Hassan S. Bazzi

Three-dimensional organization of block copolymers on "DNA-minimal" scaffolds.

Here, we introduce a 3D-DNA construction method that assembles a minimum number of DNA strands in quantitative yield, to give a scaffold with a large number of single-stranded arms. This DNA frame is used as a core structure to organize other functional materials in 3D as the shell. We use the ring-opening metathesis polymerization (ROMP) to generate block copolymers that are covalently attached to DNA strands. Site-specific hybridization of these DNA-polymer chains on the single-stranded arms of the 3D-DNA scaffold gives efficient access to DNA-block copolymer cages. These biohybrid cages possess polymer chains that are programmably positioned in three dimensions on a DNA core and display increased nuclease resistance as compared to unfunctionalized DNA cages.

Polyisobutylene-anchored N-heterocyclic carbene ligands.

The synthesis of polyisobutylene (PIB)-supported N-heterocyclic carbenes (NHCs) that are useful as ligands for recoverable/recyclable organometallic complexes is described. Both PIB-bound carbenes analogous to SIMes and IMes as well as carbene precursors bound to PIB via 1,2,3-triazoles by alkyne-azide couplings are described. Both Ag(I) and Ru(II) complexes of these carbenes are shown to be phase selectively soluble in heptane. Hoveyda-Grubbs second-generation catalysts containing a PIB-supported NHC have also been used to catalyze ring-closing metathesis.

Thermomorphic Polyethylene-Supported Olefin Metathesis Catalysts

The preparation of polyethylene-oligomer (PE(olig))-supported N-heterocyclic carbene ligands (NHCs) and their Ru complexes is described. These complexes are structurally analogous to their low molecular weight counterparts and can serve as thermomorphic, recoverable/recyclable ring-closing metathesis (RCM) catalysts. Because of the insolubility of PE(olig)-supported species at 25 °C, such complexes can perform homogeneous RCM reactions at 65 °C and, upon cooling, precipitate as solids. This allows for their quantitative separation from solutions of products.

Synthesis, spectroscopic and thermal investigations of solid charge-transfer complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane and the acceptors iodine, TCNE, TCNQ and chloranil

The solid charge-transfer complexes formed in the reaction of the electron donor 1,4,7-trimethyl-1,4,7-triazacyclononane (TMTACN) with the acceptors iodine, tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) have been isolated. These were characterized through electronic and infrared spectra as well as thermal and elemental analysis. The results show that the formed solid CT-complexes have the formulas [(TMTACN)I]I3, [(TMTACN)(TCNE)5] and [(TMTACN)(TCNQ)3] in full agreement with the known reaction stoichiometries in solution. The chloranil CT-solid complex cannot be isolated in pure form.

Synthesis and self-assembly of polymers containing dicarboximide groups by living ring-opening metathesis polymerization

Polymers with narrow molecular weight distributions and controlled architectures were generated by living ring-opening metathesis polymerization of exo-7-oxabicyclo[2.2.1] hept-5-ene-2,3-dicarboximide. The dicarboximide units have been previously shown to exhibit biological activity, can selectively bond to the nucleic acid base adenine by hydrogen-bonding, and are readily functionalizable. Block copolymers containing these moieties were generated, and underwent self-assembly into nanoscale spherical aggregates, with surface localized molecular recognition motifs.

Synthesis, spectroscopic and thermal structural investigations of the charge-transfer complexes formed in the reaction of 1-methylpiperidine with σ- and π-acceptors

The reactions of the electron donor 1-methylpiperidine (1MP) with the pi-acceptors 7,7,8,8-tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil=CHL) and iodine (I(2)) were studied spectrophotometrically in chloroform at room temperature. The electronic and infrared spectra of the formed molecular charge-transfer (CT) complexes were recorded. The obtained results showed that the stoichiometries of the reactions are not fixed and depend on the nature of the acceptor. Based on the obtained data, the formed charge-transfer complexes were formulated as [(1MP)(TCNE)(2)], [(1MP)(DDQ)].H(2)O, [(1MP)(CHL)] and [(1MP)I]I(3), while in the case of 1MP-TCNQ reaction, a short-lived CT complex is formed followed by rapid N-substitution by TCNQ forming the final reaction products 7,7,8-tricyano-8-piperidinylquinodimethane (TCPQDM). The five solids products were isolated and have been characterized by electronic spectra, infrared spectra, elemental analysis and thermal analysis.

