Osama K Abou-Zied
Sultan Qaboos University, CHEMISTRY, Faculty Member
A femtosecond fluorescence upconversion study is reported for HBO in solution, as well as for HBO incorporated in DNA. The typical time for the excited-state intramolecular proton-transfer reaction of the syn-enol tautomer in solution and... more
A femtosecond fluorescence upconversion study is reported for HBO in solution, as well as for HBO incorporated in DNA. The typical time for the excited-state intramolecular proton-transfer reaction of the syn-enol tautomer in solution and in DNA has been determined to be 150 fs. In addition, the lifetimes of the keto, the anti-enol and the ‘solvated enol’ tautomer forms were determined in protic solvents, aprotic solvents and DNA. Picosecond rise and decay components in the fluorescence transients with characteristic times between 3 and 25 ps are also observed and attributed to the effects of vibrational cooling.
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... Email: Dr. Osama K. Abou-Zied (abouzied@squ.edu.om). *Correspondence: Dr. Osama K. Abou-Zied, Department of Chemistry, Faculty of Science, Sultan Qaboos University, PO Box 36, Postal Code 123, Muscat (Sultanate of Oman), Fax: (+968)... more
... Email: Dr. Osama K. Abou-Zied (abouzied@squ.edu.om). *Correspondence: Dr. Osama K. Abou-Zied, Department of Chemistry, Faculty of Science, Sultan Qaboos University, PO Box 36, Postal Code 123, Muscat (Sultanate of Oman), Fax: (+968) 2414-1469. Publication History. ...
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The incorporation of gold nanorod-modified TiO2 nanoparticles (TiO2/AuNRs) along with cobalt-imidazolate frameworks (ZIF-67) into the photoanode of dye-sensitized solar cells resulted in a notable enhancement in energy conversion... more
The incorporation of gold nanorod-modified TiO2 nanoparticles (TiO2/AuNRs) along with cobalt-imidazolate frameworks (ZIF-67) into the photoanode of dye-sensitized solar cells resulted in a notable enhancement in energy conversion efficiency.
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Abstract Photoluminescence (PL) quenching of nanoassemblies of CuInS2/ZnS quantum dots (CIS QDs) and rhodamine 560 molecules (Rh560) is spectroscopically investigated by steady-state and femtosecond-to-nanosecond time-resolved techniques.... more
Abstract Photoluminescence (PL) quenching of nanoassemblies of CuInS2/ZnS quantum dots (CIS QDs) and rhodamine 560 molecules (Rh560) is spectroscopically investigated by steady-state and femtosecond-to-nanosecond time-resolved techniques. Fluorescence lifetime measurements of CIS QDs show a bi-exponential decay (time constants ca. 650 ns and 210 ns) that are assigned to the radiative recombination of delocalized CB electrons with localized holes, presumably associated with Cu-related defect sites. A trapped electron recombines nonradiatively with the localized hole. That means, electron trapping is the first step in the nonradiative recombination pathway in CIS QDs. The traps are of surface origin and are likely associated with unpassivated dangling bonds. In this work, we controlled the trap density by varying the amount of Rh560 on the QD surface and monitoring the electron trapping in different time scales. Transient absorption measurements of the CIS-Rh560 assemblies resolved the fast component of electron trapping that occurs in tens to hundreds of picoseconds, while fluorescence lifetime measurements resolved the slow components of trapping that occur in hundreds of nanoseconds. Unlike the case of more traditional CdSe/ZnS QDs, the PL lifetime of CIS QDs approaches the typical time scale of fluorescence intermittency. As a result, the excited state of CIS QDs is vulnerable to the blinking process. In the CIS-Rh560 assembly, trapping of CB electrons increases with dye loading which eventually prolongs the dark (or dim) period and therefore reduces the fluorescence quantum yield of CIS QDs. The appearance of short lifetime components (ca. 0.5–6.9 ns) in the QD-dye assembly hints that Auger quenching process and/or electron-phonon coupling seems to play a major role in the PL quenching process.
