Abraham Shanzer

Molecular control over Au/GaAs diodes

The use of molecules to control electron transport is an interesting possibility, not least because of the anticipated role of molecules in future electronic devices. But physical implementations using discrete molecules are neither conceptually simple nor technically straightforward (difficulties arise in connecting the molecules to the macroscopic environment). But the use of molecules in electronic devices is not limited to single molecules, molecular wires or bulk material. Here we demonstrate that molecules can control the electrical characteristics of conventional metal–semiconductor junctions, apparently without the need for electrons to be transferred onto and through the molecules. We modify diodes by adsorbing small molecules onto single crystals of n-type GaAs semiconductor. Gold contacts were deposited onto the modified surface, using a ‘soft’ method to avoid damaging the molecules. By using a series of multifunctional molecules whose dipole is varied systematically, we produce diodes with an effective barrier height that is tuned by the molecules dipole moment. These barrier heights correlate well with the change in work function of the GaAs surface after molecular modification. This behaviour is consistent with that of unmodified metal–semiconductor diodes, in which the barrier height can depend on the metals work function.

Controlled surface charging as a depth-profiling probe for mesoscopic layers

Probing the structure of material layers just a few nanometres thick requires analytical techniques with high depth sensitivity. X-ray photoelectron spectroscopy (XPS) provides one such method, but obtaining vertically resolved structural information from the raw data is not straightforward. There are several XPS depth-profiling methods, including ion etching, angle-resolved XPS (ref. 2) and Tougaards approach, but all suffer various limitations. Here we report a simple, non-destructive XPS depth-profiling method that yields accurate depth information with nanometre resolution. We demonstrate the technique using self-assembled multilayers on gold surfaces; the former contain ‘marker’ monolayers that have been inserted at predetermined depths. A controllable potential gradient is established vertically through the sample by charging the surface of the dielectric overlayer with an electron flood gun. The local potential is probed by measuring XPS line shifts, which correlate directly with the vertical position of atoms. We term the method ‘controlled surface charging’, and expect it to be generally applicable to a large variety of mesoscopic heterostructures.

Antibodies targeting the catalytic zinc complex of activated matrix metalloproteinases show therapeutic potential

Endogenous tissue inhibitors of metalloproteinases (TIMPs) have key roles in regulating physiological and pathological cellular processes. Imitating the inhibitory molecular mechanisms of TIMPs while increasing selectivity has been a challenging but desired approach for antibody-based therapy. TIMPs use hybrid protein-protein interactions to form an energetic bond with the catalytic metal ion, as well as with enzyme surface residues. We used an innovative immunization strategy that exploits aspects of molecular mimicry to produce inhibitory antibodies that show TIMP-like binding mechanisms toward the activated forms of gelatinases (matrix metalloproteinases 2 and 9). Specifically, we immunized mice with a synthetic molecule that mimics the conserved structure of the metalloenzyme catalytic zinc-histidine complex residing within the enzyme active site. This immunization procedure yielded selective function-blocking monoclonal antibodies directed against the catalytic zinc-protein complex and enzyme surface conformational epitopes of endogenous gelatinases. The therapeutic potential of these antibodies has been demonstrated with relevant mouse models of inflammatory bowel disease. Here we propose a general experimental strategy for generating inhibitory antibodies that effectively target the in vivo activity of dysregulated metalloproteinases by mimicking the mechanism employed by TIMPs.

Fluorescent siderophore-based chemosensors: iron(III) quantitative determinations.

