Friday May 5th – Presentations

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ADDITION OF PT(IPR) GROUPINGS TO RU5 CARBIDE, TRIIRON, AND TRIRUTHENIUM DODECACARBONYLS GIVE NEW MULTINUCLEAR PT- RU AND PT- FE CLUSTER COMPLEXES

 

Vincent Zollo Jr.1, Anjaneyulu Koppaka1, Sedigheh Etezadi1, Burjor Captain1.

1. Department of Chemistry, University of Miami, Coral Gables, FL 33124, USA

08:30 AM
to 08:50 AM
Inorganic Chemistry

The reaction of Pt(IPr)(SnBut3)(H), 1 [IPr = N-heterocyclic carbene ligand N,N’-bis-(2,6-(diisopropyl) phenyl)imidazole-2-ylidene], 2 with Ru­55-C)(CO)15, 3, Fe3(CO)12, 4, and Ru3(CO)12, 5, afforded several mixed-metal Pt-Ru and Pt-Fe cluster complexes. The reaction between 1 and 2 in benzene reflux in 1.2:1 (and 2.2:1) ratio afforded monoplatinum-pentaruthenium complexes PtRu­5(IPr)(µ6-C)(CO)15, 6, in 54% (10%) yield, and PtRu­5(IPr)(µ6-C)(CO)14(H)2, 7, in 6% (10%) yield, as well as Pt2Ru5(IPr)(µ6-C)(CO)15, 8, and diplatinum-pentaruthenium Pt2Ru5(IPr)26-C)(CO)15, 9, in 2% (36%) yield. Compound 6 readily reacts with, H2 in room temperature to form 7, while 9 shows dynamic activity in solution with Pt(IPr) ligands exchanging . The reaction of 1 and 3 in benzene at room temperature in a 3:1 ratio, produced two mixed-metal trigonal planar Pt-Fe complexes a monoplatinum-diiron Fe2Pt(IPr)(CO)9, 10, in 20% yield and diplatinum-monoiron FePt2(IPr)2(CO)6, 11, in 2% yield. The carbonylation of 10 shows full consumption to compound 11. In a 3:1 ratio, 1 and 5 were mixed in hexane at reflux temperature affording the trigonal bipyramidal cluster Pt2Ru3(IPr)2(CO)12, 12, in 32% yield. The synthesis, reactivity, and structural characterization of the complexes will be discussed.

Facile Synthesis of Highly Functionalized Tricycloalkanes via Enyne Cope Rearrangement

Sarah K. Scott and Alexander J. Grenning

Department of Chemistry, University of Florida, Gainesville FL 32611-7200, USA

08:50 AM
to 09:10 AM
Organic Chemistry

Terpenoid natural products and their derivatives are often targets of organic synthesis due to their promising biological activity and intriguing structural complexity. Moreover, efficient access to natural product derivatives is important for application in the drug discovery process. The work presented offers a short and simple route (four steps, three unique reactions) to access linearly fused 6/7/5 tricyclic ring systems, which represent a large class of terpenoid tricyclic natural products (dolestane, abeo-taxane, etc.). Furthermore, this work exploits the use of a rare 1,5-enyne Cope rearrangement which has previously found no application in natural product synthesis.

Voltage dependent light emitters from iClick and aurophilic interactions

Christopher. C. Beto,1 Ethan. D. Holt,1 Yajing Yang,1 James. D. Bullock,Charles W. Zeman,1  Ion Ghiviriga,1 Kirk S. Schanze,2* and Adam S. Veige1*

1University of Florida, Department of Chemistry, Center for Catalysis, P.O. Box 117200, Gainesville, FL, 32611
2University of Texas at San Antionio, Department of Chemistry, One UTSA Circle, San Antonio, TX 78249

08:50 AM
to 09:10 AM
Inorganic Chemistry

Taking advantage of inorganic click (iClick) and aurophilic interactions complexes {[PEt3Au]2(μ-N3C2)-9,9-dioctyl-9H-fluorene}2 (Au4-FO) and [PPh3Au]2(μ-N3C2)-9,9-dioctyl-9H-fluorene (Au2-FO) were inserted into OLEDs to give voltage dependent light. Varying the voltage and percent of Au present in the OLED yields light that has the colors white, blue, red, and green as confirmed using 1931 CIE color space. Au4-FO and Au2-FO were synthesized via iClick and exhibit both fluorescence and phosphorescence in solution and in the device. The device was fabricated with the structure ITO/PEDOT:PSS/PVK:PBD:Au/LiF/Al. The turn on voltage ranged depending on the percent of Au present in the emissive layer, the lowest voltage observed was 14 V.

