Chemistry Department Seminar

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Warintra "Ning" Pitsawong of the UK Chemistry Department will be presenting a seminar titled:

Fundamental elements of the catalytic mechanism of Nitroreductase, a promiscuous enzyme

Abstract: The oxygen-insensitive nitroreductase from Enterobacter cloacae (NR) catalyzes two-electron reduction of nitroaromatics to the corresponding nitroso compounds and, subsequently, to hydroxylamine products. NR has an unusually broad substrate repertoire, which may be related to protein dynamics (flexibility) and/or a simple non-selective kinetic mechanism. To investigate the possible role of mechanism in NR's broad substrate repertoire, the kinetics of oxidation of NR by para-nitrobenzoic acid (p-NBA) were investigated using stopped-flow techniques at 4°C.  The results revealed a hyperbolic dependence on the p-NBA concentration with a limiting rate of 1.9 ± 0.1 s-1, indicating one-step binding prior to the flavin oxidation step. There is no evidence for a distinct binding step in which specificity might be enforced. The reduction of p-NBA is rate-limiting in steady-state turnover (1.7 ± 0.3 s-1). The pre-steady-state reduction kinetics of NR by NADH indicate that NADH reduces the enzyme with a rate constant of 700 ± 20 s-1 and a dissociation constant of 0.51 ± 0.04 mM.  Thus we demonstrate simple transient kinetics in both the reductive and oxidative half-reactions that help to explain NR's broad substrate repertoire.

The mechanism for nitroaromatic reduction by nitroreductase requires the transfer of two electrons and two protons overall. Electrons transfer from reduced anionic FMN, which is the cofactor of NR.  One proton has been proposed to be transferred from reduced flavin (as part of hydride transfer) or transferred as a proton from solvent in electron-coupled proton transfer. The other proton must be transferred from solvent. To gain insight into the sources of protons participating in nitroaromatic reduction, X-ray crystallography has been combined with measurement of the primary and solvent kinetic isotope effects (KIEs). The transient kinetics revealed that a large primary KIE of 3.2 ± 0.2 applies to flavin reduction by NADH (reductive half-reaction) suggesting hydride transfer directly from NADH to the flavin N(5). Moreover, a primary KIE of 1.5 ± 0.05 applies to the turnover number, indicating hydride transfer from N(5) atom to p-NBA (oxidative half-reaction). The measured solvent KIEs are consistent with solvent as the source of the second proton.  Thus we have been able to characterize electron transfer events via the optical signatures of the different oxidation states of the flavin, and proton transfer steps via KIE to document the fundamental steps that make up the catalized chemical reactions of NR.

Faculty Advisor: Dr. Anne-Frances Miller

 

Date:
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Location:
CP-137

Chemistry Department Seminar

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Please note the updated time and location of this event.



Dr. Charles Chusuei of Middle Tennessee State University will be presenting a seminar titled:

Increasing Amperometric Detection of H2O2 via Controlling ZnO Nanosized Morphology

 
Abstract: Hydrogen peroxide (H2O2) is an important marker for detecting the onset of food spoilage, oxidative stress and management of cardiovascular disease.  The synthesis of zinc oxide (ZnO) nanostructures followed by attachment to multiwalled carbon tubes is shown, resulting in a composite with a unique synergistic effect. Morphology and size of ZnO was controlled using hydrothermal synthesis, varying the hydrothermal treatment temperature, prior to attachment to carboxylic acid functionalized multi-walled carbon nanotubes for sensing applications. A strong dependence of electrocatalytic activity on nanosized ZnO shape was shown.  Higher activity for H2O2 reduction was achieved when nanocomposite precursors with a roughly semi-spherical morphology (no needle-like particles present) formed at 90°C.  A 2.4-fold increase in cyclic voltammetry current accompanied by decrease in overpotential from the composites made from the nanosized, needle-like-free ZnO shapes was observed as compared to those composites produced from needle-like shaped ZnO.   Electrocatalytic activity varied with pH, maximizing at pH 7.4.  A stable, linear response for H2O2 concentrations was observed in the 1–20 mM concentration range.  The low overpotential observed in the CV demonstrates the composite’s selectivity  as it is well-separated from common interfering analytes, such as uric acid.
 

