Experimental and Computational Studies of Environmentally Persistent Free Radicals (EPFRs) Formation

The first part of the thesis investigates the use of theoretical quantum calculations for the study of EPFRs as the initial and fundamental step in the formation of polychlorinated dibenzo-p-dioxin and dibenzofuran. The computational model comprised of density functionals (B3LYP, PBE1PBE, and M06) and two types of basis set namely: LANL2DZ for all atoms and GEN (LANL2DZ for metals and aug-cc-pVDZ for non-metals). Full mechanisms of EPFRs formation over (CuO)1-8, and aluminum oxide clusters were studied. The most stable intermediates and products have been determined and compared to available experimental data. In case of (CuO)1-8 clusters, the small clusters are more exoergic and thus more reactive than larger clusters. However, Al4O6 cluster is more exoergic than Al2O3 cluster. Bader charge analysis was used to examine the degree of electron transfer from phenolic species to metal cluster. A low degree of electron transfer was observed for small clusters (CuO - Cu5O5). There was essentially no electron transfer for the large clusters studied (Cu6O6 - Cu8O8), and this suggests that the catalytic sites are likely to be small “islands of metal oxide clusters. These studies serve to refine proposed mechanisms for EPFRs formation in Prof. Dellinger laboratory. In the second part of the thesis, the dependency of EPFRs yield and their persistency on different CuO content on silica is reported. The EPFRs were generated through exposure of particles to adsorbate vapors of phenol, 2-chlorophenol, and 1,2-dichlorobenzene at 230 0C using a custom-made vacuum chamber. Adsorption resulted in the formation of surface-bound phenoxyl- and semiquinoine–type radicals with characteristic EPR spectra displaying a g-value ranging from ~2.0037 - 2.006. The highest EPFRs yield was observed for CuO concentrations between 1-3% in relation for 2-chlorophenol and phenol adsorption. The two longest lifetimes, 25 h and 23 h, were observed for phenoxyl-type radicals on 0.5% CuO and chlorophenoxyl-type radicals on 0.75% CuO, respectively. The EPFR-containing particle generated twice as much DMPO-OH compared to non-EPFR containing radical. On average, the ratio of OH radical concentration to the number of EPFRs was estimated to be 10:1 signaling a cyclic process

Solvent effects on host-guest residence time and kinetics: further insights from metadynamics simulation of Toussaintine-A unbiding from chitosan nanoparticle

This research article published by Springer Nature Switzerland AG., 2021Solvents play an important role in host-guest intermolecular interactions. The kinetics and residence time of Toussaintine-A (TouA) unbinding from chitosan was investigated by means of well-tempered metadynamics and thermodynamic integration using two solvents, polar aprotic (DMSO), and polar protic (water). The kinetic rates were found to be strongly dependent on the solvent polarity; hence, the unbinding rate proceeded much faster in DMSO compared to water. DMSO tends to participate less in a chemical reaction by weakening the intermolecular interaction between chitosan and TouA due to lack of acidic hydrogen resulting in a reduction of the transition state. On the other hand, water, which ought to donate hydrogen atoms, sustains a strong interaction and hence large barrier heights. Consequently, this reduces the unbinding rate and increases the residence time. Binding free energy from thermodynamic integration suggests a thermodynamic stable chitosan-TouA complex in water than in DMSO

Environmentally Persistent Free Radicals (EPFRs). 3. Free versus Bound Hydroxyl Radicals in EPFR Aqueous Solutions

Additional experimental evidence is presented for in vitro generation of hydroxyl radicals because of redox cycling of environmentally persistent free radicals (EPFRs) produced after adsorption of 2-monochlorophenol at 230 °C (2-MCP-230) on copper oxide supported by silica, 5% Cu(II)O/silica (3.9% Cu). A chemical spin trapping agent, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), in conjunction with electron paramagnetic resonance (EPR) spectroscopy was employed. Experiments in spiked O17 water have shown that ∼15% of hydroxyl radicals formed as a result of redox cycling. This amount of hydroxyl radicals arises from an exogenous Fenton reaction and may stay either partially trapped on the surface of particulate matter (physisorbed or chemisorbed) or transferred into solution as free OH. Computational work confirms the highly stable nature of the DMPO–OH adduct, as an intermediate produced by interaction of DMPO with physisorbed/chemisorbed OH (at the interface of solid catalyst/solution). All reaction pathways have been supported by ab initio calculations

Marine Tar Residues: a Review

Abstract Marine tar residues originate from natural and anthropogenic oil releases into the ocean environment and are formed after liquid petroleum is transformed by weathering, sedimentation, and other processes. Tar balls, tar mats, and tar patties are common examples of marine tar residues and can range in size from millimeters in diameter (tar balls) to several meters in length and width (tar mats). These residues can remain in the ocean envi-ronment indefinitely, decomposing or becoming buried in the sea floor. However, in many cases, they are transported ashore via currents and waves where they pose a concern to coastal recreation activities, the seafood industry and may have negative effects on wildlife. This review summarizes the current state of knowledge on marine tar residue formation, transport, degradation, and distribution. Methods of detection and removal of marine tar residues and their possible ecological effects are discussed, in addition to topics of marine tar research that warrant further investigation. Emphasis is placed on ben-thic tar residues, with a focus on the remnants of the Deepwater Horizon oil spill in particular, which are still affecting the northern Gulf of Mexico shores years after the leaking submarine well was capped