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Carbon nanoparticles (CNPs) are getting wide attention due to their fluorescence and low level of toxicity compared to other semiconducting photoluminescent materials. CNPs show strong 'solvatochromism', and the emission mechanism... more
Carbon nanoparticles (CNPs) are getting wide attention due to their fluorescence and low level of toxicity compared to other semiconducting photoluminescent materials. CNPs show strong 'solvatochromism', and the emission mechanism is still under discussion. Florescent carbon in the form of films would tremendously increase its potential for applications. In this work, we report for the first time the fluorescent emission characteristics of carbon films formed by aggregation of CNPs. Films of carbon were grown on glass substrates by using a novelCold Vapour Deposition System. We have performed a detailed comparative study of the emission spectra of film and CNPs (prepared using the microwave synthesis method) in various solvents. A qualitative model based on solvatochromism of CNPs is used to understand the emission pathways in the film.
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Research Interests: Chemistry, Computational Chemistry, Water, Molecular Dynamics Simulation, Quantum Theory, and 13 moreMedicine, Density Functional Theory, Physical sciences, Hydrogen Bond, CHEMICAL SCIENCES, Spectrum analysis, Molecular Conformation, Molecule, Protons, Solvents, Isomerism, Physical chemistry and chemical physics, and protonation
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Fluorescence techniques have drawn increasing attention because they provide crucial information about molecular interactions in protein-ligand systems beyond that obtained by other methods. The advantage of fluorescence spectroscopy... more
Fluorescence techniques have drawn increasing attention because they provide crucial information about molecular interactions in protein-ligand systems beyond that obtained by other methods. The advantage of fluorescence spectroscopy stems from the fact that the majority of molecules in biological systems do not exhibit fluorescence, making fluorescent probes useful with high sensitivity. Also, the fluorescence emission is highly sensitive to the local environment, providing a valuable tool to investigate the nature of binding sites in macromolecules. In this review, we discuss some of the important applications of a class of molecules that have been used as fluorescent probes in a variety of studies. Hydroxyphenyl benzazoles (HBXs) show distinct spectroscopic features that make them suitable probes for the study of certain biological mechanisms in DNA, protein and lipid. In particular, the complex photophysics of 2-(2'-hydroxyphenyl)benzoxazole (HBO) and the distinguished fluorescence signatures of its different tautomeric forms make this molecule a useful probe in several applications. Among these are probing the DNA local environment, study of the flexibility and specificity of protein-binding sites, and detecting the heterogeneity and ionization ability of the head groups of different lipidic phases. The spectroscopy of HBX molecules and some of their chemically modified structures is also reviewed.
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Tautomerism in the ground and excited states of 7-hydroxyquinoline (7HQ) was studied in different solvents using steady-state and lifetime spectroscopic measurements, density functional theory (DFT) calculations, and molecular dynamics... more
Tautomerism in the ground and excited states of 7-hydroxyquinoline (7HQ) was studied in different solvents using steady-state and lifetime spectroscopic measurements, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. Equilibrium between the enol and the keto/zwitterion tautomers exists in 7HQ, which is solvent-dependent. Of the solvents used in this study, only in water does the absorbance spectrum of 7HQ show absorption from both the enol and zwitterion tautomers. In addition, in aqueous media, fluorescence is observed from the zwitterion tautomer only, which is attributed to self-quenching of the enol fluorescence by energy transfer to the ground-state zwitterion tautomer and energetically favorable excited-state proton transfer. Solvation of the hydrogen bonding sites of 7HQ was studied in binary mixtures of 1,4-dioxane and water, and three water molecules were estimated to connect the polar sites and induce intermolecular proton transfer. The results are confirmed by DFT calculations showing that three water molecules are the minimum number required to form a stable solvent wire. Mapping the water density around the polar sites using MD simulations shows well-defined hydrogen bonds around the amino and hydroxyl groups of the enol tautomer and slightly less well-defined hydrogen bonds for the zwitterion tautomer. The presence of three-member water wires connecting the polar centers in 7HQ is evident in the MD simulations. The results point to the unique spectral signatures of 7HQ in water, which make this molecule a potential probe to detect the presence of water in nanocavities of macromolecules.
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Drug–protein interactions based on the thermodynamics approach, curve resolution analysis and computational methods at molecular levels.