A theoretical analysis of substituent electronic effects on phosphine-borane bonds

Paul A. Sibbald

Stetson University
Department of Chemistry
421 N. Woodland Blvd.
Unit 8271
DeLand, Florida 32723

08:55 AM
to 09:10 AM
Computational Chemistry

Phosphine-borane adducts are a well-known moiety in synthetic and coordination chemistry. These complexes form a dative bond in which the Lewis basic phosphorus atom donates electron density into an empty p-orbital of the Lewis acidic boron atom. However, donation of the phosphorus lone pair is not the only stabilizing interaction, as hyperconjugation and electrostatic interaction also play important roles in bonding. This presentation describes a detailed density functional theory level (B3LYP) study completed to determine the impact electron-donating and withdrawing substituents have on phosphine-borane bonds through the investigation of a series of para-substituted PAr3-BH3 and PH3-BAr3 phosphine-borane adducts. Natural bond orbital (NBO) partitioning was used to calculate the distribution of electron density between the phosphine and borane fragments. Extended transition state and natural orbitals for chemical valence (ETS-NOCV) analysis was used to isolate contributions to the overall electronic interaction of the phosphine-borane adducts. Molecular orbital composition and charge donation was calculated using AOMix. The resulting data was correlated with Hammett σ constants.

HIERARCHICAL FRACTAL ASSEMBLIES FORMED BY POLY(ETHYLENE OXIDE-b-LYSINE-b-LEUCINE)

Craig A. Machado, Kyle C. Bentz, Daniel A. Savin*

George & Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science & Engineering, Department of Chemistry, University of Florida, Gainesville, FL 32611, USA.

08:55 AM
to 09:10 AM
PMSE/POLY

Poly(ethylene oxide-b-lysine-b-leucine) was synthesized via ring-opening polymerization of N-carboxyanhydrides using amine-terminated poly(ethylene oxide) as a macroinitiator, with polymer blocks produced by sequential monomer addition. Infrared spectroscopy indicates the block polymer forms an -helix in the solid state. In solution, this polymer self-assembled into spherical and wormlike micelles with hydrodynamic radii of approximately 90 nm and radii of gyration between 220 and 310 nm in water at concentrations ranging from 0.05 to 0.20 wt% and pH values between 2 and 6.5. Upon preparation of transmission electron microscopy grids, the micelles underwent a hierarchical assembly to produce fractal assemblies. Zeta potential measurements show a slight decrease in surface charge with increasing pH, which results in more densely packed fractals due to decreased electrostatic repulsions. This system provides a facile route to nanostructured surfaces which can be used for applications such as modulating cell adhesion or promoting growth of neurons.

Elucidating Biomolecule Structural Motifs using TIMS-MS, IRMPD Spectroscopy, and Molecular Dynamics

Francisco Fernandez-Lima

Department of Chemistry and Biochemistry, Florida International University

09:05 AM
to 09:40 AM
Physical Chemistry

Recent innovations in speed, accuracy and sensitivity have established mass spectrometry (MS) based methods as a key technology for the analysis of kinetic intermediates and folding mechanism of peptides, protein, DNA and DNA-protein complexes. In particular, Ion Mobility Spectrometry – Mass Spectrometry provides a powerful tool for the identification of structural motifs, and when complemented with theoretical calculations, it permits a better understanding of the main motifs that drive the dynamics across the free energy landscape. We have recently introduced a Trapped Ion Mobility Spectrometry coupled to Mass Spectrometry (TIMS-MS) as a high-throughput technique for the study of conformational states of biomolecules, as well as the kinetic intermediates involved during their folding as a function of the molecular environment (e.g., pH, organic and salt content). While this description holds true for most contemporary IMS analyzers, the higher resolving power (e.g., R= 150-250, 3x larger than traditional IMS systems) and the unique ability to hold and interrogate molecular ions for kinetic studies (e.g., millisecond-second time scale) provides TIMS-MS with unique capabilities for the study and interrogation as a function of the time after desolvation. Recently combined with hydrogen-deuterium exchange, HDX-TIMS-MS, a more detailed description of the accessible surface area and the folding can be achieved over time.  That is, HDX-TIMS-MS has a significant advantage in the flexibility to interrogate, at the single molecule level, the molecular interactions that define the conformational space. In addition to IMS-MS separation, significant information on the type of interactions that stabilize tridimensional structures can be obtain using Action spectroscopy (IRMPD). In the present talk, recent results that reveal the kinetic intermediates and the main folding pathways for small molecules, peptides, proteins, DNA and DNA-protein complexes will be discussed.

