Isomeric distribution and catalyzed isomerization of cobalt(III) complexes with pentadentate macrocyclic ligands. Importance of hydrogen bonding

We have investigated the isomeric distribution and rearrangement of complexes of the type [CoXLn](2+,3+) (where X = Cl-, OH-, H2O, and L-n represents a pentadentate 13-, 14-, and 15-membered tetraaza or diaza-dithia (N-4 or N2S2) macrocycle bearing a pendant primary amine). The preparative procedures for chloro complexes produced almost exclusively kinetically preferred cis isomers (where the pendant primary amine is cis to the chloro ligand) that can be separated by careful cation-exchange chromatography. For L-13 and L-14 the so-called cis-V isomer is isolated as the kinetic product, and for L-15 the cis-VI form (an N-based diastereomer) is the preferred, while for the L-14(S) complex both cis-V and trans-I forms are obtained. All these complexes rearrange to form stable trans isomers in which the pendent primary amine is trans to the monodentate aqua or hydroxo ligand, depending on pH and the workup procedure. In total 11 different complexes have been studied. From these, two different trans isomers of [CoCIL14S](2+) have been characterized crystallographically for the first time in addition to a new structure of cis-V-[CoCIL14S](2+); all were isolated as their chloride perchlorate salts. Two additional isomers have been identified and characterized by NMR as reaction intermediates. The remaining seven forms correspond to the complexes already known, produced in preparative procedures. The kinetic, thermal, and baric activation parameters for all the isomerization reactions have been determined and involve large activation enthalpies and positive volumes of activation. Activation entropies indicate a very important degree of hydrogen bonding in the reactivity of the complexes, confirmed by density functional theory studies on the stability of the different isomeric forms. The isomerization processes are not simple and even some unstable intermediates have been detected and characterized as part of the above-mentioned 11 forms of the complexes. A common reaction mechanism for the isomerization reactions has been proposed for all the complexes derived from the observed kinetic and solution behavior.

Poly(hydroxyether of phenolphthalein) and its blends with poly(ethylene oxid)

Poly(hydroxyether of phenolphthalein) (PPH) was synthesized through the polycondensation of phenolphthalein with epichlorohydrin. It was characterized by Fourier transform infrared (FTIR) spectroscopy, NMR spectroscopy, and differential scanning calorimetry (DSC). The miscibility of the blends of PPH with poly(ethylene oxide) (PEO) was established on the basis of the thermal analysis results. DSC showed that the PPH/PEO blends prepared via casting from N,N-dimethylformamide possessed single, composition-dependent glass-transition temperatures. Therefore, the blends were miscible in the amorphous state for all compositions. FTIR studies indicated that there were competitive hydrogen-bonding interactions with the addition of PEO to the system, which were involved with (OHO)-O-. . .=C

Oxidation of mixed-valence CoIII/FeII complexes reversed at high pH: A kinetico-mechanistic study of water oxidation