The Effect of Placement Practices on Employee Performance in Small Service Firms in the Information Technology Sector in Kenya

Purpose: The aim of this study was to determine the effect of placement practices on employee performance in the small service firms in the information technology sector. Design/methodology/approach A descriptive research design was adopted for the study. The study selected a sample of owner managers of 36 firms using stratified random sampling technique. Data was collected using a structured questionnaire and analyzed using descriptive statistics which included frequencies and percentages. A bivariate pearson correlation was performed to determine the relationship between placement practices and employee performance. Findings The study found that most employees in the firms were average performers. The results show strong associations between performance on the one hand and job information, training and guidance on the other. Limitations/Implications A more comprehensive study needs to be done to determine the effect of Human Resource Management practices on employee performance in these organizations. Originality/value: In Kenya few studies have focussed on Human Resource Management practices in small service firms in the Information Technology Sector and so this study makes a contribution in that area

Effect of copper oxide concentration on the formation and persistency of environmentally persistent free radicals (EPFRs) in particulates

Environmentally persistent free radicals (EPFRs) are formed by the chemisorption of substituted aromatics on metal oxide surfaces in both combustion sources and superfund sites. The current study reports the dependency of EPFR yields and their persistency on metal loading in particles (0.25, 0.5, 0.75, 1, 2, and 5% CuO/silica). The EPFRs were generated through exposure of particles to three adsorbate vapors at 230 °C: phenol, 2-monochlorophenol (2-MCP), and dichlorobenzene (DCBz). Adsorption resulted in the formation of surface-bound phenoxyl- and semiquinoine-type radicals with characteristic EPR spectra displaying a g value ranging from ∼ 2.0037 to 2.006. The highest EPFR yield was observed for CuO concentrations between 1 and 3% in relation to MCP and phenol adsorption. However, radical density, which is expressed as the number of radicals per copper atom, was highest at 0.75-1% CuO loading. For 1,2-dichlorobenzene adsorption, radical concentration increased linearly with decreasing copper content. At the same time, a qualitative change in the radicals formed was observed--from semiquinone to chlorophenoxyl radicals. The two longest lifetimes, 25 and 23 h, were observed for phenoxyl-type radicals on 0.5% CuO and chlorophenoxyl-type radicals on 0.75% CuO, respectively

Ensemble-based screening of natural products and FDA-approved drugs identified potent inhibitors of SARS-CoV-2 that work with two distinct mechanisms

This research article published by Elsevier Ltd., 2021The recent outbreak of SARS-CoV-2 is responsible for high morbidity and mortality rate across the globe. This requires an urgent identification of drugs and other interventions to overcome this pandemic. Computational drug repurposing represents an alternative approach to provide a more effective approach in search for COVID-19 drugs. Selected natural product known to have antiviral activities were screened, and based on their hits; a similarity search with FDA approved drugs was performed using computational methods. Obtained drugs from similarity search were assessed for their stability and inhibition against SARS-CoV-2 targets. Diosmin (DB08995) was found to be a promising drug that works with two distinct mechanisms, preventing viral replication and viral fusion into the host cell. Isoquercetin (DB12665) and rutin (DB01698) work by inhibiting viral replication and preventing cell entry, respectively. Our analysis based on molecular dynamics simulation and MM-PBSA binding free energy calculation suggests that diosmin, isoquercetin, rutin and other similar flavone glycosides could serve as SARS-CoV-2 inhibitor, hence an alternative solution to treat COVID-19 upon further clinical validation

Luteolin: a blocker of SARS-CoV-2 cell entry based on relaxed complex scheme, molecular dynamics simulation, and metadynamics

This research article published by Springer Nature, 2021Natural products have served human life as medications for centuries. During the outbreak of COVID-19, a number of naturally derived compounds and extracts have been tested or used as potential remedies against COVID-19. Tetradenia riparia extract is one of the plant extracts that have been deployed and claimed to manage and control COVID-19 by some communities in Tanzania and other African countries. The active compounds isolated from T. riparia are known to possess various biological properties including antimalarial and antiviral. However, the underlying mechanism of the active compounds against SARS-CoV-2 remains unknown. Results in the present work have been interpreted from the view point of computational methods including molecular dynamics, free energy methods, and metadynamics to establish the related mechanism of action. Among the constituents of T. riparia studied, luteolin inhibited viral cell entry and was thermodynamically stable. The title compound exhibit residence time and unbinding kinetics of 68.86 ms and 0.014 /ms, respectively. The findings suggest that luteolin could be potent blocker of SARS-CoV-2 cell entry. The study shades lights towards identification of bioactive constituents from T. riparia against COVID-19, and thus bioassay can be carried out to further validate such observations

Enhanced S\u3csub\u3e2\u3c/sub\u3e emission in carbazole-based ionic liquids

© The Royal Society of Chemistry 2015. Ionic liquids composed of a carbazoleimidizolium-based cation and various hydrophobic anions have been synthesized and characterized. Analyses of the absorption spectra of these compounds indicate significant increases in energy gaps between the first two excited singlet states, which results in inhibition of internal conversion from the S2 to S1 states. Detailed studies of the spectral properties of these compounds support emission from multiple excited states including possible emission from the second excited singlet state (S2 emission) in combination with an intramolecular charge transfer state. This conclusion is also consistent with fluorescence lifetime data, which suggest fluorescence emission from multiple electronic excited states. In addition, theoretical calculations of the excited states support these conclusions