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Bi-Weekly Seminar

Electron Beam Characterization of YBa2Cu3O7-δ

Prof. James K. Meen

by: Prof. James K. Meen

Date: Friday August 21, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

There are significant challenges to characterizing YBa2Cu3O7-δ (Y123) by electron microbeam but the results obtained have considerable utility. Few samples of Y123 have that cation stoichiometry. Variations within individual monoliths are used to map crystallization sequence and it will be shown that this is not necessarily from a single nucleus. Melting relations under different partial pressures of oxygen are employed to show that copper is in mixed valence state in Y-Ba-Cu oxide liquids even in pure oxygen and this has its own influence on the phase relations and in crystallization of Y123. Determination of oxygen content of Y123 is critical in its characterization but is altered by procedures used to prepare samples for analysis and by aging of the samples. The Cu L self-absorption spectrum of Y123 shows marked changes with oxygen doping and varies within some samples on a micron-scale.

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Bi-Weekly Seminar

Electric Field Driven Torque in Rotary Biological Motors

Prof. John H. Miller

by: Prof. John H. Miller

Date: Friday July 24, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Rotary motors, including ATP synthase, V-type ATPases, and the bacterial flagellar motor, play crucial roles in living organisms. In humans and other eukaryotes, ATP synthase operates in the inner membranes of mitochondria to produce adenosine triphosphate (ATP), life’s chemical currency of energy. V-type ATPases utilize the energy of ATP hydrolysis to create electrochemical potential differences (usually of protons) across diverse biological membranes. I will describe our recently proposed electric field driven torque model of ion-driven rotary motors. The model predicts a scaling law relating torque to the number of ion-carrying subunits in the rotor, the number of stators, and the ion motive force across the membrane. When the FO complex of ATP synthase is coupled to F1, the model predicts a minimum proton motive force (pmf) needed to drive ATP production by F1. By contrast the model predicts a maximum pmf against which the V1 complex of a V-type ATPase can overcome the opposing torque by V0 to pump protons back across the membrane. We are also working to develop label-free electromagnetic sensors to detect activity and possible dysfunction of mitochondrial and other enzymes. Dysfunction of mitochondrial enzymes has been implicated in type-2 diabetes, cancer, heart disease, and neurodegenerative diseases, while dysfunction of V-type ATPases has been implicated in osteopetrosis, distal renal tubular acidosis, and many other diseases. Therefore, improved understanding of such enzymes is broadly significant to human health.

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Bi-Weekly Seminar

Modulation of Calmodulin Plasticity by the Effect of Macromolecular Crowding

 Margaret S. Cheung

by: Margaret S. Cheung

Date: Friday May 22, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In vitro biochemical reactions are most often studied in dilute solution, a poor mimic of the intracellular space of eukaryotic cells which are crowded with mobile and immobile macromolecules. Such crowded conditions exert volume exclusion and other entropic forces that have the potential to impact chemical equilibria and reaction rates. In this article, we used the well characterized and ubiquitous molecule calmodulin (CaM) and a combination of theoretical and experimental approaches to address how crowding impacts CaM's conformational plasticity. CaM is a dumbbell shaped molecule that contains four EF hands (two in the N-lobe and two in the C-lobe) that each could bind Ca2+ leading to stabilization of certain substates that favor interactions with other target proteins. Using coarse-grained molecular simulations, we explored the distribution of CaM conformations in the presence of crowding agents. These predictions in which crowding effects enhances the population of compact structures were then confirmed in experimental measurements using fuorescence resonance energy transfer techniques of donor/acceptor labeled CaM under normal and crowded conditions. We further explored the folding energy landscape and examined the structural characteristics of CaM at free energy basins using protein reconstruction methods. We discovered that crowding stabilizes several different compact conformations, which refects the inherent plasticity in CaM's structure. From these results, we suggest that the EF hands in the C-lobe are fexible and can be thought of as a switch, while those in the N-lobe are stiff as analogous to a rheostat. New combinatorial signaling properties may arise from the product of the differential plasticity of the two distinct lobes of CaM in the presence of crowding. We discuss the implications of these results for modulating CaM's ability to bind Ca2+ and target proteins.

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Bi-Weekly Seminar

Electropolymerizable Dendrimer and Hybrid Nanomaterials

Prof. Rigoberto  Advincula

by: Prof. Rigoberto Advincula

Date: Friday May 08, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In this talk, we describe the investigation of dendrimeric precursor and hybrid nanomaterials towards the formation of conjugated polymer nanoparticles, networks, nanopatterning, and nanobjects. Electropolymerizable precursor polymer materials have been widely reported by our group and have been used to modify electrode surfaces with the formation of conjugated polymer network films. Most of these materials are based on linear polymers and block/graft copolymers. Very few reports have been given on dendritic precursor materials utilized for their electrochemical activity. In this talk, we will describe several strategies in which we have synthesized dendritic precursor polymers. These materials have applications for conducting polymer-based energy transfer materials, nanoelectronics, and sensing.

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Bi-Weekly Seminar

Direct Probe of the Key Building Block of the Fe-based Superconductors with Scanning Tunneling Microscopy/Spectroscopy (STM/S)

Dr. Shuheng H. Pan

by: Dr. Shuheng H. Pan

Date: Thursday April 23, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The recently discovered superconductivity in iron (Fe)-based compounds is another exciting advancement in condensed matter physics since the discovery of high-Tc superconductivity in cuprates. Using a UHV Low Temperature Scanning Tunneling Microscope, we have been studying the structural and electronic properties of the parent and Co-doped BaFe2As2 compound. We find that, by low temperature in situ cleaving, we are able to expose the key building block - the Fe-As layer of this compound, where superconductivity is believed to occur. With STM/S, we directly probe this key building block with spatial resolution down to atomic scale. STM is a surface sensitive technique. Keeping this in mind, I will demonstrate how we use this high real-space resolution and surface sensitive technique to learn the structural and electronic properties within the bulk. I will also discuss our results on the density-of-states (DOS) evolution with doping, the scaling of the superconducting energy gap, and some electronic local effects that may be used to help determine the pairing symmetry.

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