STRUCTURAL AND MECHANICAL PROPERTIES OF AMYLOID BETA (40) FIBRILS: THEORY MEETS EXPERIMENTS  

Thomas J. Paul,1 Zachary Hoffmann,1 Congzhou Wang,2 Maruda Shanmugasundaram,3 Jason DeJoannis,4 Alexander Shekhtman,3 Igor K. Lednev,3  Vamsi K. Yadavalli,4 and Rajeev Prabhakar1

1. Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, FL 33146.
2. Department of Chemical and Life Science Engineering, Virginia Commonwealth University, Richmond,
Virginia 23284
3. Department of Chemistry, University at Albany, State University of New York, Albany, New York 12222
4. Dassault Systemes BIOVIA, San Deigo, California 92121

09:05 AM
to 09:25 AM
Biophysical

Amyloid Beta (Ab) peptides are biomolecules that are capable of forming a rich variety of materials under diverse conditions. Driven by intermolecular forces such as hydrogen bonds, hydrophobic interactions and π-π stacking, they can self-assemble molecule by molecule to produce supramolecular architectures (fibrils). Due to their mechanical rigidity, strength, and elasticity, biomaterials formed by amyloid fibrils can be used for novel Bio-Nano-Med applications. In a combined experimental (deep ultraviolet resonance Raman (DUVRR) spectroscopy and atomic force microscopy (AFM)) and theoretical (molecular dynamics (MD) simulations and stress−strain (SS)) study, the structural and mechanical properties of amyloid beta (Aβ40) fibrils such as β-sheet character, twist, inter-strand distance, and periodicity were found to be in agreement with experimental measurements. Furthermore, Young’s modulus (Y) = 4.2 GPa computed using SS calculations was supported by measured values of 1.79 ± 0.41 and 3.2 ± 0.8 GPa provided by two separate AFM experiments. These results revealed size dependence of structural and material properties of amyloid fibrils and show the utility of such combined experimental and theoretical studies in the design of precisely engineered biomaterials.

Environmental dynamics of dissolved black carbon in the Amazon River

Alan Roebuck

Michael Gonsior

Alex Enrich-Prast

Rudolf Jaffé

Florida International University and Southeast Environmental Research Center
University of Maryland Center for Environmental Science
Federal University of Rio De Janeiro
Florida International University and Southeast Environmental Research Center

09:05 AM
to 09:25 AM
Environmental

Dissolved black carbon (DBC) is an important component in the global carbon cycle, and constitutes a significant portion of dissolved organic carbon (DOC) in aquatic systems. While global fluxes of DBC may be well understood, little is known about systematic processing of this carbon pool in fluvial systems. Similar to DOC, DBC composition may change as it moves throughout a river continuum before it is eventually deposited into the ocean. This is especially important for large river systems that are major sources of DOC to the ocean and may have significant impacts on ocean biogeochemistry and carbon cycling. To better understand variations in DBC dynamics throughout a large fluvial system, DBC was quantified using the benzene polycarboxylic acid method (BPCA) in three major tributaries of the Amazon River, each with varying biogeochemical characteristics. In some rivers, light availability appeared to influence both DBC quantity and composition. Higher concentrations of DBC characterized by a larger, more aromatic DBC pool was found in the Rio Negro, a black water river with high levels of chromophoric dissolved organic matter and low light penetration. In the Rio Tapajos, a clear water river with higher light penetration, lower DBC concentrations characterized by higher abundances of the less polycondensed DBC pool provided evidence of photodecomposition under such conditions. The Rio Madeira is characterized as a white water river with high suspended sediment yields rich in mineral clays and metals, such as iron. This river had the lowest DBC concentrations and also had less polycondensed DBC, suggesting a preferential adsorption of the more polycondensed DBC components onto clay particles or selective coprecipitation of high molecular weight DBC with iron oxides.

Total Synthesis of (-)-Martinellic Acid Enabled by Enantioselective Copper-Catalyzed Quinoline Alkynylation

Mukesh Pappoppula, Flavio S. P. Cardoso, B. Owen Garrett, and Aaron Aponick*

University of Florida

09:05 AM
to 09:25 AM
Natural Products

 

(-)-Martinellic acid, and (+)-martinelline were isolated from martinella iquitosensis roots found in South America by Merck laboratories in 1995. These alkaloids were among the first potent, naturally occurring (non-peptide) bradykinin (BK) receptor antagonists (μm) to be reported.  Also they exhibit anti-α-adrenic activity (nm), anti-muscarinic and moderately effective antibiotic against both Gram-positive and Gram-negative bacteria. The first total synthesis of (-)-martinellic acid was reported in 2001 by Ma et al.