The outer-sphere oxidation of Fell in the mixed-valence complex trans-[(LCoNCFeII)-Co-14S-N-III(CN)(6)](-), being L-14S an N3S2 macrocylic donor set on the cobalt(III) center, has been studied. The comparison with the known processes of N-5 macrocycle complexes has been carried out in view of the important differences occurring on the redox potential of the cobalt center. The results indicate that the outer-sphere oxidation reactions with S2O82- and [Co(ox)(3)](3-) involve a great amount of solvent-assisted hydrogen bonding that, as a consequence from the change from two amines to sulfur donors, are more restricted. This is shown by the more positive values found for DeltaS(double dagger) and DeltaV(double dagger). The X-ray structure of the oxidized complex has been determined, and it is clearly indicative of the above-mentioned solvent-assisted hydrogen bonding between nitrogen and cyanide donors on the cobalt and iron centers, respectively. trans-[(LCoNCFeIII)-Co-14S-N-III(CN)(6)], as well as the analogous N-5 systems trans-[(LCoNCFeIII)-Co-14-N-III(CN)(6)], trans-[(LCoNCFeIII)-Co-15-N-III-(CN)(6)], and cis-[(LCoNCFeIII)-Co-n-N-III(CN)(6)], Oxidize water to hydrogen peroxide at pH > 10 with a rather simple stoichiometry, i.e., [(LCoNCFeIII)-Co-n-N-III(CN)(5)] + OH- - [(LCoNCFeII)-Co-n-N-III(CN)(5)](-) + 1/2H(2)O(2). In this way, the reversibility of the iron oxidation process is achieved. The determination of kinetic and thermal and pressure activation parameters for this water to hydrogen peroxide oxidation leads to the kinetic determination of a cyanide based OH- adduct of the complex. A second-order dependence on the base concentration is associated with deprotonation of this adduct to produce the final inner-sphere reduction process. The activation enthalpies are found to be extremely low (15 to 35 kJ mol(-1)) and responsible for the very fast reaction observed. The values of DeltaS(double dagger) and DeltaV(double dagger) (-76 to -113 J K-1 mol(-1) and -5.5 to -8.9 cm(3) mol(-1), respectively) indicate a highly organized but not very compressed transition state in agreement with the inner-sphere one-electron transfer from O2- to Fe-III.

Thermosetting Blends of Polybenzoxazine and Poly(ε-caprolactone): Phase Behavior and Intermolecular Specific Interactions

Polybenzoxazine (PBA-a)/poly(epsilon-caprolactone) (PCL) blends were prepared by an in situ curing reaction of benzoxazine (BA-a) in the presence of PCL. Before curing, the benzoxazine (BA-a)/PCL blends are miscible, which was evidenced by the behaviors of single and composition-dependant glass transition temperature and equilibrium melting point depression. However, the phase separation induced by polymerization was observed after curing at elevated temperature. It was expected that after curing, the PBA-a/PCL blends would be miscible since the phenolic hydroxyls in the PBA-a molecular backbone have the potential to form inter- molecular hydrogen-bonding interactions with the carbonyls of PCL and thus would fulfil the miscibility of the blends. The resulting morphology of the blends prompted an investigation of the status of association between PBA-a and PCL under the curing conditions. Although Fourier-transform infrared spectroscopy (FT-IR) showed that there were intermolecular hydrogen-bonding interactions between PBA-a and PCL at room temperature, especially for the PCL-rich blends, the results of variable temperature FT-IR spectroscopy by the model compound indicate that the phenolic hydroxyl groups could not form efficient intermolecular hydrogen-bonding interactions at elevated temperatures, i.e., the phenolic hydroxyl groups existed mainly in the non-associated form in the system during curing. The results are valuable to understand the effect of curing temperature on the resulting morphology of the thermosetting blends. SEM micrograph of the dichloromethane-etched fracture surface of a 90:10 PBA-a PCL blend showing a heterogeneous morphology.

Disulfide bond mutagenesis and the structure and function of the head-to-tail macrocyclic trypsin inhibitor SFTI-1

SFTI-1 is a novel 14 amino acid peptide comprised of a circular backbone constrained by three proline residues, a hydrogen-bond network, and a single disulfide bond. It is the smallest and most potent known Bowman-Birk trypsin inhibitor and the only one with a cyclic peptidic backbone. The solution structure of [ABA(3,11)]SFTI-1, a disulfide-deficient analogue of SFTI-1, has been determined by H-1 NMR spectroscopy. The lowest energy structures of native SFTI-1 and [ABA(3,11)]SFTI-1 are similar and superimpose with a root-mean-square deviation over the backbone and heavy atoms of 0.26 +/- 0.09 and 1.10 +/- 0.22 Angstrom, respectively. The disulfide bridge in SFTI-1 was found to be a minor determinant for the overall structure, but its removal resulted in a slightly weakened hydrogen-bonding network. To further investigate the role of the disulfide bridge, NMR chemical shifts for the backbone H-alpha protons of two disulfide-deficient linear analogues of SFTI-1, [ABA(3,11)]SFTI-1[6,5] and [ABA(3,11)]SFTI-1[1,14] were measured. These correspond to analogues of the cleavage product of SFTI-1 and a putative biosynthetic precursor, respectively. In contrast with the cyclic peptide, it was found that the disulfide bridge is essential for maintaining the structure of these open-chain analogues. Overall, the hydrogen-bond network appears to be a crucial determinant of the structure of SFTI-1 analogues.

