Ishrat M. Khan

A DENSITY FUNCTIONAL TREATMENT OF ORGANOLITHIUM COMPOUNDS: COMPARISON TO AB INITIO, SEMIEMPIRICAL, AND EXPERIMENTAL RESULTS

Density functional calculations on several classes of organolithium compounds are described. The compounds studied include lithium bonds to carbon, oxygen, and nitrogen and are representative of most types of organolithium compounds that have appeared in the recent literature. The computational results are compared to those using MNDO, which has been shown to have some serious deficiencies in compounds involving carbon–lithium bonds, and to PM3 results, which offer some improvement over MNDO for many organolithium compounds. Most of the density functional calculations with a large basis set are in good agreement with available ab initio and experimental data. Calculated carbon–lithium bond lengths were slightly shorter than those calculated by other ab initio methods and were substantially longer than those calculated by MNDO, which is known to underestimate carbon–lithium bond lengths severely. Dimerization energies of methyllithium, calculated by DMol, were also in good agreement with those of other ab initio calculations. Lithium–nitrogen bonds in lithium amides were calculated to be slightly shorter by DMol than by MNDO, although the two methods were in qualitative agreement for this type of compound.

Synthesis of Optically Active Helical Poly(2-methoxystyrene). Enhancement of HeLa and Osteoblast Cell Growth on Optically Active Helical Poly(2-methoxystyrene) Surfaces

Poly(2-methoxystyrene)s (P2MS) were synthesized using n-BuLi in THF and toluene at various temperatures. At −20°C and higher temperatures, toluene was an effective polymerization solvent for synthesizing poly(2-methoxystyrene). Under these conditions, polymers with good yields and reasonable molecular weight distributions were obtained. Polymers synthesized under all conditions were isotactic; the most highly isotactic polymer was obtained in toluene at −20°C. The m (isotactic dyad) content of the polymers synthesized in toluene at 0°C and −20°C was 0.65 and 0.74, respectively. Optically active helical (+) and (−) P2MS were synthesized by asymmetric polymerization utilizing (+) or (−) [2,3-dimethoxy1,4(dimethylamino)butane] (DDB)/tolyl lithium initiating complex in toluene. Asymmetric polymerizations were also carried out at 0°C to synthesize optically active polymers. The optical rotations of the polymers were found to be dynamic and reversible, strongly suggesting contribution of the chiral higher ordered structure to the overall optical rotation. Geometry optimization carried out using various force fields such as MM+, AMBER and CHARMM suggests that isotactic P2MS form low energy stable helical conformations. HeLa cells showed better growth on surfaces prepared using chiral polymers compared to the surfaces prepared with achiral polymers. Similarly, chiral P2MS surfaces were also more effective as supports for mouse and human osteoblast cells. The cell attachment and growth data demonstrate that chiral P2MS surfaces were better supports compared to achiral P2MS surfaces. Atomic force microscopy (AFM) studies on surfaces prepared using chiral poly(2-methoxystyrene) showed more discrete topography features compared to surfaces prepared with achiral polymers. Thus, the surface topography may play a role in determining polymer–cell interactions.

Non-covalent nano-adducts of co-poly(ester amide) and poly(ethylene glycol): preparation, characterization and model drug-release studies.

Biodegradable, biocompatible poly(ester amide)s (co-PEAs), composed of amino acids, fatty diols and carboxylic acids, have been synthesized. To improve the performance of co-PEAs in Federal Drug Administration-approved solvents such as water and ethanol, these polymers were complexed with poly(ethylene glycol) (PEG) of 10 kDa molecular mass have been prepared by solution blending. The non-covalent adducts were purified by precipitation into hexanes. Co-PEAs are soluble in organic solvents but are insoluble in water and ethanol; however, the co-PEA/PEG (0.8:1, w/w) adducts are soluble in ethanol and slightly soluble in water. 2D-NOESY NMR spectroscopy suggests that the non-covalent adducts are held together by multiple non-covalent interactions between the –CH2– groups of the two polymers (co-PEA and PEG). Differential scanning calorimetry studies indicate that the two polymers are interacting in the non-covalent adducts; the thermal properties of the adducts are different from those of the pure polymers. The solid-state adduct structures have been determined by atomic force microscopy (AFM). By one sample preparation method, nanoscale pancake-like structures were observed with an average diameter of 260 nm and an average height of 16 nm. Films of co-PEAs and (co-PEA)/PEG adducts containing Rhodamine B Base (RhBB), a model hydrophobic drug, were prepared. From the adduct/RhBB film containing 3% RhBB, 20% of the total RhBB was released within the first 2 h. Film and adduct composition may be varied to obtain different release profiles. The studies reported here demonstrate that non-covalent conjugation is a relatively easy and effective approach in developing new materials for application as biomaterials.