A highly enantioselective alkynylation of quinolinium salts was developed using the chiral imidazole-based biaryl P,N-ligand StackPhos and copper bromide to establish the absolute stereochemistry of the martinella core from a 4-substituted quinoline. The three-component reaction between a quinoline, a terminal alkyne, and ethyl chloroformate afforded the desired products in high yields with excellent enantioselectivities (up to 98% ee). The development of this method and its application to the enantioselective total synthesis of (-)-martinellic acid will be described.

Designing New Scaffolds for Organoactinide Chemistry: Synthesis, Characterization and Reactivity of Phosphazide and Phosphasalen Ligands

Paul G. Hayes, Connor S. MacNeil, Tara K. K. Dickie, Christopher P. Forfar

University of Lethbridge
Department of Chemistry & Biochemistry
4401 University Drive, Lethbridge, Alberta, Canada T1K 3M4

09:10 AM
to 09:45 AM
Inorganic Chemistry

Although salicylaldimine (salen) ligands are ubiquitous in coordination and organometallic chemistry, much less work has been conducted with the phosphinimine (RN=PR3) analogues. Thus, we have prepared a series of hexadentate phosphazidosalen and tetradentate phosphasalen ligands via reaction between the vicinal diazide 1,2-diazidobenzene and the requisite phosphine. The alkali metal salts of these scaffolds react with UCl4(dme)to afford a range of uranium complexes that feature unusual structures and bonding. Intriguingly, we observed stepwise N2 elimination from the phosphazidosalen complex with concomitant U–Cl bond activation. The synthesis, structures and reaction chemistry of these species will be discussed in detail.

Enantioselective Tsuji allylation of Enol Acetates

Ji Liu, Sourabh Mishra, Aaron Aponick

University of Florida

09:10 AM
to 09:30 AM
Organic Chemistry

The convenient construction of all-carbon quaternary chiral centers which are ubiquitous in natural products
is a major challenge in organic synthesis. Enantioselective allylation to form α-quaternary ketones has recently
become an efficient tool targeting this challenge. A new method of enantioselective intermolecular deacylative
allylation from easily available enol acetates has been developed. In this process, enol acetates act as
prenucleophiles as well as an activators of allylic alcohols via simple transesterification which allows for direct use of
allyl alcohols. Various allylic alcohols and enol acetates have been demonstrated to give excellent enantioselectivity
and yields. The method of this development will be presented.

Development of a many-body carbon dioxide potential and its application to the electronic and vibrational structure of CO2 clusters

Olaseni Sode,1 Jasmine Cherry,1 Murat Keçeli2 and Samuel Maystrovsky1

1 The University of Tampa, Department of Chemistry, Biochemistry and Physics, Tampa, FL, 33606.
2 Argonne National Laboratory, Chemical Sciences and Engineering Division, Argonne, IL, 60439.

09:10 AM
to 09:30 AM
Computational Chemistry

A “first principles” potential energy function with flexible monomers is developed for carbon dioxide (CO2) gas phase systems. This function is constructed through a fit to the electronic energies of CO2 monomers and dimers at the CCSD(T)-F12b/aug-cc-pVTZ level and trimers at the CCSD(T)/aug-cc-pVDZ level. Thousands of CO2 configurations were used to train the potential function, which was then used to optimize the structures of CO2 clusters ranging in size from 3 – 13 molecules. The anharmonic vibrational frequencies of the minimum energy structures were obtained using the vibrational self-consistent-field (VSCF) and vibrational configuration-interaction (VCI) methods.

Acid-Degradable Segmented Hyperbranched Polymers by RAFT Polymerization of Imine-Containing Crosslinkers

Michael B. Sims, Kush Y. Patel, Mallika Bhatta, Soma Mukherjee, Brent S. Sumerlin

George & Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science & Engineering, Department of Chemistry, University of Florida, PO Box 117200, Gainesville, FL 32611-7200, USA.

