Search

The optoelectronic properties of polydiacetylenes can be strongly modulated by torsions along the polymer chains. These as well as other distortions of the nominally coplanar polydiacetylene backbones result in the major color changes observed for these materials in response to a variety of external, low-energy stimuli; such color changes form the basis for the many applications of polydiacetylenes as sensor materials. There has been little theoretical work related to backbone distortions in polydiacetylenes; actually, previous estimates of the torsional barriers in these systems differ by an order of magnitude. Understanding the impact that polymer torsions have upon the properties of polydiacetylenes necessitates accurate estimates of the torsion potentials. Here, by using computationally efficient, wave-function-based electronic structure methods on increasingly larger oligomers, we present reliable estimates of the torsional barriers in model diacetylene oligomers and provide an accurate extrapolation of these values to the polymer limit.

Nanoporous zeolitic imidazolate frameworks (ZIFs) form structural topologies equivalent to zeolites. ZIFs containing only one type of imidazole linker show separation capability for limited molecular pairs. We show that the effective pore size, hydrophilicity, and organophilicity of ZIFs can be continuously and drastically tuned using mixed-linker ZIFs containing two types of linkers, allowing their use as a more general molecular separation platform. We illustrate this remarkable behavior by adsorption and diffusion measurements of hydrocarbons, alcohols, and water in mixed-linker ZIF-8(x)-90(100-x) materials with a large range of crystal sizes (338 nm to 120 μm), using volumetric, gravimetric, and PFG-NMR methods. NMR, powder FT-Raman, and micro-Raman spectroscopy unambiguously confirm the mixed-linker nature of individual ZIF crystals. Variation of the mixed-linker composition parameter (x) allows continuous control of n-butane, i-butane, butanol, and isobutanol diffusivities over 2-3 orders of magnitude and control of water and alcohol adsorption especially at low activities.

Electron spin echo envelope modulation (ESEEM) and extended X-ray absorption fine structure (EXAFS) spectroscopic studies of oxygenated cobalt (oxyCo) [tetraphenylporphyrin(TPP)][1-methylimidazole (1-MeIm)], an active site model of oxyCo-substituted globins (functional and EPR-active [S = 1/2] analogues of oxygen carrying hemoproteins), are carried out in order to examine the correlation of oxygen affinity with electron-nuclear coupling parameters and metal−ligand bond lengths. ESEEM demonstrates that the magnitude of the electron-nuclear hyperfine and nuclear quadrupole couplings to the directly-coordinated 14N of 1-MeIm decrease (Aiso, from 3.54 to 3.04 MHz; e2qQ, from 2.39 to 2.08 MHz) as the solvent compositon is varied from 0 to 50% (v/v) dichloromethane in toluene. For oxyCo[(o-R)1TPP][1-MeIm] (where R = −H, −NHCOC(CH3)3, −NHCOCH3, or −NHCONHC6H5, an ortho substitutent on one of the four meso phenyls of TPP), couplings to the axial nitrogen decrease (Aiso, from 3.54 to 3.07 MHz; e2qQ, from 2.39 to 2...

The two available crystallographic structures of cobalamin dependent enzymes, the 27 kDa fragment of the methylcobalamin-dependent enzyme, methionine synthase, from Escherichia coli [Drennan, C. L. et al. Science 1994, 266, 1669] and the 5‘-deoxyadenosylcobalamin-dependent enzyme methylmalonyl-coenzyme A mutase from Propionibacterium shermanii [Mancia, F. et al. Structure 1996, 4, 339], show striking similarities despite the differences in reaction mechanism. In particular, the 5,6-dimethylbenzimidazole group is detached and replaced by a histidine group of the enzyme. Here we present an analysis of Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic data for both 5‘-deoxyadenosylcobalamin and aquocobalamin bound to methylmalonyl-coenzyme A mutase in the absence of substrate. The analysis is conducted with a suite of programs called AUTOFIT 1.0 [Chance, et al. Biochemistry 1996, 35, 9014], which allows an evenhanded comparison of the goodness-of-fit of the EXAFS data to a varied grid of simulat...