Pavement Subgrade Stabilization Using Polymers: Characterization and Performance

This paper reports a laboratory investigation aimed at evaluating the potential of polymer binders to stabilize pavement subgrades in Qatar. The conclusions regarding the impact of the polymer stabilizers are based on comparisons with selected physical, chemical, mechanical, and microstructural properties of natural Qatari subgrade soil and soils stabilized with the traditional standard, portland cement (PC). The results demonstrate that the polymer binders modify the Qatari subgrade soils resulting in more favorable engineering properties: for example, the compressive strengths of the polymer-stabilized soils are superior to those of the unstabilized soils and those stabilized using PC. The mechanical properties of the stabilized and unstabilized soils were incorporated in the analysis of asphalt pavement sections using the state-of-the-art protocol for pavement design and analysis. Results of the analysis further demonstrated the benefits of using stabilized subgrade soils in improving pavement performance. Of particular practical importance is that polymer subgrade stabilization significantly reduces subgrade rutting. Consequently, polymer subgrade stabilization is a key consideration for perpetual pavements, especially considering the extremely high truck traffic in Qatar. Although the work reported in this paper was carried out with Qatari soil, the results are expected to be relevant to other types of soil and weather conditions experienced throughout the Middle East and in similar climates throughout the world.

Synthesis and spectroscopic studies of the charge transfer complexes of 2- and 3-aminopyridine.

The interactions of the electron donors 2-aminopyridine (2APY) and 3-aminopyridine (3APY) with the pi-acceptors tetracyanoethylene (TCNE), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), 2-chloro-1,3,5-trinitrobenzene (picryl chloride, PC), and 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil) were studied spectrophotometrically in chloroform at room temperature. The electronic and infrared spectra of the formed molecular charge transfer (CT) complexes were recorded. Photometric titration showed that the stoichiometries of the reactions were fixed and depended on the nature of both the donor and the acceptor. The molecular structures of the CT-complexes were, however, independent of the position of the amino group on the pyridine ring and were formulated as [(APY)(TCNE)], [(APY)(DDQ)], [(APY)(PC)], and [(APY) (chloranil)]. The formation constants (K(CT)), charge transfer energy (E(CT)) and molar extinction coefficients (epsilon(CT)) of the formed CT-complexes were obtained.

Synthesis and spectroscopic studies on charge-transfer molecular complexes formed in the reaction of imidazole and 1-benzylimidazole with σ- and π-acceptors.

The spectrophotometric characteristics of the solid charge-transfer molecular complexes (CT) formed in the reaction of the electron donors imidazole (IML) and 1-benzylimidazole (BIML) with the σ-acceptor iodine and π-acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetracyanoethylene (TCNE) and 2,3,5,6-tetrachloro-1,4-benzoquinone (CHL) have been studied in chloroform at 25 °C. These were investigated through electronic and infrared spectra as well as elemental analysis. The results show that the formed solid CT-complexes have the formulas [(IML)2 I]I3, [(IML)(DDQ)], [(IML)2(TCNE)5] and [(IML)(CHL)] for imidazole and [(BIML) I]I3, [(BIML)(DDQ)2], [(BIML)(TCNE)2] and [(BIML)(CHL)2] for 1-benzylimidazole in full agreement with the known reaction stoichiometries in solution as well as the elemental measurements. The formation constant KCT, molar extinction coefficient ɛCT, free energy change ΔG0, CT energy ECT and ionization potential Ip have been calculated for the CT-complexes [(IML)2 I]I3, [(IML)(DDQ)], [(IML)(CHL)], [(BIML) I]I3, [(BIML)(DDQ)2], [(BIML)(TCNE)2] and [(BIML)(CHL)2].

“DNA–Teflon” sequence-controlled polymers

Perfluorocarbons (PFCs) are a promising class of molecules for medical applications: they are detectable through 19F nuclear magnetic resonance (NMR) and they assemble separately from water or lipophilic phases, thus bringing unique supramolecular interactions into nanostructures. We report the ready insertion of PFCs into nucleic acids, as well as non-natural polymers in a sequence-defined fashion. This is achieved via an automated and efficient synthetic pathway using phosphoramidite chemistry. Modulating the PFC tail length of “DNA–Teflon” block copolymers resulted in micelles that are almost monodisperse, have a low critical micelle concentration (CMC), are traceable by 19F NMR and are responsive to external stimuli. Strong fluorine–fluorine interactions in DNA duplexes allowed remarkable melting temperature increases and provided nuclease resistance. Finally, PFC insertion into siRNA was achieved, and the conjugates were efficient for gene silencing, outlining that these modifications are highly suitable for oligonucleotide therapeutics and bioimaging tools.