Synthesis and Characterization of α,ω‐bi[2,4‐dinitrophenyl (DNP)] poly(2‐methoxystyrene) Functional Polymers. Initial Evaluation of the Interaction of the Functional Polymers with RBL Mast Cells

Two series of functional polymers, α,ω‐bi[2,4‐dinitrophenyl][poly(ethylene oxide)‐b‐poly(2‐methoxystyrene)‐b‐poly(ethylene oxide)] (DNP‐PEO‐P2MS‐PEO‐DNP) and α,ω‐bi[2,4‐dinitrophenyl caproic][poly(ethylene oxide)‐b‐poly(2‐methoxystyrene)‐b‐poly(ethylene oxide)] (CDNP‐PEO‐P2MS‐PEO‐CDNP), were synthesized by anionic living polymerization. The polymers were characterized by FT‐IR, 1H‐NMR and Gel Permeation Chromatography (GPC). The molecular weight distributions for the lower molecular weight functional polymers were slightly broad (1.3–1.5). However, the molecular weight distributions for higher molecular weight polymers were narrower (1.1–1.2). Differential scanning calorimetry (DSC) studies showed thermal transitions indicative of the presence of microphases in the polymer solid state. The polymers were white powders and soluble in tetrahydrofuran. The binding affinity of DNP‐PEO‐P2MS‐PEO‐DNP ligands towards anti DNP IgE was determined by titrations with fluorescently labeled FITC‐IgE. A water soluble CDNP‐PEO‐P2MS‐PEO‐CDNP/DMEG (dimethoxyethylene glycol) complex binds and achieves steady state binding with solution IgE within a few seconds. This strongly suggests that CDNP functional polymers with improved water solubility have potential in therapeutics. Higher molecular weight (water insoluble) CDNP‐PEO‐P2MS‐PEO‐CDNP polymers were electrosprayed as fibers (500 nm) on silicon surface. Fluorescence spectroscopy clearly showed that RBL mast cells were interacting with the fibers suggesting that the cell‐surface receptors were clustered along the fiber surface. These observations suggest that the functional polymers hold promise for developing an antibody detection device.

Effect of polymer stereoregularity on polystyrene/single-walled carbon nanotube interactions

We use a combination of computational and experimental studies to elucidate the effect of polymer stereoregularity on the capability of polystyrene interacting with single-walled carbon nanotube (SWNT) surfaces. Calculated binding energies on complexes of slightly oxidized SWNT with isotactic and atactic polystyrene favor the former, which suggests that the isotactic polymer interacts more effectively with the SWNT. The glass transition temperature (Tg) of the isotactic polystyrene/SWNT matrix increases from 90.9 to 100.5 °C as the SWNT content is increased to 0.5%, whereas the glass transition temperature of the atactic polystyrene/SWNT matrix is invariant with increasing SWNT content. Rotating frame 13C T1ρ relaxation rates for the isotactic polymer/SWNT matrix increases from 2.15 to 2.43 ms as the SWNT content is increased from 0.25 to 1.0%. However, the rotating frame 13C T1ρ relaxation rates for the atactic polymer/SWNT matrix decreases from 2.50 to 1.60 ms as SWNT content is increased from 0.25 to 1.0%. Our results demonstrate that the SWNTs are better dispersed within the isotactic polystyrene and the better dispersion is associated with a more effective interaction of the isotactic polymer with the SWNT surface.

Structure, bonding, and asymmetric induction in (R,R)-2,3-dimethoxy-1,4-bis(dimethylamino)butane complexes with organolithium compounds: A semiempirical computational study

Abstract Semiempirical PM3 calculations were performed on several organolithium complexes with (R,R)-2,3-dimethoxy-1,4-bis(dimethylamino)butane. The calculations showed that the ligand binding mode is dependent on the aggregation state of the organolithium, and may bind via the nitrogen atoms, oxygen atoms, or both. The complex with t-butyllithium exists as a monomer, while the complexes with several other organolithium compounds exist primarily as dimers. Complexes between the various chelated forms of butyllithium and propanal were examined in order to better understand the mechanism of asymmetric induction.

