chemistry seminar

Synthesis of Metal Oxide Surface and Interface Arrays by a Combined Solid-Liquid- Vapor/Vapor-Liquid-Solid Approach

Date: 
Wednesday, July 15, 2020 - 1:00pm to 2:00pm
Location: 
Zoom

This project was motivated by an in situ heating experiment in the transmission electron
microscope (TEM) in which gold (Au) nanoparticles were observed to dissolve tin dioxide (SnO2)
nanowires (NWs) under vacuum. The explanation for this observation was that the hightemperature
and low-pressure environment of the TEM caused the reverse reaction of the wellknown
vapor-liquid-solid (VLS) method commonly used to grow NWs. In the VLS process, a
metal catalyst absorbs reactant vapor until it becomes supersaturated. The precipitation of the NW
occurs at the liquid-solid interface, which ceases when there is no longer reactant vapor, and the
diameter of the NW is determined by the diameter of the original catalyst. The reverse process, the
solid-liquid-vapor (SLV) method occurs when atoms in a solid NW diffuse into the metal catalyst.
Eventually, the metal catalyst becomes supersaturated and the vapor escapes at the liquid-vapor
interface. In this dissertation we demonstrate the combination of the SLV and the VLS mechanisms
to create embedded heterogeneous interfaces in a variety of metal oxides. Metal catalysts are first
used to etch metal oxide surfaces producing hollow channels that we term “negative nanowires”,
and after etching the metal catalyst is reused to grow a NW of a different material from within the
channel to form a crystalline interface. Understanding the chemical structure at these interfaces is
both crucial and fascinating because diverse materials may interact in a variety of ways, including
atomic mixing of the two structures and/or the formation of an abrupt crystalline interface or gap.
We present our approach, therefore, towards gaining a comprehensive understanding of the
structure-function relationship of these materials, focusing on particular on the interfacial region,
to allow the design of new nanomaterials with tailored functionality.

Thinking Cells as Macromolecules: A Chemist's Pondering Upon Cell Biology

Date: 
Friday, December 6, 2019 - 4:00pm to 5:00pm
Location: 
CP-114
Tags/Keywords:
Type of Event (for grouping events):

Abstract:  Conventionally physical chemistry is a field that mainly investigates physicochemical phenomena at atomic and molecular levels. Noticing the analogy between molecular (especially macromolecular) dynamics and cellular dynamics, in the past few years my lab has focused on introducing and generalizin

g the techniques and concepts of physical chemistry into cell biology studies. In this talk I will first discuss a long-standing Nobel-Prize winning puzzle on olfaction. Each olfactory sensory neuron stochastically expresses one and only one type of olfactory receptors, but the molecular mechanism remained unanswered for decades. I will show how simple physics taught in introductory physical chemistry textbook explains this seemingly complex problem, and briefly mention our ongoing efforts of investigating chromosome dynamics with a CRISPR-dCas9-based live cell imaging platform. 

In the second part of my talk, I will discuss our efforts on developing an emerging new field of single cell studies of the dynamics of cell phenotypic transition (CPT) processes, in parallel to single molecule studies in  chemistry. Mammalian cells assume different phenotypes that can have drastically different morphology and gene expression patterns, and can change between distinct phenotypes when subject to specific stimulation and microenvironment. Some examples include stem cell differentiation, induced reprogramming (e.g., iPSC) and others. In many aspects a CPT process is analogous to a chemical reaction. Using the epithelial-to-mesenchymal transition as a model system, I will present an integrated experimental-computational platform, and introduce concepts from chemical rate theories such as transition state, transition path, and reactive/nonreactive trajectories to quantitatively study the dynamcis of CPT processes.

Research: https://www.csb.pitt.edu/Faculty/xing/

 

Zirconium in the Spotlight: New Pathways to Photoluminescent Molecules

Date: 
Friday, December 7, 2018 - 12:00pm to 12:50pm
Location: 
CP-114
Tags/Keywords:
Type of Event (for grouping events):

Zirconium in the Spotlight: New Pathways to Photoluminescent Molecules

Carsten Milsmann

C. Eugene Bennett Department of Chemistry, West Virginia University

The efficient utilization of sunlight as a carbon-neutral, renewable energy source remains a challenge for scientists across many disciplines. The enormous scope of solar energy conversion on a global scale requires the development of photoactive materials from readily available and economically viable resources. Photoluminescent complexes based on earth-abundant early transition metals or main group elements present an attractive low-cost, low-toxicity alternative to precious metal photosensitizers commonly used in photochemical processes and solar energy conversion. However, the fundamentally different electronic structures of these elements requires the development of new approaches to light-induced charge separation and electron transfer.

