Bioinspired catalysts: Synthesis, characterisation and some applications

Our recent work concerning the synthesis, characterisation and some applications of bioinspired electron-transfer catalysts is reviewed in this contribution. The catalysts were various mono- or heterobimetallic complexes having either Cu(II) or Cu(II) and Zn(II) as central ions and amino acids, their derivatives or various N-containing organic molecules as ligands. Emphasis was based upon the solid support immobilised versions of these complexes. They were built or anchored onto various kinds of supports (silica gel, montmorillonite, Merrifield’s resin) with different methods (adsorption, ion exchange, covalent grafting). The resulting materials were characterised by a variety of instrumental (FT-IR, Raman, EPR [electron paramagnetic resonance] and atomic absorption spectroscopies, thermogravimetry) as well as computational methods. Their superoxide dismutase, catecholase and catalase activities were tested and some of them were found to be promising candidates as durable electron-transfer catalysts being close to the efficiency of the mimicking enzymes

Hydrogen-bonding interactions in the crystalline-phase structures of cinnamic acid derivatives

Secondary interactions responsible for the crystalline-phase structures of various cinnamic acids and cinnamic acid esters drawn from the Cambridge Structural Database were studied with the help of the Cerius package implemented on an SGI workstation. Primarily hydrogen-bonding interactions were sought, but deviation from planarity was also measured. For the acids, the main structural feature is the strong hydrogen bonding between the carboxyl groups. The dimers are interconnected by C-H ... O intermolecular hydrogen bonds. In most cases the C atom of the C-H unit was a member of the aromatic ring. Intramolecular (olefinic) C-H ... O bonds were found to be frequent, fixing synperiplanar and antiperiplanar C=C-C=O conformations with about the same abundance in the acids and exclusively synperiplanar conformations in the esters. The carbonyl group of the ester is always involved in C-H ... O hydrogen bonding. Here, the C atom of the C-H unit was either a member of the aromatic ring or the olefinic group, or was attached to the alcoholic O atom of the ester group. The beta-phenyl and the carboxyl or the ester groups are almost coplanar. The crystals have a layered structure and in the most frequent parallel arrangement the phenyl groups are offset by varing amounts (but always to a small extent) in neighbouring layers. The common hydrogen bonds are mostly within a layer

Conformations of 2-phenyl-3-pyridylpropenoic acid (alpha-phenyl pyridylcinnamic acid) dimers – A computational study

Motivation. Cinnamic acid analogs are not only important parts of the shikimic acid metabolic pathway of higher plants but it is possible to assemble, particularly from those containing oxygen or nitrogen heteroatoms, various patterned structures kept together with CH...O or CH...N hydrogen bonds. The fundamental unit of these structures is the acid dimer, e.g., the dimer of E and Z–2–phenyl–3–pyridylpropenoic acids of this study, which may exist in many conformations. As a preparation for a detailed conformational analysis of the patterned structures, it was decided to study the conformational behavior of these acid dimers, containing the N heteroatom in all possible positions of the aromatic ring. The conformational behavior of any cinnamic acid analogs in the dimeric form has not been studied before. Method. The conformational search module of the HyperChem package was used for the conformational analysis of the acid dimers with the PM3 semiempirical method. Calculations were performed for isolated dimers, i.e., without solvent. Results. The conformational search identified many conformers of the acid dimers. Although their numbers amounted to hundreds, they were found to fill the conformational space unevenly, in a highly symmetric nature. The distribution patterns were typical for the stereoisomers, but resembled to each other irrespective to the position of the nitrogen atom. Conclusions. It was proved to be possible to study the conformational behavior of cinnamic acid analogs in their dimeric forms for the first time. Large number of conformers was identified and they were found the fill the conformational space in a patterned way

