Reaction of glucose catalyzed by framework and extraframework tin sites in zeolite beta

The isomerization of glucose into fructose is a large-scale reaction for the production of high-fructose corn syrup, and is now being considered as an intermediate step in the possible route of biomass conversion into fuels and chemicals. Recently, it has been shown that a hydrophobic, large pore, silica molecular sieve having the zeolite beta structure and containing framework Sn⁴⁺ (Sn-Beta) is able to isomerize glucose into fructose in aqueous media. Here, I have investigated how this catalyst converts glucose to fructose and show that it is analogous to that achieved with metalloenzymes. Specifically, glucose partitions into the molecular sieve in the pyranose form, ring opens to the acyclic form in the presence of the Lewis acid center (framework Sn⁴⁺), isomerizes into the acyclic form of fructose and finally ring closes to yield the furanose product. Akin to the metalloenzyme, the isomerization step proceeds by intramolecular hydride transfer from C2 to C1. Extraframework tin oxides located within hydrophobic channels of the molecular sieve that exclude liquid water can also isomerize glucose to fructose in aqueous media, but do so through a base-catalyzed proton abstraction mechanism. Extraframework tin oxide particles located at the external surface of the molecular sieve crystals or on amorphous silica supports are not active in aqueous media but are able to perform the isomerization in methanol by a base-catalyzed proton abstraction mechanism. Post-synthetic exchange of Na⁺ with Sn-Beta alters the glucose reaction pathway from the 1,2 intramolecular hydrogen shift (isomerization) to produce fructose towards the 1,2 intramolecular carbon shift (epimerization) that forms mannose. Na⁺ remains exchanged onto silanol groups during reaction in methanol solvent, leading to a near complete shift in selectivity towards glucose epimerization to mannose. In contrast, decationation occurs during reaction in aqueous solutions and gradually increases the reaction selectivity to isomerization at the expense of epimerization. Decationation and concomitant changes in selectivity can be eliminated by addition of NaCl to the aqueous reaction solution. Thus, framework tin sites with a proximal silanol group are the active sites for the 1, 2 intramolecular hydride shift in the isomerization of glucose to fructose, while these sites with Na-exchanged silanol group are the active sites for the 1, 2 intramolecular carbon shift in epimerization of glucose to mannose.

Current dependence of the energy gap in superconductors : a direct measurement

From the tunneling characteristics of a tin-tin oxide-lead junction, a direct measurement has been made of the energy-gap variation for a superconductor carrying a current in a compensated geometry. Throughout the region investigated – several temperatures near T_c and down to a reduced temperature t = 0.8 –the observed current dependence agrees quite well with predictions based on the Ginzburg-Landau-Gor’kov theory. Near T_c the predicted temperature dependence is also well verified, though deviations are observed at lower temperatures; even for the latter, the data are internally consistent with the temperature dependence of the experimental critical current. At the lowest temperature investigated, t = 0.8, a small “Josephson” tunneling current allowed further a direct measurement of the electron drift velocity at low current densities. From this, a preliminary experimental value of the critical velocity, believed to be the first reported, can be inferred in the basis of Ginzburg-Landau theory. For tin at t = 0.8, we find v_c = 87 m/sec. This value does not appear fully consistent with those predicted by recent theories for superconductors with short electronic mean-free-paths.

Recognition of double helical DNA by purine oligonucleotides via triple helix formation

Oligonucleotide-directed triple helix formation is one of the most versatile methods for the sequence specific recognition of double helical DNA. Chapter 2 describes affinity cleaving experiments carried out to assess the recognition potential for purine-rich oligonucleotides via the formation of triple helices. Purine-rich oligodeoxyribonucleotides were shown to bind specifically to purine tracts of double helical DNA in the major groove antiparallel to the purine strand of the duplex. Specificity was derived from the formation of reverse Hoogsteen G•GC, A•AT and T•AT triplets and binding was limited to mostly purine tracts. This triple helical structure was stabilized by multivalent cations, destabilized by high concentrations of monovalent cations and was insensitive to pH. A single mismatched base triplet was shown to destabilize a 15 mer triple helix by 1.0 kcal/mole at 25°C. In addition, stability appeared to be correlated to the number of G•GC triplets formed in the triple helix. This structure provides an additional framework as a basis for the design of new sequence specific DNA binding molecules.

