Ethylene Dehydroaromatization over Ga-ZSM-5 Catalysts: Nature and Role of Gallium Speciation

Bifunctional catalysis in zeolites possessing both Brønsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here, we examine the impact of framework and extra-framework gallium species on enriched aromatics production in zeolite ZSM-5. We compare three distinct methods of preparing Ga-ZSM-5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post-synthesis methods. Using a combination of state-of-the-art characterization, catalyst testing, and density functional theory calculations, we show that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework)–Ga(extra-framework) pairings, which can only be achieved in materials prepared by direct synthesis, are the most energetically favorable sites for reaction pathways leading to aromatics. Calculated acid site exchange energies between extra-framework Ga at framework sites comprised of either Al or Ga reveal a site-specific preference for stabilizing Lewis acids, which is qualitatively consistent with experimental measurements. These findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, which can have a marked impact on catalyst performance relative to conventional H-ZSM-5.     

Target patterns in reaction-diffusion systems

We consider the general reaction-diffusion system At = F(A) + ε D_M ²A + εg(→x, A), 0 < ε 1, where the small term εg(→x, A) represents the effects of localized impurities. We assume that the system A_t = F(A) has a stable time-periodic solution. Then we construct stable target pattern solutions of the full system. For typical initial conditions we find that these target patterns will arise only if g(→x, A) 0. Finally, we determine how target patterns interact and show that higher frequency target patterns eventually engulf neighboring lower frequency target patterns.     

the distribution of exit times for weakly colored noise

We analyze the exit time (first passage time) problem for the Ornstein-Uhlenbeck model of Brownian motion. Specifically, consider the positionX(t) of a particle whose velocity is an Ornstein-Uhlenbeck process with amplitudeσ/ρ and correlation time ε², $$dX/dt = \sigma Z/\varepsilon , dZ/dt = - Z/\varepsilon ^2 + 2^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} \xi (t)/\varepsilon $$ whereξ(t) is Gaussian white noise. Let the exit timet _ex be the first time the particle escapes an interval ?A, given that it starts atX(0)=0 withZ(0)=z ₀. Here we determine the exit time probability distributionF(t)≡Prob {t _ex>t} by directly solving the Fokker-Planck equation. In brief, after taking a Laplace transform, we use singular perturbation methods to reduce the Fokker-Planck equation to a boundary layer problem. This boundary layer problem turns out to be a half-range expansion problem, which we solve via complex variable techniques. This yields the Laplace transform ofF(t) to within a transcendentally smallO(e ^?A/εσ +e ^?B/εσ error. We then obtainF(t) by inverting the transform order by order in ε. In particular, by lettingB→∞ we obtain the solution to Wang and Uhlenbeck's unsolved problem b; throughO(ε²σ²/A ¹) this solution is $$F(t) = Erf\left\{ {\frac{{A + \varepsilon \sigma \alpha + \varepsilon \sigma z_0 }}{{2\sigma (t - \varepsilon ^2 \kappa )^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} }}} \right\} + ... for \frac{t}{{\varepsilon ^2 }} > > 1$$ andF=1 otherwise. Here, α=∥ξ(1/2)∥=1.4603?, where ξ is the Riemann zeta function, and the constant κ is 0.22749?.     

Resonant enhancement of two-photon absorption in substituted fluorene molecules

The degenerate and nondegenerate two-photon absorption (2PA) spectra for a symmetric and an asymmetric fluorene derivative were experimentally measured in order to determine the effect of intermediate state resonance enhancement (ISRE) on the 2PA cross section delta. The ability to tune the individual photon energies in the nondegenerate 2PA (ND-2PA) process afforded a quantitative study of the ISRE without modifying the chemical structure of the investigated chromophores. Both molecules exhibited resonant enhancement of the nonlinearity with the asymmetric compound showing as much as a twentyfold increase in delta. Furthermore, the possibility of achieving over a one order of magnitude enhancement of the nonlinearity reveals the potential benefits of utilizing ND-2PA for certain applications. To model ISRE, we have used correlated quantum-chemical methods together with the perturbative sum-over-states (SOS) expression. We find strong qualitative and quantitative correlation between the experimental and theoretical results. Finally, using a simplified three-level model for the SOS expression, we provide intuitive insight into the process of ISRE for ND-2PA.     

A novel kinetic trap for NO release from cytochrome c': a possible mechanism for NO release from activated soluble guanylate cyclase

Flash photolysis studies on the five-coordinate heme nitrosyl of Alcaligenes xylosoxidans cytochrome c' were carried out to investigate the ramifications of its proximal nitrosyl ligand on NO release. Delta absorbance spectra recorded 5 ms after photolysis indicate that approximately 5% of the photolyzed hemes are converted to a five-coordinate high spin ferrous state, revealing that reattachment of the endogenous His ligand is fast enough to trap some of the photolyzed heme. Analysis of NO rebinding suggests that the photolyzed ferrous protein is initially in a strained conformation, which relaxes on a millisecond time scale. The strained ferrous heme appears to contain a significantly labilized Fe-His bond, which allows direct second-order rebinding to the proximal face at high NO-concentrations. In contrast, the NO-binding properties of the relaxed conformation are similar to those previously observed in stopped-flow studies, which showed that a five-coordinate heme-nitrosyl is formed via a six-coordinate intermediate. The discovery of a rapid proximal His ligand reattachment to NO-dissociated heme reveals a novel "kinetic trap" mechanism for lowering the five-coordinate heme nitrosyl population in response to decreased ambient NO concentrations. Thus, NO dissociation from the five-coordinate heme nitrosyl, whether thermal or photochemical, is followed by rapid, and only slowly reversible, His reattachment which acts to kinetically trap the heme in its five-coordinate ferrous state. Because return to the five-coordinate heme nitrosyl requires two NO-dependent steps, the protein uses a kind of kinetic amplification of the thermodynamic dissociation that occurs in response to decreased NO concentrations. The implications of this "kinetic-trap" mechanism for NO release from soluble guanylate cyclase are discussed.     