Functional and structural mimics of superoxide dismutase enzymes

Superoxide dismutase (SOD) enzymes form important defence line in living organisms. Through a dismutation reaction they transform the highly reactive superoxide radical ion to oxygen and hydrogen peroxide. The latter compound is further transformed by catalase or peroxidase enzymes to water and oxygen. The overall structure of the enzymes and those of the active sites are largely known, thus, it has been revealed that in eukaryotes Cu(II) and Zn(II) ions act as cofactors and they are connected with an imidazolate bridge and this structural unit is coordinated with amino acids. In prokaryotes the SOD enzymes contain Mn(II) or Fe(II) or Ni(II) in their active centres. In order to learn about the working mechanism of SOD enzymes at the molecular level various structural mimics were prepared and their structural transformations during the dismutation reaction was followed. Gathering adequate amount of information allowed the preparation of functional mimics that are not necessarily copies of the active sites of the enzymes, nevertheless, display considerable SOD activity. Both functional and structural mimics are comprehensively dealt with in this review. Although enzymes may seem to be attractive catalysts for promoting real-life reactions effectively with high selectivity, they can seldom if ever be used under industrial conditions, i.e. at high temperatures and pressures. The SOD enzymes for promoting oxygen transfer reactions are not durable enough under these conditions either. The complexes mimicking SOD activities perform better in this respect, however, their reusabilities are limited, because of separation problems. A solution can be the immobilisation of these SOD mimicking complexes on solid or semi-solid supports. Even if the activity is not better then the support-free complexes, the catalyst can be filtered at the end of the reaction and can easily be recycled. Attempts for immobilisation are also comprehensively reviewed and immobilised complexes with surprisingly high SOD activities are reported as well. Full characterisation of these materials is given and rationalisation of their exceptionally high activities is offered

Deuterium distribution in two major products obtained during the cyclic dimerisation of methyloxirane over acidic molecular sieves

The ring-transformation reactions of methyloxirane on DZSM-5 and DAlMCM-41 aluminosilicates were studied in a pulse microreactor at 363 K. Cyclic dimerisation was found to be the major reaction pathway. Deuterium distribution in two important products of this reaction route was monitored. The positions of the deuterium atoms were determined giving help in proposing a more accurate dimerisation mechanism

A computational study on the adsorption of the major ring-transformation products of methyloxirane in HY and HZSM-5 zeolites

The adsorption of dioxolane and dioxane derivatives in HY and HZSM-5 zeolites was studied computationally. The molecules are the main ring-transformation products of methyloxirane over these zeolites. Experimental work showed that the cyclic dimerisation reaction starts with the adsorption of the monomers over Brønsted acid sites through their ring oxygens. Computations revealed that these sites were close enough even in the high-silica HZSM-5 zeolite and the dimers comfortably fit even in the β-cage of HY as well as in the channel of the HZSM-5 zeolites. The stereochemical outcome of the dimerisation was also studied and the optimum conformations of the dimers were calculated as well

Towards a model of the Al13-pillared layer clay - a computational study

A step-by-step construction of an Al13-Keggin ion pillared montmorillonite segment is shown with the use of the MM+ general force field. The starting structures for the montmorillonite as well as the Al13-Keggin ion were obtained as pdb files. The two layers of the clay were dissected and segmented, then optimised with the MM+ force field. The Keggin ion was optimised with the PM3 semiempirical method. Then, the Keggin ion was attached to the layer through chemical bonds and was optimised with the MM+ force field once again. Finally, the structure was capped with the second layer and was optimised once again with the MM+ force field

SOD activity of immobilized enzyme mimicking complexes

A binuclear, imidazolato-bridged, possible superoxide dismutase-mimicking complex (Cu(II)-diethylenetriamino-μ-imidazolato-Zn(II)-tris-aminoethylamine-triperchlorate) was prepared and immobilized on silica gel or among the layers of montmorillonite. The superoxide dismutase (SOD) activity of the complex before and after immobilization was studied by a SOD assay. It was found that the SOD activity of the host-free complex decreased somewhat when montmorillonite was the host, however, using silica gel as host it increased

The formation of carbonaceous layer from ethylene over various transition metal catalysts – an FT-IR study

The ethylene-derived carbonaceous overlayers were studied over silica-supported Pt, Pd, Rh and Cu catalysts by FT-IR spectroscopy under desorption conditions. As a general feature it was observed over all catalysts that upon increasing the desorption temperature the overlayer gradually became poor in hydrogen. The structure of the overlayers was similar over the silica-supported Pt, Pd and Rh. On increasing the reaction temperature the σ-adsorbed half-hydrogenated species are transformed to adsorbed ethylidyne. On the Cu catalyst prepared with ion exchange a similar picture emerged, while on those prepared with precipitation ethylidyne soon became the predominant species. Hydrogen swept off the carbonaceous species from the transition metals, however, copper retained large portions of it