We demonstrate the first application of L-edge X-ray absorption spectroscopy (XAS) to the electronic characterization of biological photolysis products. The experimental L-edge XAS spectra of deoxymyoglobin (deoxy Mb), oxymyoglobin (MbO2), carbonmonoxymyoglobin (MbCO), and the low-temperature photoproducts (Mb*CO and Mb*O2) are presented and compared to simulated spectra using a ligand field multiplet calculation. This analysis indicates that MbCO and MbO2 are both low spin and does not support some previous studies which suggest that MbO2 has an intermediate spin. Both photoproducts, Mb*CO and Mb*O2, are different from deoxy Mb in the FeII electronic structure. In addition, different low-temperature photolysis intermediates are suggested for MbCO and MbO2. The L-edge XAS spectra for FeIII in aquometmyoglobin (met Mb) and azidomet myoglobin (MbN3) provide a comparison of the ferrous versus ferric myoglobin species. Finally, the special advantages of using soft X-ray absorption spectroscopy for understandi...

The effect of pH on the inner barrier to oxygen binding in myoglobin has been examined through a study of the pH-dependence of the low-temperature photoproduct yield for horse oxymyoglobin (MbO2). ...

The corrinoid/iron-sulfur protein (C/Fe-SP) from Clostridium thermoaceticum is an 88 kDa [alpha][beta]-dimer that cycles between a Co(I) and methyl-Co(III) form as it transfers a methyl group from methyltetrahydrofolate (methyl-H[sub 4]folate) to carbon monoxide dehydrogenase. Extended X-ray absorption fine structure (EXAFS) and X-ray edge spectroscopy of the as-isolated C/Fe-SP indicates that the inactive Co(II) state of the corrinoid has a four-coordinate distorted square-planar structure. To the authors' knowledge, this is the first observation of a four-coordinate Co(II) corrinoid. This unusual coordination state would be expected to poise the Co(II) state of the C/Fe-SP for facile reduction to four-coordinate Co(I), which is the active nucleophile that acts as the acceptor of the methyl group of methyl-H[sub 4]folate. Additionally, the first-derivative X-ray edge spectra for both the methyl-Co(III) and Co(II) forms of the C/Fe-SP show an unexpected shift to lower energy when compared to analogous free cobalamins indicating a potential role for the protein in mediating electronic as well as structural properties of the enzyme bound methylcobamide. 42 refs., 6 figs., 3 tabs.

Although a number of (nonalkyl)cobalamins are considered essential to human nutrition, providing starting materials in the synthesis of alkylcobalamin coenzymes, their structures are either not available or are very old and unreliable. We present cobalt-ligand distance information in solution for methylcobalamin, cyanocobalamin, and aquocobalamin using extended X-ray absorption fine Structure (EXAFS) spectroscopy. Methylcobalamin was examined as a control to verify our ability to accurately assign cobalt-ligand distances, since this compound has a known X-ray structure

The valence effective Hamiltonian (VEH) technique is used to compute ionization potentials, optical transition energies, and electron affinities of oligomers and polymers in four conjugated systems: polyacetylene, poly@-phenylene), polythiophene, and polypyrrole. The theoretical results compare very favorably with experimental data on gas-phase ionization potentials, optical absorption, and electrochemical redox potentials. The latter case is especially important, and the calculated oxidation and reduction potentials are in remarkably good agreement with experiment. For polyacetylene the predicted oxidation potential is 0.4 V vs. SCE, and the predicted reduction potential is -1.1 V, both of which are in good agreement with experimentally observed oxidation and reduction onsets. In these systems, the electronic and electrochemical properties predicted by VEH theory for the oligomers extrapolate to those of the polymer with an inverse chain-length dependence.