Enhanced activity for supported Au clusters: Methanol oxidation on Au/TiO2(110)

Gold clusters supported on TiO₂(110) exhibit unusual activity for the oxidation of methanol to formaldehyde. Temperature programmed desorption studies of methanol on Au clusters show that both Au and titania sites are necessary for methanol reaction. Isotopic labeling experiments with CD₃OH demonstrate that reaction occurs via OH bond scission to form a methoxy intermediate. When the TiO₂ surface is oxidized with ¹⁸O₂ before or after Au deposition, methanol reaction produces H₂¹⁸O below 300 K, indicating that oxygen from titania promotes OH bond scission and is incorporated into desorbing products. XPS experiments provide additional evidence that during methanol reaction on the Au/TiO₂ surface, methanol adsorption occurs on TiO₂, given that the titania support becomes slightly oxidized after exposure to methanol in the presence of Au clusters. While the role of TiO₂ is to dissociate the OH bond and form the reactive methoxy intermediate, the role of the Au sites is to remove hydrogen from the surface as H₂, thus preventing the recombination of methoxy and hydrogen to methanol. The decrease in formaldehyde yield with increasing Au coverage above 0.25 ML suggests that reaction occurs at Au–titania interfacial sites; scanning tunneling microscopy images of various Au coverages confirm that the number of interfacial sites at the perimeter of the Au clusters decreases as the Au coverage is increased between 0.25 and 5 ML.     

Engineered biosynthesis of geldanamycin analogs for Hsp90 inhibition

Geldanamycin, a polyketide natural product, is of significant interest for development of new anticancer drugs that target the protein chaperone Hsp90. While the chemically reactive groups of geldanamycin have been exploited to make a number of synthetic analogs, including 17-allylamino-17-demethoxy geldanamycin (17-AAG), currently in clinical evaluation, the "inert" groups of the molecule remain unexplored for structure-activity relationships. We have used genetic engineering of the geldanamycin polyketide synthase (GdmPKS) gene cluster in Streptomyces hygroscopicus to modify geldanamycin at such positions. Substitutions of acyltransferase domains were made in six of the seven GdmPKS modules. Four of these led to production of 2-desmethyl, 6-desmethoxy, 8-desmethyl, and 14-desmethyl derivatives, including one analog with a four-fold enhanced affinity for Hsp90. The genetic tools developed for geldanamycin gene manipulation will be useful for engineering additional analogs that aid the development of this chemotherapeutic agent.     

Free radical reactions at the Ru(0 0 0 1) surface: Atomic oxygen and dissociated NH3

X-ray photoelectron spectroscopy was used to study the effect of atomic oxygen on Ru(0 0 0 1), and the effect of dissociated ammonia on RuO₂/Ru(0 0 0 1), in UHV conditions at ambient temperature. The Ru(0 0 0 1) surface was exposed, at ambient temperature, to a mixed flux of atomic and molecular oxygen generated by dissociation of O₂ in a thermal catalytic cracker, with 45% dissociation efficiency. The detailed study of the XPS spectra shows the formation of a disordered multilayer oxide (RuO₂). No formation of higher oxides of Ru was observed. The formation of RuO₂ proceeded without saturation for total oxygen exposures of up to 10⁵ Langmuir, at which point an average oxide thickness of 68 Å was observed. RuO₂ formed by the reaction with atomic oxygen was exposed to a flux of NH_x (x = 1, 2) + H generated by the cracker. The reduction of RuO₂ to Ru metal was observed by XPS. An exposure of 3.6 × 10² L of NH_x + H, resulted in the observation of adsorbed H₂O and OH, but no evidence of lattice oxide. The chemisorbed species were removed by additional NH_x + H exposure. No nitrogen adsorption was observed.     