09:10 AM
to 09:25 AM
PMSE/POLY

The degradation of higher-order polymeric architectures is an important environmental and biological consideration for practical applications of these systems. Both the variety of pH-sensitive functional groups and the ubiquity of pH gradients in nature make the incorporation of acid- and base-reversible linkages an effective method for facilitating this degradation. Imines are particularly attractive candidates due to their facile preparation and tunable acid stability depending on the identity of the N-substituent. To this end, segmented hyperbranched polymers containing acid-labile imine crosslinks were synthesized by reversible addition-fragmentation chain transfer polymerization of three novel divinyl compounds containing oxime, semicarbazone, and acyl hydrazone moieties. The branched polymers degraded into well-defined linear polymers under aqueous acidic conditions, and the rate of degradation depended on the hydrolytic stability of the imine crosslinks. Furthermore, precise control over the degradation rate was achieved through the incorporation of different imines at various ratios into the same branched polymer.

TOWARDS TERPENOID SCAFFOLDS VIA MULTI-FUNCTIONALIZATION OF kNOEVENAGEL ADDUCTS

Peter Vertesaljai, Matthew Williams, and Alexander J. Grenning

Department of Chemistry, University of Florida, Gainesville FL 32611-7200, USA

09:25 AM
to 09:45 AM
Natural Products

Knoevenagel adducts obtained from inexpensive ketones and malonic acid derivatives readily undergo selective transformations under mild conditions which makes them attractive building blocks. Our research focuses on the utility of Knoevenagel adducts in scalable, divergent syntheses towards terpenoid natural products and their analogs. A sequence of alkylation/[3,3]-sigmatropic rearrangement/α-alkylation/ring-closing metathesis was devised to construct a library of angular polycyclic carbon frameworks. Diversity is introduced by proper choice of starting materials as well as late stage functional group interconversion.
Another study based on selective functionalization allowed for the synthesis of highly substituted perhydroindoles. Experiments were aimed at outlining the scope and understanding the reactivity of such structures.

Structure and Dynamics of Thermo-reversible Colloidal Gels near the Gel Boundary 

Divya Bahadur1, Qingteng Zhang2, Eric Dufresne2, Piotr Maj3, Suresh Narayanan2, Subramanian Ramakrishnan1, Alec Sandy2

1 Department of Chemical and Biomedical Engineering, FAMU - FSU College of Engineering, Tallahassee, FL
2 Advanced Photon Source, Argonne National Laboratory, Lemont, IL
3 AGH University of Science and Technology, Kraków, Poland

09:25 AM
to 09:45 AM
PMSE/POLY

We examined gel formation and dissolution processes for thermo-reversible colloidal gels by studying the structure, dynamics and mechanical properties of the gel as it traverses the gel boundary via x-ray photon correlation spectroscopy (XPCS) and rheology. Octadecyl silica particles in decalin (f = 0.2) at three different particle sizes (56 nm, 82 nm and 110 nm) were made to reversibly gel upon cooling. Gels were formed by rapidly quenching the system at a range of temperatures close to the gel point (ΔT ~ 2K) and the dynamics and mechanical properties were probed as they evolved in time. A slow heating ramp from a well formed gel state was carried out to identify the gel transition temperature. The transition is characterized by a slowing down of dynamics upon gel formation in the intermediate scattering function, emergence of low q up - turns in the structure factor as well as a rapid increase in the modulus a function of quench depth and aging time while the inverse is observed upon melting. The transition times and temperatures observed via XPCS are in good agreement with rheological measurements. Similar length and time scales emerge upon both gel formation and melting which were scaled using universal scaling criteria suggested by the mode coupling theories.

STUDY OF THE CONFORMATIONAL DYNAMICS AND INTRAMOLECULAR NETWORK OF DNA OLIGOMERS USING TRAPPED ION MOBILITY SPECTROMETRY -MASS SPECTROMETRY

Jacob Porter1 and Francisco Fernandez-Lima1,2

1Department of Chemistry and Biochemistry, Florida International University, Miami, USA
2Biomolecular Sciences Institute, Florida International University, Miami, USA

09:25 AM
to 09:45 AM
Biophysical

Multi-stranded DNA topologies, including quadruplexes, triplexes and duplexes, are present in human telomeric DNA, and provide important insights into pathological mechanisms. Assessing their stability is important for drug discovery and biosensing. With the recent development of Trapped ion mobility spectrometry coupled to mass spectrometry (TIMS-MS), biomolecular ions can be separated by mass, charge and size in the gas phase as a function of their tridimensional structure, providing insight into conformeric subpopulations and folding pathways on the millisecond timescale not accessible using traditional techniques. In the current research, model oligonucleotides shown to adopt multi-stranded topologies were analyzed using TIMS-MS. Multiple charge states and oligomeric states were observed (e.g., [M+H]+, [2M+2H]2+, [3M+3H]3+ and [4M+4H]4+). Multiple mobility bands were detected as a function of the charge states, indicating a series of partially-folded DNA geometries. Conformational dynamics were studied as a function of the time after desolvation on the millisecond timescale. These data provide insight into DNA folding mechanisms and oligomeric complex stability, which can be further supported with candidate structure generation.

