Chemistry Department                      

       Abdel Monem Rawashdeh, Assistant Professor

 

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Research Interests:

 

  • THEORETICAL AND COMPUTATIONAL WORK

    1- DNA intercalation of 3a,9a-diazaperylenium dication

    DNA binding molecules regulate mechanisms central to cellular function, including DNA replication and gene expression. Many small molecules that mimic or block these processes offer potential therapeutic agents. Critical to the understanding of the function of such molecules is the characterization of their binding modes, which are usually investigated through ensemble measurements. Understanding how complexation affects both the structural and mechanical properties of DNA is an important step towards understanding the functional mechanisms of binding agents, and may provide a key to more rational drug design.

     

        Many classes of synthetic and naturally occurring low molecular weight agents are known to interact with DNA through a variety of distinct mechanisms, including non-covalent (reversible) or covalent fixation processes.1 However, most drug-based strategies have exploited the antigen approach, where double-stranded DNA is targeted directly by a ligand molecule so as to interfere with template transcription function or replication processes. Binding mechanisms typically involve either interaction in the minor or major grooves of the host duplex, 2 or intercalation between stacked base pairs, although mixed-mode binding is also often evident, biological response is primarily governed by the effective residence time of a bound molecule with cytotoxic effects arising from cellular events that require the unimpeded DNA template.

    ( DAP )

    3a,9a-diazaperylenium dication (DAP) which has been synthesized recently3 may find an application as a fluorescent probe or in analogy to its quinolizinium building block, and as a DNA binder with possible antileukemic action.

        Our basic idea came from structural similarity of those quinolizinium salt with our compound and the fact that our target molecule is flat, which will facilitate this type of interaction.4

     

    Intercalation of a planar molecule in the DNA base pairs stack

     

     

    2-Computing the Redox Potentials of phenothiazine derivatives.

           Phenothiazines with an N-aminopropyl side chain and various ring substituents are potent neuroleptics. With their ability to cross the blood barrier in the brain; their high solubility in aqueous and nonaqueous solvents, and their relatively low toxicity and non-genotoxicity, phenothiazines have been in clinical use for many years to treat mental disorders such as schizophrenia, paranoia, and psychosis, additionally not only that but they are used as antimicrobial drugs, and as an anti-oxidant, anti-tumor.

          Cyclic voltammetry of both PTZ (Phenothiazine) and MPTZ (N-methylphenothiazine) show that both compounds are oxidized reversibly under the specified conditions. so that the new values are 846 mV for PTZ and 962 mV for MPTZ (vs. NHE). The redox values reveal something very appealing; MPTZ is harder to oxidize than PTZ. Although alkylation is known to increase the electron density and thus render aromatic systems more easily oxidized.

          Semiempirical Austin Model 1 (AM1), ab initio Hartree-Fock (HF) and Density Functional Methods (DFT) were employed in the geometry optimization and in the computation of the one-electron oxidation potentials of phenothiazine and N-methylphenothiazine. The calculations show that N-methylphenothiazine radical cation is nonplanar but the phenothiazine radical cation is flat. Using the Continuum Solvation Module (PCM) for prediction of the solvation energies; the DFT/ B3PW91/6-311+g* method gave the best agreement with the experimental redox potential for both compounds.

                

     

                Phenothiazine (PTZ)                                    N-methylphenothiazine (MPTZ)

     

     

                

            (PTZ+·) radical cation                                           (MPTZ+·) radical cation

     

     

    The structures of Phenothiazine, N-methylphenothiazine and their radical Cations

    Reference:

     A. M. Rawashdeh. “Computing the Redox Potentials of Phenothiazine and N-methylphenothiazine” Abhath Alyarmouk “Basic Science and Engineering” 2005, 14(2) 195-208

     

  • AEROGEL CHEMISTRY

     

    Doped Aerogels as platforms of gas sensors

         The compounds shown below were synthesized, characterized in solution and frozen matrices, and evaluated as dopants of sol-gel materials.  The intramolecular quenching efficiency of 4-benzoyl-N-methylpyridinium cation in solution depends on the solvent.  In frozen matrices or absorbed on the surfaces of silica aerogel, both Ru(II) complex/electron acceptor dyads are photoluminescent. The photoluminescence of our Ru(II) complex dyads adsorbed on aerogel is quenchable by O2 diffusing through the mesopores. Thus, in the presence of O2, aerogels doped with dyads based on 4-benzoyl-N-methylpyridinium can modulate their photoluminescence over a wider dynamic range than aerogels doped with dyads based on viologen, and both are more sensitive than aerogels doped with Ru(II) tris(1,10-phenanthroline). Furthermore, in contrast to frozen solutions, the luminescent moieties in the bulk of aerogels kept at low temperatures are still accessible, leading to more sensitive platforms for oxygen sensors than their room temperature counterparts.

     

     

     

     

     

     

     

     

     

     

     


     

     

     

     

     

     

     

     

     

     

     

    Room temperature response of a silica aerogel doped with 1 under an alternating stream

        of N2 and O2. The fastest switching rate is 15 s; emission was monitored at 643 nm.

          

     

     

     

     Differences in the emission spectra at 77 K of silica aerogels doped with

    Ru(phen)3]2+ (A), 1 (B) and 2 (C) under nitrogen and under oxygen

    Reference:

    N. Leventis; A. M. Rawashdeh; I. A. Elder; J. Yang; A. Dass; C. Sotiriou-Leventis “Synthesis of Ru(II) tris(1,10-phenanthroline)- electron acceptors dyads incorporating the 4-benzoyl-N-methylpyridinium cations or N-benzyl-N´-methylviologen. Characterization in fluid and frozen solutions, and on silica aerogels” Chem. Mater. 2004, 16(8), 1493-1506

     

     

     

  • ORGANIC ELECTROCHEMISTRY

     

    The effect of substitution on stepwise multi-electron processes

          Dendrimers are self-repeating globular branched star molecules, whose fractal structure continues to fascinate, challenge, and inspire. Functional dendrimers may incorporate redox centers, and potential applications include antennae molecules for light harvesting, sensors, mediators, and artificial biomolecules.

     

    The e-transfer across the perimeter of dendrimers should depend on their rigidity, but it is unclear whether it would be more or less efficient than e-transfer along the branches. As these questions have important implications for molecular design, they were investigated with star systems 1-4, serving as models of first- and second-generation redox dendrimers.

     

     

     

          The rigidity of the star 1, provides a complementary view of the fact that fast e-transfer along the perimeter of core-branch systems requires flexible branches. From a practical viewpoint, redox equivalents emerging from the core of a rigid light-harvesting system would be localized at the tips of the branches they emerge from core of a rigid light-harvesting system would be localized at the tips of the branches they emerge from, creating issues of efficient bimolecular e-transfer to redox quenchers in their immediate environment. Flexible branches may not only facilitate e-transfer along the perimeter but may also fold, rendering internal redox centers more accessible.

    Reference:

    J. Yang; A. M.  Rawashdeh; W. Oh; C. Sotiriou-Leventis; N. Leventis “Redox-active star molecules incorporating the 4 benzoylpyridinium cation: implications for the charge transfer efficiency along branches vs. across the perimeter in dendrimers” J. Am. Chem. Soc. 2004, 126(13), 4094-4095