Research Interests: Thermodynamics, Chemistry, Medicine, Fluorescence Resonance Energy Transfer, Humans, and 13 moreCircular Dichroism, Albumin, Physical sciences, CHEMICAL SCIENCES, Hydrogen Bonding, Coumarins, Chalcone, Calorimetry, Protein Binding, Static Electricity, Serum albumin, Physical chemistry and chemical physics, and binding sites
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Human serum albumin (HSA) is the most abundant protein in blood plasma. It has high relevance for the lipid metabolism, and its ability to bind a large variety of natural and pharmaceutical compounds makes it a crucial determinant of drug... more
Human serum albumin (HSA) is the most abundant protein in blood plasma. It has high relevance for the lipid metabolism, and its ability to bind a large variety of natural and pharmaceutical compounds makes it a crucial determinant of drug pharmaco-kinetics and -dynamics. The drug binding properties of HSA can be characterized by spectroscopic analysis of bound probe molecules. We have recently characterized the subdomain IIA binding site of HSA using three hydroxyquinoline derivatives. In this work, we extend our study by combining data from energy transfer experiments, ligand docking, and long molecular dynamics (MD) simulations. Multiple possible binding locations are found within the subdomain IIA site, and their solvent accessibility and interactions with ligands are analyzed in detail. Binding pockets appear well hydrated during simulations, with ligands in direct contact to water molecules at all times. Binding free energies in good agreement to experiment are calculated. The HSA apoprotein is found to exhibit significant conformational flexibility over 250 ns of simulation time, but individual domains remain structurally stable. Two rotamers of Trp214 were observed on a time scale longer than 50 ns in the MD simulations, supporting the experimental observation of two fluorescence lifetime components. The flexible protein structure and heterogeneous nature of its binding sites explain the ability of HSA to act as a versatile molecular transporter. The combination of experimental and computational molecular distance information allows the conclusion that hydroxyquinoline probes bind in a binding mode similar to the anticoagulant drug warfarin.
Research Interests: Engineering, Chemistry, Molecular Dynamics Simulation, Medicine, Molecular Dynamics, and 12 moreFluorescence Resonance Energy Transfer, Humans, Physical sciences, CHEMICAL SCIENCES, Molecule, Ligands, Binding Site, Molecular Structure, Serum albumin, binding sites, fluorescent dyes, and Human serum albumin
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Abstract Environmental friendly and biocompatible fluorescent carbon nanoparticles (CNPs) show great potential for various applications. The functional groups on the surface of the NPs dramatically influence the absorption and emission... more
Abstract Environmental friendly and biocompatible fluorescent carbon nanoparticles (CNPs) show great potential for various applications. The functional groups on the surface of the NPs dramatically influence the absorption and emission characteristics of the particles. However, a conclusive emission mechanism of CNPs and the exact role of functional groups are still lacking. Herein, a systematic investigation has been carried out by quantifying the oxygen-based function groups (–OH, –CHO, –COOH, –CO, –COO) present on the surface of activated carbon (AC) nanoparticles to identify their role in the emission characteristics. AC samples, prepared by the activation process of green waste of mandarin peels, were subjected to further treatments to achieve different levels of surface functionality with oxygen containing functional groups such as carbonyl, carboxyl, phenol and lactone. Various characterization techniques were employed to identify and quantify the functional groups. A detailed emission study revealed the role of specific functional groups in the emission process. Our results suggests that various emissive pathways in CNPs can be controlled by selective surface functionalization. This study shows significant results, which could shed more light on the emission mechanism of CNPs and the pivotal role of surface functional groups.
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This work evaluates and compares alkyl mono- and di-glucoside sugar vesicles as potential delivery vehicles for small hydrophilic compounds.