Sol-gel encapsulated horseradish peroxidase: a catalytic material for peroxidation

This study addresses the viability of sol-gel encapsulated HRP (HRP:sol-gel) as a recyclable solid-state catalytic material. Ferric, ferric-CN, ferrous, and ferrous-CO forms of HRP:sol-gel were investigated by resonance Raman and UV-visible methods. Electronic and vibrational spectroscopic changes associated with changes in spin state, oxidation state, and ligation of the heme in HRP:sol-gel were shown to correlate with those of HRP in solution, showing that the heme remains a viable ligand-binding complex. Furthermore, the high-valent HRP:sol-gel intermediates, compound I and compound II, were generated and identified by time-resolved UV-visible spectroscopy. Catalytic activity of the HRP:sol-gel material was demonstrated by enzymatic assays by using I(-), guaiacol, and ABTS as substrates. Encapsulated HRP was shown to be homogeneously distributed throughout the sol-gel host. Differences in turnover rates between guaiacol and I(-) implicate mass transport of substrate through the silicate matrix as a defining parameter in the peroxidase activity of HRP:sol-gel. HRP:sol-gel was reused as a peroxidation catalyst for multiple reaction cycles without loss of activity, indicating that such materials show promise as reusable catalytic materials.     

Direct introduction of single protein channels and pores into lipid bilayers

We have developed a mechanical method for inserting single pores and channels into lipid bilayers. A hand-operated hydrogel probe, coated with a layer of proteins, is mechanically engaged with the lipid bilayer. The two major classes of membrane proteins (beta barrels and alpha-helix bundles) that can be inserted, thereby demonstrating the wide applicability of the approach. Recordings from the proteins show that they retain electrical properties that are the same as those of proteins inserted from solution. Protein-loaded probes can be used repeatedly, allowing individual pores to be rapidly screened one at a time. The method has implications for fundamental studies of cell membranes, array fabrication, and chemical screening.     

Adducts of thianthrene- and phenoxathiin cation radical tetrafluoroborates to 1-alkynes. Structures and formation of 1-(5-thianthreniumyl)- and 1-(10-phenoxathiiniumyl)alkynes on alumina leading to alpha-ketoylides and alpha-ketols

[structure: see text] Thianthrene cation radical tetrafluoroborate (Th*+ BF4(-)) added to the terminal alkynes 1-pentyne, 1-hexyne, 1-heptyne, 1-octyne, 1-nonyne, and 1-decyne to form trans-1,2-bis(5-thianthreniumyl)alkene tetrafluoroborates (1-6). Similarly, addition of phenoxathiin cation radical tetrafluoroborate (PO*+ BF4(-)) to the same alkynes gave 1,2-bis(10-phenoxathiiniumyl)alkene tetrafluoroborates (7-12). The trans configuration of two of the adducts (1 and 4) was shown with X-ray crystallography. When solutions of 1-6 in chloroform were stirred with activated alumina, cis elimination of a proton and thianthrene (Th) occurred with the formation of 1-(5-thianthreniumyl)alkyne tetrafluoroborates (1a-6a). Similar treatment of 8-12 caused elimination of a proton and phenoxathiin (PO) with formation of 1-(10-phenoxathiiniumyl)alkene tetrafluoroborates (8a-12a). Stirring of 1a-6a with alumina for short periods of time caused their conversion into 5-[(alpha-keto)alkyl]thianthrenium ylides (1b-6b) and alpha-ketols, RC(O)CH2OH (1c-6c).     

Catalytic subunit of protein kinase A caged at the activating phosphothreonine

Caged reagents are photoactivatable molecules with applications in biological research. While a great deal of work has been carried out on small caged molecules, less has been done on caged macromolecules, such as proteins. Caged proteins would be especially useful in signal transduction research. Since most proteins involved in cell signaling are regulated by phosphorylation, a means to cage phosphorylated proteins would be generally applicable. Here we show that the catalytic subunit of protein kinase A can be activated by thiophosphorylation at Thr-197. The modified protein can then be caged with 4-hydroxyphenacyl bromide to yield a derivative with a specific catalytic activity that is reduced by approximately 17-fold. Upon photolysis at near UV wavelengths, an approximately 15-fold increase in activity is observed, representing an approximately 85-90% yield of uncaged product with a quantum yield phi(P) = 0.21. Because protein kinases belong to a superfamily with structurally related catalytic domains, the protein chemistry demonstrated here should be applicable to a wide range of signaling proteins.