Heteroepitaxial thin film of iron phthalocyanine on Ag(1 1 1)

Ordering of submonolayer iron phthalocyanine (FePc) molecules deposited on Ag(1 1 1) was investigated using scanning tunneling microscopy. The room temperature deposition of FePc alone, without any annealing, results in no ordered overlayers. However, posterior annealing the substrate to 475 K leads to the formation of a two-dimensional oblique lattice with the lattice constants of 16.2 ± 0.3 Å and the angle of 78 ± 1° between them. The resulting FePc lattice is commensurate to the substrate lattice. In addition, the nearest neighbor distance in the lattice is significantly increased through a distinctive molecular orientation of the FePc molecules within the unit cell. The commensurate lattice with a large intermolecular distance is in sharp contrast to that observed from a close-packed square lattice that many other metallo-phthalocyanine molecules often self-assemble into. A possible reasoning behind this intriguing structure is discussed.     

Universal relaxational dynamics of gapped one-dimensional models in the quantum sine-Gordon universality class

A semiclassical approach to the low-temperature real-time dynamics of generic one-dimensional, gapped models in the sine-Gordon model universality class is developed. Asymptotically exact universal results for correlation functions are obtained in the temperature regime T < Delta, where Delta is the energy gap.     

Delayed luminescence analysis (DLA) of purine and pyrimidine ribose and deoxyribose nucleotide triphosphates in picomole quantities

A discovery is reported of a new system that enables one to quantitate the amounts of separated nucleotide triphosphates in picomole quantities. This system of delayed luminescence analysis (DLA) is sensitive to both purine and pyrimidine ribose and deoxyribose nucleotide triphosphates. A crude luciferin-luciferase (substrate-enzyme) preparation from firefly lanterns, in the presence of nucleotide triphosphate, is utilized to generate light that is detected by a liquid scintillation counter with the coincidence of the photomultiplier tubes turned off. Light is produced in a delayed fashion, the maximum emission being dependent on the type of nucleotide. Purine nucleotides (GTP, ITP, dATP, dGTP) give maximal light emission at approximately 2 mins; with the pyrimidine nucleotides the time required for maximal light emission was 5 min for UTP, dUTP, and TTP, 10 min for CTP, and 12 min for dCTP. A linear relationship on a log-log plot of light emission vs. concentration of nucleotide is demonstrated with ITP, dATP, UTP, and CTP.     

Unusual liquid state of hard-core bosons on the pyrochlore lattice

We study the physics of hard-core bosons with unfrustrated hopping (t) and nearest-neighbor repulsion (V) on the three dimensional pyrochlore lattice. At half-filling, we demonstrate that the small V/t superfluid state eventually becomes unstable at large enough V/t to an unusual insulating state which displays no broken lattice translation symmetry. Equal time and static density correlators in this insulator are well described by a mapping to electric field correlators in the Coulomb phase of a U(1) lattice gauge theory, allowing us to identify this insulator with a U(1) fractionalized Mott-insulating state. The possibility of observing this phase in suitably designed atom-trap experiments with ultracold atoms is also discussed, as are specific experimental signatures.     

Synthesis of a Systematic 64-Membered Heparan Sulfate Tetrasaccharide Library

Heparan sulfate (HS) has multifaceted biological activities. To date, no libraries of HS oligosaccharides bearing systematically varied sulfation structures are available owing to the challenges in synthesizing a large number of HS oligosaccharides. To overcome the obstacles and expedite the synthesis, a divergent approach was designed, where 64 HS tetrasaccharides covering all possible structures of 2-O-, 6-O- and N-sulfation with the glucosamine-glucuronic acid-glucosamine-iduronic acid backbone were successfully produced from a single strategically protected tetrasaccharide intermediate. This extensive library helped identify the structural requirements for HS sequences to have strong fibroblast growth factor-2 binding but a weak affinity for platelet factor-4. Such a strategy to separate out these two interactions could lead to new HS-based potential therapeutics without the dangerous adverse effect of heparin-induced thrombocytopenia.