Faculty Host: Dr. David Atwood

 

Date:
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Location:
CP-137

Chemistry Department Seminar

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Dr. Yujun Shi of the University of Calgary will be presenting a seminar titled: 

Gas-phase and Surface Chemistry in the Hot-wire Chemical Vapor Deposition Process of Silicon Carbide

Abstract: Hot-wire chemical vapor deposition (HWCVD) has received continued research interest as a powerful technique for the deposition of semiconductors, metal oxide nanoparticles, and polymer coatings. Extensive studies can be found on optimizing the process conditions and characterizing the properties of final products, however  a lot of work remains to understand the reactions in the gas phase and on the wire/substrate surfaces for a complete understanding of the film growth mechanisms and for a rational and efficient improvement of the processes. Our group has studied the gas-phase reaction chemistry when using open-chain alkylsilane and cyclic silacyclobutane molecules, the two important type of single-source organosilicon precursors for silicon carbide formation. In this seminar, results will be presented on our investigation of the complex reaction chemistry of these two types of source gases using laser ionization mass spectrometric diagnostics in combination with isotope labeling and chemical trapping. In addition, the alloying of metal filaments, which serve as catalysts in HWCVD, will also be discussed. The nature of the alloys formed is closely linked to the gas-phase chemistry exhibited by the precursor gases. The phase changes in the alloying processes depend heavily on the interplay of Si/C diffusion into the wire and their evaporation from the wire surface.  
 

Faculty Host: Dr. Dennis Clouthier

Date:
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Location:
CP-137

Chemistry Department Seminar

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Dr. David R. McMillan of Purdue University will be presenting a seminar titled:

The Ins and Outs of Cationic Porphyrins Binding to DNA Platforms

Abstract: The well known cationic porphyrin, meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (H2T4), binds effectively to DNA. However, literature studies reveal the binding motif varies with the base make up of double-stranded hosts. Characterizing the binding interactions is of interest from the point of view of fundamental supramolecular chemistry as well as with regard to potential therapeutic applications. Meso-N-methylpyridinium-4-yl substituents of H2T4 play a critical role in shaping the binding because they affect the steric properties, overall charge, and water solubility of the ligand. In search of additional spectroscopic handles and more consistent binding, the McMillin group has used absorbance, emission, and circular dichroic methods to characterize the binding interactions of copper(II) derivatives of sterically less demanding, dicationic systems like 5,15-di(N-methylpyridinium-4-yl)porphyrin (H2tD4), as well as the H2T4 control. It turns out the substituents affect not only the mode of binding but the reactivity as well. When copper(II) is present, luminescence is especially useful for establishing binding motifs to a variety of single- and multi-stranded DNA hosts. Binding patterns emerge, and ligand-ligand interactions occur, especially at high drug load. The Pd(T4) and Pd(tD4) analogues are more useful for sensitizing singlet oxygen, because the triplet lifetimes extend to hundreds of microseconds in deoxygenated solution. Surprisingly, the efficiency of singlet oxygen production is similar for intercalated and externally bound forms of the bulky Pd(T4) system. However, energy-transfer quenching is a strikingly more efficient process with the sterically friendly analogue, Pd(tD4), which exclusively binds by intercalation. Better orbital overlap in the precursor complex may account for the enhanced energy-transfer rate.

Faculty Host: Dr. Edith Glazer

 

Date:
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Location:
CP-137

Perfect Fusion: Teaching English as a Second Language

The visiting South Korean teachers currently studying at UK are part of a long line of successful educational partnerships for the University’s Center for English as a Second Language (CESL) program. The experience for these teachers, however, is a bit different: they are the first to receive graduate credit by participating UK’s newly-formed master’s program in Teaching English as a Second Language (TESL).

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