In work described in Chapter 3, the triplet specificities and required strand orientations of two classes of DNA triple helices were combined to target double helical sequences containing all four base pairs by alternate strand triple helix formation. This allowed for the use of oligonucleotides containing only natural 3'-5' phosphodiester linkages to simultaneously bind both strands of double helical DNA in the major groove. The stabilities and structures of these alternate strand triple helices depended on whether the binding site sequence was 5'-(purine)_m (pyrimidine)_n-3' or 5'- (pyrimidine)_m (purine)_n-3'.

In Chapter 4, the ability of oligonucleotide-cerium(III) chelates to direct the transesterfication of RNA was investigated. Procedures were developed for the modification of DNA and RNA oligonucleotides with a hexadentate Schiff-base macrocyclic cerium(III) complex. In addition, oligoribonucleotides modified by covalent attachment of the metal complex through two different linker structures were prepared. The ability of these structures to direct transesterification to specific RNA phosphodiesters was assessed by gel electrophoresis. No reproducible cleavage of the RNA strand consistent with transesterification could be detected in any of these experiments.

I. The rate and mechanism of the vapor-phase reaction between water and parts-per-million concentrations of nitrogen dioxide. II. The rate and mechanism of the air oxidation of parts-per-million concentrations of nitric oxide in the presence of water vapor.

Part I

A study of the thermal reaction of water vapor and parts-per-million concentrations of nitrogen dioxide was carried out at ambient temperature and at atmospheric pressure. Nitric oxide and nitric acid vapor were the principal products. The initial rate of disappearance of nitrogen dioxide was first order with respect to water vapor and second order with respect to nitrogen dioxide. An initial third-order rate constant of 5.5 (± 0.29) x 10⁴ liter² mole^-2 sec^-1 was found at 25˚C. The rate of reaction decreased with increasing temperature. In the temperature range of 25˚C to 50˚C, an activation energy of -978 (± 20) calories was found.

The reaction did not go to completion. From measurements as the reaction approached equilibrium, the free energy of nitric acid vapor was calculated. This value was -18.58 (± 0.04) kilocalories at 25˚C.

The initial rate of reaction was unaffected by the presence of oxygen and was retarded by the presence of nitric oxide. There were no appreciable effects due to the surface of the reactor. Nitric oxide and nitrogen dioxide were monitored by gas chromatography during the reaction.

Part II

The air oxidation of nitric oxide, and the oxidation of nitric oxide in the presence of water vapor, were studied in a glass reactor at ambient temperatures and at atmospheric pressure. The concentration of nitric oxide was less than 100 parts-per-million. The concentration of nitrogen dioxide was monitored by gas chromatography during the reaction.

For the dry oxidation, the third-order rate constant was 1.46 (± 0.03) x 10⁴ liter² mole^-2 sec^-1 at 25˚C. The activation energy, obtained from measurements between 25˚C and 50˚C, was -1.197 (±0.02) kilocalories.

The presence of water vapor during the oxidation caused the formation of nitrous acid vapor when nitric oxide, nitrogen dioxide and water vapor combined. By measuring the difference between the concentrations of nitrogen dioxide during the wet and dry oxidations, the rate of formation of nitrous acid vapor was found. The third-order rate constant for the formation of nitrous acid vapor was equal to 1.5 (± 0.5) x 10⁵ liter² mole^-2 sec^-1 at 40˚C. The reaction rate did not change measurably when the temperature was increased to 50˚C. The formation of nitric acid vapor was prevented by keeping the concentration of nitrogen dioxide low.

Surface effects were appreciable for the wet tests. Below 35˚C, the rate of appearance of nitrogen dioxide increased with increasing surface. Above 40˚C, the effect of surface was small.