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Research Interests: Biochemistry, Biology, Transmission Electron Microscopy, Medicine, Lantibiotics, and 13 moreBacteriocins, Streptococcus Pyogenes, Gram Positive Bacteria, Membrane, Anti-Bacterial Agents, Nisin, Microbial Sensitivity Tests, Amino Acid Sequence, Cell Wall, Peptidoglycan, Cytoplasm, Micrococcus luteus, and Cell Envelope
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The 2,2'-bipyridine-3,3'-diol molecule (BP(OH)2) was investigated as a potential photophysical probe in... more
The 2,2'-bipyridine-3,3'-diol molecule (BP(OH)2) was investigated as a potential photophysical probe in inclusion and biological studies. Binding of BP(OH)2 to cyclodextrins (CDs) and human serum albumin (HSA) was studied by following the changes in its absorption and fluorescence spectra. The stoichiometric ratios and binding constants of the complexes were deduced by fitting the changes in the spectral intensity to binding isotherms. The stoichiometric ratio in the BP(OH)2/(alpha-CD) complex is dominated by 1:2, whereas in all other CDs and in HSA this ratio is 1:1. The structure of the BP(OH)2:(alpha-CD)2 complex, calculated using ab initio methods, indicates that the inclusion of the BP(OH)2 molecule is axial and centered between the two cavities of alpha-CD with van der Waals and electrostatic interactions dominating the binding. Analysis of these results along with the inclusion results of BP(OH)2 in beta-CD, methyl-beta-CD, 2,6-di-O-methyl-beta-CD, and gamma-CD shows that absorption and fluorescence of BP(OH)2 are very sensitive to the change in the cavity size of CD and its hydrophobicity. This change is reflected in the form of a decrease in the intensity of the absorption peaks of the BP(OH)2/water complex in the region 400-450 nm and a red shift in the fluorescence peak as the cavity size decreases and its hydrophobicity increases. Binding of BP(OH)2 as a probe ligand to HSA, a prototype protein, reflects the hydrophobic interior of HSA in a similar manner. The spectral changes indicate that BP(OH)2 binds in the hydrophobic cavity of HSA's subdomain IIA. The results presented here show that BP(OH)2 can be used in binding sites and biological systems as a microenvironment-sensitive probe.
Research Interests: Engineering, Chemistry, Physical Chemistry, Electrochemistry, Fluorescence, and 14 moreMedicine, Humans, Absorption spectroscopy, Physical sciences, CHEMICAL SCIENCES, Stoichiometry, Cyclodextrins, Molecular Conformation, Protein Binding, Molecule, Ligands, Serum albumin, Isomerism, and Human serum albumin
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The S2-S0 fluorescence excitation spectra of xanthione (XT) and azulene (AZ) complexed with 1 or 2 molecules of the C1 to C10 n-alkanes and the C1 to C6 perfluoro-n-alkanes have been measured. The 1 : 1 complexes exhibit microscopic... more
The S2-S0 fluorescence excitation spectra of xanthione (XT) and azulene (AZ) complexed with 1 or 2 molecules of the C1 to C10 n-alkanes and the C1 to C6 perfluoro-n-alkanes have been measured. The 1 : 1 complexes exhibit microscopic solvent shifts, Delta-nu, which are larger for the alkanes than the corresponding perfluoroalkanes, despite the larger molecular polarizability of the latter.
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The inclusion of azulene (AZ) inside the cavities of beta-cyclodextrin (beta-CD) and gamma-cyclodextrin (gamma-CD) was studied using absorption, fluorescence and induced-circular dichroism spectroscopy. The inclusion of AZ into the cavity... more
The inclusion of azulene (AZ) inside the cavities of beta-cyclodextrin (beta-CD) and gamma-cyclodextrin (gamma-CD) was studied using absorption, fluorescence and induced-circular dichroism spectroscopy. The inclusion of AZ into the cavity of beta-CD has a stoichiometry of 1:1, whereas that of AZ/gamma-CD complex is 1:2. The equilibrium constants for the formation of the two complexes were calculated to be 780+/-150 M(-1) for AZ:beta-CD and (4.5+/-0.86)x10(5) M(-2) for AZ:(gamma-CD)(2). The latter is due to a stepwise equilibrium mechanism in which a 1:1 complex is formed with a binding constant of 775 M(-1), followed by the formation of a 1:2 complex with a binding constant of 580 M(-1). The difference between the two binding constant values is slight, indicating an almost equal contribution from each of the gamma-CD molecules to the overall binding in AZ:(gamma-CD)(2). From the induced-circular dichroism spectra, the inclusion of AZ was found to be axial in AZ:beta-CD and nearly axial in AZ:(gamma-CD)(2).
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... The keto-amine tautomers are the dominant base pair forms which are responsible for high-sequence specificity in duplex DNA. Double-proton transfer reaction along two parallel hydrogen bonds joining the DNA chains could originate rare... more
... The keto-amine tautomers are the dominant base pair forms which are responsible for high-sequence specificity in duplex DNA. Double-proton transfer reaction along two parallel hydrogen bonds joining the DNA chains could originate rare tautomers as shown in Fig. ...