluster was covalently attached to specific sites on Fab', F(ab')2 and whole IgG molecules such that each carried 11-33 gold atoms without significant loss of native immunospecificity. Gold cluster labeled 17-1A monoclonal F(ab')2 antibody fragments showed 80% immunoreactivity compared to native antibody fragments in binding to human colon carcinoma cells in vitro. Radioactive gold in vivo biodistributions in nude mice with human tumors are also reported. By using clusters, potentially a larger destructive payload can be carried per antibody.

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.