Alkali hydroxide-induced gelation of pectin

The mechanism of pectin gelation depends on the degree of methoxylation. High methoxyl pectin gels due to hydrophobic interactions and hydrogen bonding between pectin molecules. Low methoxyl pectin forms gels in the presence of di- and polyvalent cations which cross link and neutralise the negative charges of the pectin molecule. Monovalent cations normally do not lead to gel formation with high methoxyl pectin solutions free of divalent cations, especially Ca. The present study found that alkali (NaOH or KOH) added to high methoxyl pectin leads to gel formation in a concentration-depended manner. It was also found that monovalent cations (Na and K) induce gelation of low methoxyl pectin and the time required for gel formation (setting time) depends on the cation concentration. The results indicate that a combined char-e neutralisation and ionic strength effect is responsible for the monovalent cation-induced gelation of pectin. (C) 2003 Elsevier Ltd. All rights reserved.

Interfacial interactions and structure of polyurethane intercalated nanocomposite

Understanding the interfacial interactions between the nanofiller and polymer matrix is important to improve the design and manufacture of polymer nanocomposites. This paper reports a molecular dynamic Study on the interfacial interactions and structure of a clay-based polyurethane intercalated nanocomposite. The results show that the intercalation of surfactant (i.e. dioctadecyldlmethyl ammonium) and polyurethane (PU) into the nanoconfined gallery of clay leads to the multilayer structure for both surfactant and PU, and the absence of phase separation for PU chains. Such structural characteristics are attributed to the result of competitive interactions among the surfactant, PU and the clay surface, including van der Waals, electrostatic and hydrogen bonding.

Factors influencing the stability of cyclotides: Proteins with a circular backbone and cystine knot motif

Cyclotides are a large family of mini-proteins that have the distinguishing features of a head-to-tail cyclised backbone and a cystine knot formed by six conserved cysteine residues. They are present in plants from the Rubiaceae, Violaceae and Cucurbitaceae families. The unique structural features of the cyclotides make them extremely resistant to chemical, thermal and proteolytic degradation. In this article we review recent Studies from our laboratory that dissect the role of the individual structural elements in defining the stability of cyclotides. The resistance of cyclotides to chemical and proteolytic degradation is in large part due to the cystine knot, whereas the thermal stability is I composite of several features including the cystine knot, the cyclic backbone and the hydrogen bonding network. A range of biological activities of cyclotides is critically dependent oil the presence of the cyclic backbone.

Kinetic and structural evidence for the importance of Tyr236 for the integrity of the Mo active site in a bacterial sulfite dehydrogenase

The sulfite dehydrogenase from Starkeya novella is the only known sulfite-oxidizing enzyme that forms a permanent heterodimeric complex between a molybdenum and a heme c-containing subunit and can be crystallized in an electron transfer competent conformation. Tyr236 is a highly conserved active site residue in sulfite oxidoreductases and has been shown to interact with a nearby arginine and a molybdenum-oxo ligand that is involved in catalysis. We have created a Tyr236 to Phe substitution in the SorAB sulfite dehydrogenase. The purified SDHY236F protein has been characterized in terms of activity, structure, intramolecular electron transfer, and EPR properties. The substituted protein exhibited reduced turnover rates and substrate affinity as well as an altered reactivity toward molecular oxygen as an electron acceptor. Following reduction by sulfite and unlike SDHWT, the substituted enzyme was reoxidized quickly in the presence of molecular oxygen, a process reminiscent of the reactions of the sulfite oxidases. SDHY236F also exhibited the pH-dependent CW-EPR signals that are typically observed in vertebrate sulfite oxidases, allowing a direct link of CW-EPR properties to changes caused by a single-amino acid substitution. No quantifiable electron transfer was seen in laser flash photolysis experiments with SDHY236F. The crystal structure of SDHY236F clearly shows that as a result of the substitution the hydrogen bonding network surrounding the active site is disturbed, resulting in an increased mobility of the nearby arginine. These disruptions underline the importance of Tyr236 for the integrity of the substrate binding site and the optimal alignment of Arg55, which appears to be necessary for efficient electron transfer.