Metal Halide Assisted Hydrogenation and Etherification of Furfural and Furfuryl Alcohol in the Liquid Phase

Susanna Ogozaly, Lindsey A. Welch, Ph.D.

Dept. of Chemical and Physical Sciences, Cedar Crest College, Allentown, PA

09:25 AM
to 09:45 AM
Environmental

Furfural is considered one of the platform chemicals derived from the dehydration of xylose from biomass.  Selective transformations of furfural can lead to products relevant to biofuel as well as fine chemical industries.  Our work investigated the selective transformation furfural and furfuryl alcohol to ether and hydrogenation products in the liquid phase using light alcohol solvents.  The selectivity is attributed to a synergistic interaction between the supported Pd/C catalyst with metal halide additives acting as Lewis acids.  This presentation will hightlight our results from both ambient and high pressure conditions. By optimizing the reaction conditions, highly selective processes were identified.   

Modular entry to the fusicoccane nucleus via a photoinduced pericyclic reaction cascade

Anna E. Salvati, James A. Law, Josue Liriano, and James H. Frederich

Florida State University

09:30 AM
to 09:50 AM
Organic Chemistry

The fusicoccanes are a group of diterpenes that have attracted considerable attention for their capacity to stabilize protein-protein interactions. Herein, we describe a unified strategy to prepare this family of natural products, and a range of synthetic variants, in a practical fashion. This chemistry provides a uniquely flexible synthetic platform to explore the rich biological activity of this chemotype.

BROADBAND ABSORPTION SPECTRA FROM TIME-DEPENDENT COUPLED-CLUSTER THEORY

Daniel R. Nascimento and A. Eugene DePrince, III

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306-4390

09:30 AM
to 09:45 AM
Computational Chemistry

We present some of our contributions to the advancement of explicitly time-dependent coupled-cluster theory towards the efficient computation of linear absorption spectra of molecular systems. Our newly developed explicitly time-dependent equation-of-motion coupled-cluster (TD-EOM-CC) formalism based on the propagation of the CC dipole function possesses great potential for the efficient computation of linear absorption spectra over arbitrarily wide energy windows. This approach introduces no approximations and requires only half of the computational effort of TD-CC methods based on the propagation of the wave function. The TD-EOM-CC formalism is particularly useful when dealing with molecular systems with high density-of-states, or if the spectral region of interest spans a large energy range (> 10 eV). We perform illustrative calculations of the UV-Vis, NEXAFS, and electronic circular dichroism (ECD) spectra of several molecular species relevant to the study of atmospheric chemistry and astrochemistry. Furthermore, we exploit extrapolation techniques based on Pade approximants, these techniques are shown to significantly reduce the computational effort required to simulate NEXAFS spectra.

THE MECHANISM OF BREVETOXIN INDUCED OXIDATIVE STRESS AND EFFECT OF ANTIOXIDANT.

Anupama Tuladhar; Kathleen Rein*

Department of Chemistry and Biochemistry, Florida International University, Miami, Fl, 33199.

10:15 AM
to 10:35 AM
Natural Products

Karenia brevis, a red tide dinoflagellate, commonly blooms in the Gulf of Mexico and west coast of Florida. It is associated with massive fish and marine mammal poisoning and is known to produce neurotoxins known as brevetoxins (PbTx). Marine mammals and human cell lines exposed to red tide bloom have also shown signs of oxidative stress associated with PbTx exposure. These indicators include increased Reactive Oxygen Species (ROS) and DNA damage. Our group has established that PbTx-2 (but not PbTx-3) is an inhibitor of mammalian thioredoxin reductase (TrxR). The role of TrxR is the maintenance of redox homeostasis by reduction of thioredoxin and elimination of ROS via peroxiredoxins. Our hypothesis is that the selenocysteine of mammalian TrxR adds to PbTx-2 which have α,β-unsaturated bond in a Michael addition. Adducts between selenocysteine and PbTx-2 as well as cysteine and PbTx-2 have been easily prepared and characterized by FTICR-MS. Compromising the selenocysteine of TrxR results in the formation of a SecTRAP (Selenium Compromised Thioredoxin Reductase-derived Apoptotic Proteins), a pro-oxidant known to induce cell death. Our efforts to confirm the formation of an adduct between TrxR and PbTx-2 will be described. The effect of PbTx-2 induced oxidative stress is studied in the human lymphoblast cells and antioxidant such as such as vitamin C and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (TROLOX) are used to mitigate the oxidative stress. This research thus provides the mechanism of inhibition of TrxR and potential antioxidant treatment for PbTx exposed humans and marine mammals.