Abstract A highly sensitive and selective method is described for a rapid and easy determination of iron(III). This procedure is based on fluorimetric detection combined with the attractive properties of siderophores and biomimetic ligands, which are strong and selective ferric chelators. Azotobactin δ, a bacterial fluorescent siderophore, three fluorescent derivatives of desferriferrioxamine B with a linear structure (NBD-, MA-, NCP-desferriferrioxamine B) and one tripodal biomimetic ligand of desferriferrichrome carrying an anthracenyl fluorescent probe were examined. A very efficient static quenching mechanism by iron was observed for all the ligands considered in this work. Our results identify azotobactin δ as the most promising chemosensor of ferric traces in water, more sensitive than the NBD-desferriferrioxamine B fluorescent ligand. Under more lipophilic conditions, the anthryl-desferriferrichrome biomimetic analogue showed similar analytical potential and was found to be more sensitive than the lipophilic MA- and NCP-desferriferrioxamine B. Their detection limits were respectively 0.5 ng mL–1 for azotobactin δ and 0.6 ng mL–1 for the anthryl tripodal chelator. The calibration curves were linear over the range 0–95 ng mL–1 and 0–180 ng mL–1. Various foreign cations have been examined and only copper(II) and aluminium(III) were shown to interfere when present in similar concentrations as iron(III). The developed procedure using fluorescent siderophores or biomimetic ligands of iron(III) may be applied (1) to monitor iron(III)-dependent biological systems and (2) to determine iron(III) quantitatively in natural waters and in biological systems.

Molecular control of a GaAs transistor

Abstract The interactions between adsorbed organic molecules and the electronic charge carriers in specially made GaAs structures are studied by time- and wavelength-dependent measurements of the photocurrent. The adsorption of the molecules modifies the photocurrent decay time by orders of magnitude. The effects are molecularly specific, as they depend on the electronic properties and absorption spectrum of the molecules. These observations are rationalized by assuming that new surface states are created upon adsorption of the molecules and that the character of these states is controlled by the relative electronegativity of the substrates and the adsorbed molecules. The relevance for surface passivation and for construction of semiconductor-based sensors is indicated.

Characterisation of non-covalent complexes by electrospray mass spectrometry

Abstract Electrospray Mass Spectrometry (ESMS) has been used to analyse protein/metal ion complexes directly in solution. A synthetic siderophore analogue and two sulphur-iron proteins have been used as models for the study of protein/iron interactions. These experiments were successfully extended to a protein/cofactor complex interaction model, myohemoglobin. Our results open the door to the characterization of weak interactions between large molecules by ESMS.

The ferrichrome uptake pathway in Pseudomonas aeruginosa involves an iron release mechanism with acylation of the siderophore and recycling of the modified desferrichrome.

The uptake of iron into Pseudomonas aeruginosa is mediated by two major siderophores produced by the bacterium, pyoverdine and pyochelin. The bacterium is also able of utilize several heterologous siderophores of bacterial or fungal origin. In this work, we have investigated the iron uptake in P. aeruginosa PAO1 by the heterologous ferrichrome siderophore. (55)Fe uptake assays showed that ferrichrome is transported across the outer membrane primarily (80%) by the FiuA receptor and to a lesser extent (20%) by a secondary transporter. Moreover, we demonstrate that like in the uptake of ferripyoverdine and ferripyochelin, the energy required for both pathways of ferrichrome uptake is provided by the inner membrane protein TonB1. Desferrichrome-(55)Fe uptake in P. aeruginosa was also dependent on the expression of the permease FiuB, suggesting that this protein is the inner membrane transporter of the ferrisiderophore. A biomimetic fluorescent analogue of ferrichrome, RL1194, was used in vivo to monitor the kinetics of iron release from ferrichrome in P. aeruginosa in real time. This dissociation involves acylation of ferrichrome and its biomimetic analogue RL1194 and recycling of both modified siderophores into the extracellular medium. FiuC, an N-acetyltransferase, is certainly involved in this mechanism of iron release, since its mutation abolished desferrichrome-(55)Fe uptake. The acetylated derivative reacts with iron in the extracellular medium and is able to be taken up again by the cells. All these observations are discussed in light of the current knowledge concerning ferrichrome uptake in P. aeruginosa and in Escherichia coli.

Origin of the iron(III) binding and conformational properties of enterobactin.

