Jacqueline Libman

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.

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.

Controlling surfaces and interfaces of semiconductors using organic molecules

Abstract Control over semiconductor surface energetics can be achieved using different chemisorbed organic molecules with diverse electronic properties. We find evidence of such control over CdTe upon adsorption of dicarboxylic acid derivatives with different substituted phenyl rings. FT-IR measurements show that the dicarboxylic acid derivatives bind as carboxylates to form approximately one monolayer. Such chemisorption modifies both the band bending and the electron affinity (up to 500 and 700 mV, respectively), as measured by contact potential difference WPM Changes in band bending result from a coupling between molecular orbitals and surface states close to the valence band and depend on the withdrawing character of the phenyl substituent. A model is presented to interpret and explain the data.

Iron uptake in Ustilago maydis: studies with fluorescent ferrichrome analogues

Iron uptake by the phytopathogenic fungus Ustilago maydis was studied using synthetic biomimetic ferrichrome analogues and their fluorescently labelled derivatives as structural and dynamic probes, respectively. The use of structurally distinct analogues enabled determination of the structural requirements for recognition by the fungal iron-uptake system. The application of fluorescently labelled derivatives which convert from a non-fluorescent to a fluorescent state upon iron (III) release enabled monitoring of iron uptake in real time both fluorimetrically and microscopically. Different rates of 55Fe uptake were found for two structurally distinct synthetic analogues, B9 and B5, which differ in their amino acid building blocks. B9 mediated uptake of 55Fe at a higher rate than B5. The behaviour of the fluorescent derivatives B9-Ant (anthracene-labelled B9) and B5-Ant (anthracene-labelled B5) paralleled that of their non-labelled precursors. Exposure of fungal cells to B9-Ant led to a higher increase of fluorescence in the medium than exposure to B5-Ant, indicating a more effective iron uptake from B9-Ant. By using fluorescence microscopy it was possible to trace the label of B9-Ant. Fluorescence was localized in regularly shaped vesicles in the treated cells. The rate of fluorescence appearance within the cells lagged behind the rate of iron uptake, suggesting use of the siderophores for iron storage.

Multiple weak forces in ion-binding molecules

Natural ion binding molecules, specifically iron(ll1 carriers, are used as guiding advantage of the evolutionary edge of natural molecules and aims at reproducing the essential characteristics of the most potent natural compounds with the simplest possible synthetic structures. The potent Enterobactin siderophore is examined as a guiding model. Its H-bonding (hydrogen-bonding) network and elements of preorganization are reproduced with all-synthetic, C3-symmetric, chiral Triscatecholates to provide efficient and selective binders of high optical purity. The structural principles operating in iron(lllb Entero actin are then applied to the synthesis of Ferrichrome analogs, but with a shift of emphasis to conformational characteristics and biological activity. Comparison between two homologous families of Trishydroxamates demonstrates the role of H-bonding networks and van-der-Waals interactions in dictating the complexes’ isomeric and optical purity, and highlights the conformational consequences of H-bonds. Some analogs of Ferrichrome reproduce the function of the natural compound as microbial iron(ll1) carriers by binding to both membrane receptors and transport proteins. Other derivatives inhibit the performance of the natural carriers by blocking the membrane receptors. The three strands of the C3-symmetric ligands are extended, each carrying two hydroxamates, to become ditopic ion binders. These ligands form triple-stranded, helical metal complexes which are stabilized by H-bond networks and allow systematic extension to polyn uclea r com plexes. Our design was governed by a few basic principles. We separated functional elements from structural elements, and used chiral elements as structural probes. To allow systematic modifications we adopted a modular synthetic strategy, and made use of variable modules as building blocks. As a common theme we applied multiple weak forces, particularly H-bond networks, to shape the individual molecules, stabilize their complexes and possibly favor their assembly to supramolecular architectures. Throughout all stages of this work physicochemical and biological testing of the compounds was joined by theoretical calculations in order to put experimental observations into a coherent structural framework. models for the preparation of synthetic binders. I, his biomimetic approach takes

Synthetic ferrichrome analogues with growth promotion activity for Arthrobacterflavescens

Two families of trihydroxamic acid analogues of ferrichrome were chemically synthesized and tested for biological activity with Arthrobacter flavescens. Compounds using a tertiary amine as anchor showed little activity. Several compounds using tetrahedral carbon as anchor showed activity approaching or equalling that of the natural siderophore, ferrichrome. The biological activity is discussed in relation to physical and chemical properties of the analogues.

Chiral linear hydroxamates as biomimetic analogues of ferrioxamine and coprogen and their use in probing siderophore-receptor specificity in bacteria and fungi.

SummaryLinear hydroxamate derivatives, possessing chiral α-amino acid moieties, were synthesized and their iron transport activities were studied in bacteria and fungi. No growth-promoting activity could be detected in the Gram-positive hydroxamate-auxotrophAureobacterium flavescens JG9. However, Gram-negative enterobacteria, such asEscherichia coli, Pantoea agglomerans andHafnia alvei were able to utilize iron from these analogues. Uptake of55Fe-labeled analogues was inhibited by sodium azide, suggesting an active transport process. The receptors involved during uptake in enterobacteria were identified by using appropriate indicator organisms which are defective in the transport of either ferrioxamines (P. agglomerans FM13), coprogens (H. alvei), or both of these siderophore classes (E. coli fhuE). Our data suggest that the chiral hydroxamates are recognized by the ferrioxamine receptor (FoxA) and the coprogen receptor (FhuE) at a ratio which depends on the opticalν/δ isomer fraction and the nature of side chains. Transport was also observed in the fungusNeurospora crassa, known to take up coprogen rather than ferrioxamines, suggesting that in this fungus the synthetic analogues behave like coprogen.

ENGINEERING THE INTERFACE ENERGETICS OF SOLAR CELLS BY GRAFTING MOLECULAR PROPERTIES ONTO SEMICONDUCTORS

The electronic properties of semiconductor surfaces can be controlled by binding tailor-made ligands to them. Here we demonstrate that deposition of a conducting phase on the treated surface enables control of the performance of the resulting device. We describe the characteristics of the free surface of single crystals and of polycrystalline thin films of semiconductors that serve as absorbers in thin film polycrystalline, heterojunction solar cells, and report first data for actual cell structures obtained by chemical bath deposition of CdS as the window semiconductor. The trend of the characteristics observed by systematically varying the ligands suggests changes in work function rather than in band bending at the free surface, and implies that changes in band line-up, which appear to cause changes in band bending, rather than direct, ligand-induced band bending changes, dominate.