M. Lahav

Nanostructured Gold Colloid Electrodes

Recent advances in the assembly of nanoparticle superstructures on electrodesare addressed here. Methods for the assembly and characterization of these arrays are summarized and their electronic, photoelectrochemical, and sensor applications are discussed. The Figure shows a one-layer architecture of Au nanoparticles and tetracationic cyclophanes on an amine-functionalized ITO support.

Abrogation by prostaglandin F2α of LH-stimulated cyclic AMP accumulation in isolated rat corpora lutea of pregnancy

Abstract Corpora lutea explanted from rats on the sixth day of pregnancy responded to luteinizing hormone (LH; 5 μg/ml) in vitro with a two- to five-fold increase in cellular cyclic AMP (cAMP) concentration. The maximal cAMP level was reached within 60 min and maintained to the end of the 2 hr-incubation. On incubation with prostaglandin F 2α (PGF 2α ) in addition to LH, this rise in cAMP accumulation was prevented. For significant suppression, 1.4 × 10 −5 M PGF 2α was required. In the absence of LH, PGF 2α (4.2 × 10 −5 M) caused no change in cellular cAMP. Addition of PGF 2α (4.2 × 10 −5 M) to the incubation medium after the maximal response to LH was attained, caused the cAMP concentration to return to its basal level within 15 min. This abrogation of LH-stimulated cAMP accumulation represents the earliest and hence possibly the triggering event in PGF 2α -induced luteolysis.

Imprinting of specific molecular recognition sites in inorganic and organic thin layer membranes associated with ion-sensitive field-effect transistors

Molecular recognition sites were imprinted in inorganic TiO2 films, and acrylamide–acrylamidephenylboronic acid copolymer membranes, associated with ion-sensitive field-effect transistors, ISFETs, that act as transduction devices for the association of the substrates to the imprinted membranes. Molecular structures of carboxylic acids, e.g. 4-chlorophenoxyacetic acid (1), 2,4-dichlorophenoxyacetic acid (2), fumaric acid (3), and maleic acid (4), are imprinted in TiO2 films. The imprinted sites reveal high specificity, and substrates, structurally-related to the imprinted compounds are fully differentiated by the imprinted membranes. The specificity of the imprinted sites originates from the complementary structural fitting and ligation of the guest carboxylic acid residues to the Ti(IV)–OH sites in the host carboxylic acids to the imprinted cavities. An acrylamide–acrylamidephenylboronic acid copolymer acts as a functional polymer for the imprinting of nucleotides, e.g. adenosine 5′-monophosphate, AMP, (7), guanosine 5′-monophosphate, GMP, (8), or cytosine 5′-monophosphate, CMP, (9). The specificity of the imprinted nucleotide sites originates from the cooperative binding interactions between the nucleotides and the boronic acid ligand and acrylamide H-bonds. The detection regions and sensitivities for sensing of the different substrates by the functional polymers are determined.

Crystallinity of the Double Layer of Cadmium Arachidate Films at the Water Surface

A crystalline counterionic layer at the interface between an electrolyte solution and a charged layer of insoluble amphiphilic molecules was observed with grazing incidence synchrotron x-ray diffraction. Uncompressed arachidic films spread over 10–3 molar cadmium chloride solution (pH 8.8) spontaneously form crystalline clusters with coherence lengths of ∼1000 angstroms at 9�C. Ten distinct diffraction peaks were observed, seven of which were attributed to scattering only from a crystalline Cd2+ layer and the other three to scattering primarily from the arachidate layer. The reflections from the Cd2+ layer were indexed according to a 2 x 3 supercell of the arachidate lattice with three Cd2+ ions per cadmium unit cell.

Electrochromic Behavior of a Self-Propagating Molecular-Based Assembly

A metallo-supramolecular network undergoes reversible redox chemistry on indium-tin oxide (ITO) coated glass substrates with concurrent color change. The switching time, long-term stability, and coloration efficiency are competitive with polymeric materials such as the industrially important PEDOT.

Imprinting of Chiral Molecular Recognition Sites in Thin TiO2 Films Associated with Field-Effect Transistors: Novel Functionalized Devices for Chiroselective and Chirospecific Analyses

(R)- or (S)-2-Methylferrocene carboxylic acids, (R)-1 or (S)-1, (R)- or (S)-2-phenylbutanoic acid, (R)-2 or (S)-2, and (R)- or (S)-2-propanoic acid, (R)-3 or (S)-3, can be imprinted in thin TiO2 films on the gate surface of ion-sensitive field-effect transistor (ISFET) devices. The imprinting is performed by hydrolyzing the respective carboxylate TiIV butoxide complex on the gate surface, followed by washing off the acid from the resulting TiO2 film. The imprinted sites reveal chiroselectivity only towards the sensing of the imprinted enantiomer. The chiral recognition sites reveal not only chiroselectivity but also chirospecificity and, for example, the (R)-2-imprinted film is active in the sensing of (R)-2, but insensitive towards the sensing of (R)2-phenylpropanoic acid, (R)-3, which exhibits a similar chirality. Similarly, the (R)-3-imprinted film is inactive in the analysis of (R)-2. The chiroselectivity and chirospecificity of the resulting imprinted films are attributed to the need to align and fit the respective substrates in precise molecular contours generated in the cross-linked TiO2 films upon the imprinting process.

The structure and symmetry of crystalline solid solutions: a general revision.