Enterobactin (1) is one of the most efficient natural binders of ferric ions known to date. Structural analogues of enterobactin have been synthesized, including the tribenzamide (TBA) 7, which differs from enterobactin only by lacking the catechol hydroxyl groups. The analogues have been studied by a combination of IR, NMR, and CD spectroscopy, X-ray diffraction, and empirical-force-field calculations. These studies elucidated the origin of enterobactin’s unique binding properties and of its complex’s right-handed chirality. TBA 7 in its most stable conformation, preferred in nonpolar solvents, possesses C3 symmetry, its benzamide side chains are in axial positions, hydrogen bonds are formed between the amide hydrogen and the ring oxygen, and the phenyl rings are arranged in a right-handed (A) orientation. Uncomplexed enterobactin (1) is shown to resemble closely TBA 7. The relation of the preferred A chirality of TBA 7 to the observed A chirality of ( F e e ~ ~ t ) ~ is discussed. A comparison between enterobactin and the hitherto best synthetic binder, a tricatecholamide derivative of mesitylene 3, is presented. Enterobactin’s superiority is partly due to its lower molecular strain upon binding and partly due to the lower conformational freedom of uncomplexed enterobactin. The binding strain of (Fe~ent)~? resides more in the catecholamides than in the trilactone ring, while in the synthetic analogue 3 the mesitylene ring is more strained than the catecholamides. Metal ions are essential for the maintenance of living systems. The alkali metal ions such as sodium control the transmission of nerve impulses, the alkaline earth metal ions such as calcium act as secondary messengers, and the transition-metal ions such as iron and copper are involved in enzymatic redox processes.’ A large variety of ion carriers exist in nature to control the metal ion balance.* Carriers for the transition-metal ions are rare, with the exception of iron, for which protein and non-protein chelates exist. For iron, two families of low molecular weight carriers, or siderophores, are known: those utilizing hydroxamate groups and those utilizing catechol groups as binding sites.3 Among the latter carriers, enterobactin (1) assumes a unique p ~ s i t i o n . ~ Produced by enteric bacteria when grown in iron-deficient media, enterobactin is one of the most efficient binders and carriers known. It has a binding constant of log (Kbind) = 52 for the reaction Fe3+ + en@(Feent)3where “ent6-” is the sixfold deprotonated en ter~bac t in .~ Chemically, enterobactin is a tripodlike molecule. It consists of a trilactone ring, composed of three L-serine residues, each with an attached catechol ligand. Information on the conformation of enterobactin is limited to NMR studies in MezSO at elevated temperatures5 or to IR studies in water.6

Monitoring of iron(III) removal from biological sources using a fluorescent siderophore.

We present here the physicochemical and biochemical properties of NBD-DFO, the 7-nitrobenz-2-oxa-1,3-diazole (NBD) derivative of the siderophore, desferrioxamine B (DFO) (Lytton et al., Mol. Pharmacol. 40, 584, 1991). Modification of DFO at its terminal amine renders it more lipophilic, imparts to it fluorescent properties, and is conservative of the high-affinity iron(III) binding capacity. NBD-DFO partitions readily from aqueous solution into n-octanol (Pcoeff = 5) and displays solvent-induced shifts in absorption and fluorescence spectra. The relative quantum yield of the probes fluorescence increases over a 10-fold range with decreasing dielectric constant of the solvent. Fluorescence is quenched upon binding of iron(III) to the probe. We demonstrate here the application of NBD-DFO for the specific detection and monitoring of iron (III) in solutions and iron(III) mobilization from cells. Interactions between fluorescent siderophore and the ferriproteins ferritin and transferrin were monitored under physiological conditions. Iron removal from ferritin was evident by the demonstrable quenching of NBD-DFO fluorescence by scavenged iron(III). Quantitation of iron sequestered from cells by NBD-DFO or from other siderophore-iron(III) complexes was accomplished by dissociation of NBD-DFO-Fe complex by acidification and addition of excess ethylenediamin-etetraacetic acid. The sensitivity of the method and the iron specificity indicate its potential for monitoring chelatable iron under conditions of iron-mediated cell damage, iron overload, and diseases of iron imbalance such as malaria.

Functional Monolayers with Coordinatively Embedded Metalloporphyrins

Perpendicularly oriented iron porphyrins are absorbed onto a gold surface when interconnected long-chain diimidazolyl groups coordinate axially to the metal center from either side of the ring plane (see schematic representation). The stacking of the rings is simultaneously prevented. The monolayers have been characterized structurally and electrochemically.