On the equilibrium and dynamic behavior of alcohol vapors in activated carbon

As alcohol molecules such as methanol and ethanol have both polar and non-polar groups, their adsorption behavior is governed by the contributions of dispersion interaction (alkyl group) and hydrogen bonding (OH group). In this paper, the adsorption behavior of alcohol molecules and its effect on transport processes are elucidated. From the total permeability (B-T) of alcohol molecules in activated carbon, an adsorption mechanism is proposed, describing well the experimental data, by taking combination effects of clustering, entering micropores, layering and pore filling processes. Unlike the case of non-polar compounds, it was found that at low pressures there are two rises in the BT of alcohol molecules in activated carbon. The first rise is due to the major contribution of surface diffusion to the transport (which is the case of non-polar molecules) and the second one may be associated with cluster formation at the edge of micropores and entering micropores when the clusters are sufficiently large enough to induce a dispersive energy. In addition the clusters formed may enhance surface diffusion at low pressures and hinder gas phase diffusion and flow in meso/macropores. (c) 2006 Elsevier Ltd. All fights reserved.

1,3-Dimethylisoguanine

The structure of 1,3-dimethylisoguanine [ or 6-amino-1,3-dimethyl-1H-purin- 2(3H)- one], C7H9N5O, has been redetermined and the correct assignment of H atoms on the heterocycle is now reported. Intermolecular hydrogen-bonding interactions confirm that this form is the correct molecular structure; this form is also in agreement with an earlier reported structure of the trihydrate form.

Electrostatically mediated specific adsorption of small molecules in metallo-organic frameworks

We investigate the interaction of ethylene and ethane with a Cu-tricarboxylate complex and show that at low loadings the lighter molecule has a higher binding energy as a result of an increased interaction with the framework Cu and stronger hydrogen bonding with the basic framework oxygens. This leads to selective adsorption of ethylene by a factor of about 2 at low pressure, which is overcome by the stronger van der Waals interaction of ethane at high loadings, explaining recent literature data. The results suggest the possibility of separation of light hydrocarbons at low pressures or in trace amounts.

Characterization of titanium phosphate as electrolytes in fuel cells

Titanium phosphate is currently a promising material for proton exchange membrane fuel cells applications (PEMFC) allowing for operation at high temperature conditions. In this work, titanium phosphate was synthesized from tetra iso-propoxide (TTIP) and orthophosphoric acid (H3PO4) in different ratios by a sol gel method. High BET surface areas of 271 m(2).g(-1) were obtained for equimolar Ti:P samples whilst reduced surface areas were observed by varying the molar ratio either way. Highest proton conductivity of 5.4 x 10(-2) S.cm(-1) was measured at 20 degrees C and 93% relative humidity (RH). However, no correlation was observed between surface area and proton conductivity. High proton conductivity was directly attributed to hydrogen bonding in P-OH groups and the water molecules retained in the sample structure. The proton conductivity increased with relative humidity, indicating that the Grotthuss mechanism governed proton transport. Further, sample Ti/P with 1:9 molar ratio showed proton conductivity in the order of 10(-1) S.cm(-1) (5% RH) and similar to 1.6x10(-2) S.cm(-1) (anhydrous condition) at 200 degrees C. These proton conductivities were mainly attributed to excess acid locked into the functionalized TiP structure, thus forming ionisable protons.