This presentation will highlight the Milsmann group’s efforts to establish design principles for the generation of luminescent early transition metal complexes that can undergo photo-induced single electron transfer (SET) reactions upon irradiation with visible light. A particular focus will be on phosphorescent zirconium complexes that exhibit exceptionally long triplet excited state lifetimes. Our results show that these complexes can not only replace precious metal photosensitizers in photoredox catalytic reactions, but may exhibit optical properties that complement those of traditional late metal compounds. In addition, research towards the development of metal-free phosphorescent molecules based on silicon will be discussed.


Two component structure of Graphene Oxide: New insights in the oxygen surface coverage

Date: 
Friday, November 30, 2018 - 12:00pm to 12:50pm
Location: 
CP-114
Tags/Keywords:
Type of Event (for grouping events):

Abstract:

The structure of graphene oxide (G-O) is still a challenging topic for developing useful and novel applications, where a compatibility, affinity or even covalent linkage is required at the interface. There is still a no consensus about the essential details, especially relating to the epoxy groups as main complexes in the basal plane, as well as the simultaneous presence of G-O sheets and oxidative debris (OD), with large difference in their oxygen content between both entities. The present seminar  deeps on this topic, through the characterization the base-washed G-O (bwGO) sheets, the OD and the humic fraction of the OD obtained by base digestion, when the parent G-O was previously dried or not, and previously sonicated or not. It was found that the presence of lactols at graphene edges as the dominant surface complexes agrees with all the characterization techniques, and also explains the high decrease of oxygen surface coverage in bwGO, where no carboxylic group removal was observed. These findings suggest that the Hummers-Offeman reaction produces a chemical scissor-effect during the water/hydrogen peroxide quenching step, yielding a broad size distribution of G-O sheets, with little in-plane oxidation, and the vast majority of edges being oxidized to form 7-ring lactol-type heterocyclic functionalities. Therefore, OD consists essentially of the very low sheet-size fractions, highly oxidized poly aromatic hydrocarbons, with a very high oxygen:carbon ratio due to the very high edge to weight ratio. 

Biological macromolecules in crowded cellular environments: weak transient interactions have consequences

Date: 
Friday, November 16, 2018 - 12:00pm to 12:50pm
Location: 
CP-114
Tags/Keywords:
Type of Event (for grouping events):

Abstract:

Biological macromolecules function in dense, crowded cellular environments.

Early studies of crowding effects have emphasized volume exclusion effects, but it is becoming clear that frequent non-specific interactions between proteins, nucleic acids, and metabolites may be the more important factor in modulating the structure and dynamics of biomolecules. Computer simulation studies at different scales of a series of models ranging from concentrated homogeneous protein solutions to models of bacterial cytoplasms are presented to explore the effects of non-specific quinary protein-protein interactions on protein stability and dynamics.  One focus is on the formation of transient clusters that determine diffusive properties and lead to liquid-liquid phase transitions. The computational results are related to existing experimental data and the challenges and opportunities to expand the current studies to whole-cell modeling in molecular detail are discussed.

Ice is cool!

Date: 
Friday, November 9, 2018 - 12:00pm to 12:50pm
Location: 
CP-114
Tags/Keywords:
Type of Event (for grouping events):

Abstract:

When an aqueous solution freezes at atmospheric conditions, essentially pure crystals of hexagonal ice are formed, and the solutes are threaded between the ice grains in ice boundary grooves or in puddles formed on the surface.

Certainly, there are several questions to ask about this process: What is the microstructure of ice with brine like? What is the chemical state and immediate environment of the solutes there? And, most importantly, how is the reactivity of the compounds influenced by freezing?