 

Further Understanding of Electronic Defects in p-Type CuGaO2 Delafossite Nanocrystals

Alexandria Combs, Byron H. Farnum

Department of Chemistry and Biochemistry, Auburn University

10:15 AM
to 10:35 AM
Materials Chemisry

The synthesis and electronic characterization of nanocrystalline p-type CuGaO2 has been investigated for its application as a transparent hole transport material in thin film solar cells. CuGaO2, along with other CuMO2 materials (M = AlIII, InIII, CrIII, ScIII, BIII), is attractive because of its wide band gap and two-dimensional delafossite crystal structure; the latter of which results in a large mobility of valence band holes, an uncommon feature for p-type metal oxides. Research in our group has focused on increasing our understanding of the electronic structure of this nanocrystalline material through physical methods such as electrochemistry and spectroscopy. Through these studies we hope to gain insight on what factors control hole transport and the physical nature of defect states in this class of materials.

Metal-organic frameworks with embedded basic sites for heavy metal capture from aquatic environments

Trevor A. Makal

The University of Virginia's College at Wise

10:15 AM
to 10:35 AM
Inorganic Chemistry

Heavy metals present in contaminated water sources present a significant threat to humans and wildlife alike. The development of new materials to efficiently remove targeted heavy metal toxins from aquatic environments is essential to securing the future of our already stressed potable water supplies. Previous efforts in the development of materials for heterogeneous water purification and remediation have relied upon highly porous (eg activated charcoal) or functionalized materials (eg layered double hydroxides). In this work, we aimed to design and synthesize highly porous and water-stable metal-organic frameworks (MOFs) with embedded basic sites using hard-soft acid-base theory. A series of five UiO-66-based MOFs were synthesized, characterized, and analyzed for water stability and heavy metal uptake capabilities in order to probe the role of thiol and thioether functionalities within a porous MOF toward heavy metal capture and selectivity. Two of the newly synthesized MOFs exhibit exceptional stability, even to exposure to strongly acidic and basic environments, and >97% removal of Ag(I) from aqueous solutions. Another material exhibits >98% removal of Hg(II).

Quantum Size Effects in Optically Pumped NMR in Strained GaAs/AlGaAs Quantum Wells

John T. Tokarski III, 1 Stephen A. McGill,2 Gary Sanders,3 Chris Stanton,3 John L. Reno,4 and Clifford R. Bowers1

1University of Florida, Department of Chemistry, Gainesville, FL, 32611, USA
2National High Magnetic Field Laboratory, Tallahassee, FL, 32360, USA
3University of Florida, Department of Physics, Gainesville, FL, 32611, USA
4Sandia National Laboratories, Albuquerque, NM, 87123, USA

10:15 AM
to 10:35 AM
Physical Chemistry

Recently we reported the effects of elastic interactions and resulting bowing moments on strain in bulk GaAs/Si composites1 using optically pumped nuclear magnetic resonance (OPNMR).2 We have also reported the effects of strain in GaAs/AlGaAs quantum wells, where the heavy-hole/light-hole energy splitting was affected by both strain and quantum confinement.3 In that 30 nm wide GaAs well, our calculations show that only modest quantum confinement effects were present, and strain effects dominated the modifications of the electronic band structure, optical absorption, and OPNMR action spectrum. Here we present our progress-to-date on an OPNMR study of a series of GaAs/AlGaAs quantum wells where the well-width is varied. In collaboration with Sandia National Laboratories, a series of quantum well arrays (QWA) with widths of 28, 14, 7, and 4 nm were grown via molecular beam epitaxy.  By comparing the photon energy dependences of the OPNMR, magneto-optical absorption and photoluminescence spectra to electronic band structure calculations, we expect a more complete understanding of the OPNMR photo-physics to emerge. The QWA films were epoxy-bonded to a transparent, single crystal sapphire wafer, and the sacrificial GaAs substrate was etched down to the stop-etch layer using selective wet chemical etching. Magneto-photoluminescence experiments ensured the complete removal of the bulk GaAs growth substrate. The exploitation of strain, quantum confinement, and magnetic field to enhance and control the optically pumped nuclear spin hyperpolarization could have applications to nuclear spin electronics and quantum computing.