Archetypical Conductive Polymer Structure for Specific Interaction with Proteins

A series of 2,4-dinitrophenyl (DNP) functionalized polypyrrole terpolymers, capable of specific binding to IgE antibodies (proteins), have been synthesized using oxidizing initiator, ammonium persulfate and were characterized by 1H-NMR, IR, DSC, Light Scattering etc. The terpolymers were composed of monomers and macromonomers: monomer (pyrrole), macromonomer A (pyrrole with pendant ethylene glycol) and macromonomer B (pyrrole with pendant DNP) with specific functionality of conductance, processiblity and binding, respectively. The terpolymers are found to be semiconductive, (5 × 10−6 S cm−1) by itself without the addition of doping agents. Molecular dynamics simulation of terpolymer shows that the DNP-(2,4-dinitrophenyl) functional group extends out from the polymer backbone and thus, is available for binding. The DNP functional groups on the terpolymer achieve steady state binding with anti-DNP IgE proteins at nanomolar concentrations in solution. The terpolymer was blended with sulfonated polystyrene and processed in fibers which exhibited effective specific binding to fluorescently tagged IgE proteins and therefore, possessed the potential to be an active component in biosensor.

Non-covalent adducts of sodium poly(α, L-glutamate) with poly(N-vinyl pyrrolidone): Methods of preparation and characterization of structure

Non-covalent adducts of poly(N-vinyl pyrrolidone) (PVP) (mol. wt. 10 K & 29 K) with sodium poly(α,L-glutamate)(PGNA) (mol. wt. 32 K) are prepared by evaporation of aqueous mixtures (EAM), ultra-centrifugation (UC) and dehydration of reverse micelles (DRM). The EAM and UC adducts contain nearly equal amounts of PVP while the DRM adduct has lower amounts. Higher-molecular-weight PVP favored greater PVP content in the adducts regardless of the method of preparation. DSC thermograms, and FT-IR and CD spectra of the three adducts in the solid state revealed that PVP and PGNA are intimately mixed and the PGNA is in a random conformation. Hydrophobic interactions between PGNA and PVP are evident in dilute aqueous solutions of all three adducts, while Na + ions of PGNA remain as free ions. 2D-NOESY 1 H NMR spectra of the EAM and UC adducts are very similar and show a strong correlation between the a-proton of PGNA with a pyrrolidone ring (no. 3 and no. 4 protons) and β-protons of PGNA with a pyrrolidone ring (no. 5 proton). In contrast, regarding the DRM adduct. only the α-proton of PGNA interacts with the pyrrolidone ring (no. 3 and no. 4 protons), presumably due to the orientation of the pyrrolidone ring at the organic phase-water interface of the reverse micelle, which causes the proton in position 5 of the ring to be buried in the organic phase. All three adducts dissociate in water to form free PVP and PGNA. However, the DRM adduct dissociates faster than other two, presumably due to reduced hydrophobic interactions. Differences in composition and properties observed for the non-covalent adducts may be attributed to the differences in intermolecular (hydrophobic) interfactions imposed on the two components, PGNA and PVP, during each method of preparation.

Alkaline Degradation of Resorcinol-Formaldehyde Resins: Solid-State NMR, Thermal Adsorption and Desorption Analysis, and Molecular Modeling

Abstract Resorcinol-formaldehyde resins, used for alkali metal-cation exchange, were shown to degrade more rapidly in the metallated form compared to the protonated form. Solid-state 13C NMR showed additional peaks in the partially degraded resin corresponding to quinone and other carbonyl functionalities, which are absent in the undegraded resin. Semiempirical molecular orbital calculations were used in conjunction with the experimental data to elucidate the degradation mechanism.

A computational study of mixed aggregate formation in lithio azaallylic systems

Abstract Ab initio and semiempirical molecular orbital calculations were used to study the formation of mixed aggregates of lithioazaallylic systems with lithium alkoxides and lithium amides. Both the ab initio and semiempirical calculations predicted the existence of two isomeric mixed aggregates, differing in the nature of the lithium bonding to the azaallylic p-orbital. The effects of molecular structure and solvation on the relative energies of the two isomers were examined.