Mixed single crystals composed of host and guest organic molecules of similar structures and shapes are shown to comprise sectors with different host-guest distributions and to have symmetries lower than that of the host crystal. These properties are determined by the structure of the guest and the surface structures of the crystal faces through which the guest molecules are occluded. This general concept is illustrated by studies of three mixed crystal systems,(E)-cinnamamide—(E)-2-thienylacrylamide, (E)-cinnamamide—(E)-3-thienylacrylamide, and(S)-asparagine—(S)-aspartic acid, with x-ray and neutron diffraction and solid-state photochemistry.

Racemic β-Sheets as Templates of Relevance to the Origin of Homochirality of Peptides: Lessons from Crystal Chemistry

The origin of life is a historical event that has left no relevant fossils; therefore, it is unrealistic to reconstruct the chronology of its occurrence. Instead, by performing laboratory experiments under conditions that resemble the prebiotic world, one might validate feasible reaction pathways and reconstruct model systems of artificial life. Creating such life in a test tube should go a long way toward removing the shroud of mystery over how it began naturally. The riddle of the appearance of natural proteins and nucleic acids--that is, biopolymers wholly consisting of homochiral subunits (L-amino acids and D-sugars, respectively)--from the unanimated racemic prebiotic world is still unsolved. There are two hypotheses concerning the sequence of their emergence: one maintains that long homochiral (isotactic) peptides must have been formed after the appearance of the first living systems, whereas the other presumes that such biopolymers preceded the primeval enzymes. The latter scenario necessitates, however, the operation of nonlinear synthetic routes, because the polymerization of racemates in ideal solutions yields chains composed of residues of either handedness. In this Account, we suggest applying lessons learned from crystal chemistry, in which molecules from isotropic media are converted into crystals with three-dimensional (3D) periodic order, to understand how the generation of homochiral peptides from racemic alpha-amino acids might be achieved, despite its seemingly overwhelming complexity. We describe systems that include the self-assembly of activated alpha-amino acids either in two-dimensional (2D) or in 3D crystals, followed by a partial lattice-controlled polymerization at the crystal-aqueous solution interface. We also discuss the polymerization of mixtures of activated hydrophobic racemic alpha-amino acids in aqueous solutions, as initiated by primary amines or thiols. The distribution of the diastereomeric oligopeptides was analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and MS/MS with monomers enantioselectively tagged with deuterium. The reaction performed in aqueous solutions encompasses the following sequential steps: (i) formation of a library of short racemic peptides enriched with isotactic diastereoisomers during the early stages of the polymerization, and (ii) self-assembly of oligopeptides into racemic beta-sheet colloidal-like aggregates that are delineated by enantiotopic sites or rims; these operate as templates (nuclei) for regio-enantioselective growth in the ensuing steps of chain elongation. Desymmetrization of the racemic mixtures of peptides was achieved with enantiopure alpha-amino acid esters as initiators. The enantiomeric excess of the isotactic peptides, not including the initiator, varies with chain length, the result of a cross-enantiomeric impeding mechanism. Our results suggest a feasible scenario in which primitive homochiral peptides might have emerged early in the prebiotic world.

An enlarged bis-bipyridinium cyclophane-Au nanoparticle superstructure for selective electrochemical sensing applications

Highly stable electrostatically-linked superstructures of 13 nm Au-colloids have been constructed by the use of the tetracationic cyclophanes cyclobis(paraquat-p-phenylene) and cyclobis(paraquat-p-biphenylene). These architectures have been characterized by optical and electrochemical means and exhibit sensing capabilities that are shown to depend on the crosslinking cyclophane. While superstructures linked by cyclobis(paraquat-p-phenylene) sense hydroquinone derivatives and not ferrocene derivatives, those linked by the larger receptor cyclobis(paraquat-p-biphenylene) sense only the ferrocene derivatives. This high degree of selectivity is based on the topological and supramolecular fit of the analyte in the cavity of the receptor. Arrays containing both cyclophanes are shown to exhibit sensing characteristics that are dependent on the position of each cyclophane in the superstructure, possibly as a consequence of the limited porosity of the composite.

A Cyclic Continuous Process for Converting Conglomerates into Optically Pure Enantiomers by Crystallization and Dissolution with the Assistance of ‘Tailor-made’ Polymers

Abstract An industrially feasible kinetic process for the conversion of d , l racemic mixtures that crystallize in the form of conglomerates into optically pure materials was elaborated. The method takes into consideration the presence of a single-enantiomer polymer that causes crystals that match it in chirality to lag far behind their enantiomers both in growth and in dissolution. This phase lag is exploited in a repeated cycle of growth and dissolution to collect crystals of one kind after partial growth and of the other before complete dissolution. Three major steps are involved: (a) Kinetically controlled crystallization of a racemic conglomerate in the presence of chiral polymers at temperature T 1 , modeled on the basis of the structure and morphology of the 3D crystals. The polymer inhibits the precipitation of the undesired enantiomorph, collecting the desired one by filtration; (b) Preferential dissolution of added racemic mixture of crystals of the substrate in the mother liquor enriched with the undesired enantiomer and the chiral polymer at temperature T 2 . This step takes advantage of concentration-gradients of the two enantiomers in the filtrate and the enatioselective dissolution induced by the chiral polymer; (c) The separation by filtration followed by racemization of the undesired enantiomer for additional cycles of resolution. The process is illustrated for the conversion of d , l methionine·HCl that crystallizes in the form of a conglomerate that displays twinning of enantiomorphous lamellae into the corresponding l or d Met·HCl in kilograms scale.