I would like to invite you to search for the answers with me, and I will be very pleased with your interest. So far, I have applied environmental scanning electron microscopy and optical spectroscopies in seeking indications of the aggregation, pH jumps, and electrical “freezing potential” formed at the interfaces of ice grains upon freezing. My goal is to explain the (photo) reactivity of compounds in environmental ices and during laboratory-based and industrial freezing procedures. I am currently a visiting scholar on sabbatical here at the University of Kentucky, and would appreciate any suggestions on facilities or methods available here.

 

Mapping Allosteric Communication Pathways in Protein Conformational Ensembles

Date: 
Friday, November 2, 2018 - 12:00pm to 12:50pm
Location: 
CP-114
Tags/Keywords:
Type of Event (for grouping events):

Abstract:

Detailed understanding of how conformational dynamics orchestrates function in allosteric regulation of recognition and catalysis at atomic resolution remains ambiguous. The three dimensional structure of protein is not always adequate to provide a complete understanding of protein function. We use atomistic molecular dynamics simulations to complement experiments to understand how protein conformational dynamics are coupled to allosteric function. We analyze multi-dimensional simulation trajectories by mapping key dynamical features within individual macrostates as residue-residue contacts. In this talk, we will discuss computational studies on members of a ubiquitous family of enzymes that regulate many sub-cellular processes. The effects of distal mutations and substrate binding are observed at locations far beyond the mutation and binding sites, implying their importance in allostery. The results provide insights into the general interplay between enzyme conformational dynamics and catalysis from an atomistic perspective that have implications for structure based drug design and protein engineering.

 

Improved Synthesis of Two-Dimensional Covalent Organic Frameworks

Date: 
Friday, October 26, 2018 - 12:00pm to 12:50pm
Location: 
CP-114
Tags/Keywords:
Type of Event (for grouping events):

Abstract:

Polymerizing monomers into periodic two-dimensional (2D) networks provides structurally precise, layered macromolecular sheets that exhibit desirable mechanical, optoelectrotronic, and molecular transport properties. 2D covalent organic frameworks (COFs) offer broad monomer scope but are generally isolated as powders comprised of aggregated nanometer-scale crystallites. I will discuss 2D COF formation using a two-step procedure, in which monomers are added slowly to pre-formed nanoparticle seeds. The resulting 2D COFs are isolated as single-crystalline, micron-sized particles. Transient absorption spectroscopy of the dispersed COF nanoparticles provides two to three orders of magnitude improvement in signal quality relative to polycrystalline powder samples and suggests exciton diffusion over longer length scales than those obtained through previous approaches. These findings will enable a broad exploration of synthetic 2D polymer structures and properties.

Driving forces of greasy protein association in greasy membranes

Date: 
Friday, October 19, 2018 - 12:00pm to 12:50pm
Location: 
CP-114
Tags/Keywords:
Type of Event (for grouping events):

ABSTRACT: What are the thermodynamic driving forces that influence the free energy of membrane protein folding and association in lipid bilayers? For soluble proteins, the burial of hydrophobic groups away from aqueous interfaces is a major driving force, but membrane-embedded proteins cannot experience hydrophobic forces, as the lipid bilayer lacks water. A fundamental conundrum thus arises: how does a greasy protein surface find its greasy protein partner in the greasy lipid bilayer to fold faithfully into its native structure? Recently, a structurally stable and functional monomeric form of the normally homodimeric Cl-/H+ antiporter CLC-ec1 was designed by introducing tryptophan mutations at the dimer interface. We have used this to develop a new model system for studying reversible dimerization in membranes for free energy measurements, which encompasses the thermodynamic properties of protein interactions in the membrane environment. To quantify monomer vs. dimer populations across a wide range of protein densities, we developed a method that quantifies the capture of subunits into liposomes from large equilibrium membranes single-molecule photobleaching by total internal reflection microscopy.  With this, we are able to determine that CLC-ec1 has a free energy of dimerization of -11 kcal/mole in 2:1 POPE/POPG membranes.  We are now investigating why this complex is so stable, dissecting the changes in enthalpy and entropy while varying protein interactions or the composition of the lipid solvent.  The results from this study will provide a physical foundation for the development of informed strategies aimed at correcting protein mis-folding or regulating protein interactions in membranes in physiologically and pathological situations.

Dawson Lecture

Date: 
Friday, October 12, 2018 - 4:00pm to 4:50pm
Location: 
JSB 321
Type of Event (for grouping events):

Pages

Subscribe to RSS - chemistry seminar