 

 

[1] Wood, R.M.; Tokarski III, J.T.; McCarthy, L.A.; Stanton, C.J; Bowers, C.R., Characterization of elastic interactions in GaAs/Si composites by optically pumped nuclear magnetic resonance, Journal of Applied Physics, 2016, 120, 085104.

[2] Kuhns, P. L.; Kleinhammes, A.; Schmiedel, T.; Moulton, W. G.; Chabrier, P.; Sloan, S.; Hughes, E.; Bowers, C. R., Magnetic-field dependence of the optical Overhauser effect in GaAs, Physical Review B, 1997, 55, 7824-7830.

[3] Wood, R. M.; Saha, D.; McCarthy, L. A.; Tokarski, J. T.; Sanders, G. D.; Kuhns, P. L.; McGill, S. A.; Reyes, A. P.; Reno, J. L.; Stanton, C. J.; Bowers, C. R., Effects of strain and quantum confinement in optically pumped nuclear magnetic resonance in GaAs: Interpretation guided by spin-dependent band structure calculations. Physical Review B, 2014, 90 155317.

Solution phase synthesis of teixobactin

Yu Yuan

University of Central Florida

10:15 AM
to 10:40 AM
Organic Chemistry

The globally ever-increasing antimicrobial resistance has become a serious threat to human health and it demands immediate attention to develop novel therapeutic agents. Recently, a novel cyclic depsipeptide antibiotic, teixobactin, has been discovered through screening of uncultured bacteria. Teixobactin exhibits excellent activities against an array of Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin resistant Enterococcus (VRE) and Mycobacterium tuberculosis.

We have reported a solid-phase peptide synthesis (SPPS) based approach for the preparation of teixobactin and its analogues. In the reported route, both fragments were produced by SPPS, and coupled by a serine ligation. Further medicinal development requires large scale synthesis of the parent molecule and straightforward methods to access various analogues. To address these challenges, our laboratory initiated an exploration of second generation, homogenous approach to the scalable synthesis of the depsipeptide. In the second generation synthesis, we retained the original disconnection at Ser7 position to allow maximum convergence, which also provided a convenient handle for analogue synthesis. The cyclic peptide synthesis commenced from Ala residue. After C-terminal was masked by an allyl group, D-Thr was attached followed by Ser/Cys. The subsequent ester formation was much smoother compared to SPPS and the unnatural amino acid (AA), enduracididine, was the last AA incorporated into the fragment. By this synthetic sequence, a highly convergent synthesis of the cyclic fragment was achieved in good yield.

The linear peptide D-N-Me-Phe1 to Ile6 was prepared by HATU coupling of 2 tripeptides. Permutation of various AAs gives direct access to different analogues. In the first generation approach, the C-terminal salicylaldehyde ester was produced by ozonolysis, but this condition was not compatible with reducing side chain functional groups, e.g. Cys. We used an alternative masking strategy to gain the access to such C-terminal group in the presence of reducing side chain

pH dependent Conformational Reorganization due to Ionizable Residues in a Hydrophobic Protein Interior

Ankita Sarkar1, Pancham Lal Gupta2, Adrian E. Roitberg2

1 Department of Physics, University of Florida, Gainesville, USA
2 Department of Chemistry, University of Florida, Gainesville, USA

10:15 AM
to 10:35 AM
Biophysical

Internal ionizable residues, despite being inherently incompatible with hydrophobic environments, play major roles in energy transduction and enzyme catalysis. These buried ionizable residues display anomalous experimental pKa values.  In the present work, we study the pH-dependent conformational reorganizations associated with the ionization of lysine residues in the L25K and L125K  variants of staphylococcal nuclease (SNase). We carried out constant pH replica exchange molecular dynamics simulations (pH-REMD) in explicit solvent using AMBER, implemented to run in GPUs. Our calculations show that the pKa values of Lys25 and Lys125 are significantly deviated from their pKa values in bulk water and are in good agreement with experimental values. A study of the water proximity to the lysine residues at different pH indicate that the lysine residues move towards the protein-water interface when protonated and prefer to remain deprotonated when buried in the hydrophobic protein pocket. The root mean square fluctuations display signatures of pH dependent conformational fluctuations in L25K, in contrast to the relatively pH independent conformational changes observed in L125K. The present computational study, besides offering a detailed atomistic understanding of the structural determinants of the shifted pKa values displayed by internal ionizable residues, aids in bolstering the experimental findings.