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At least 109 records · Page 6Linked to original sources
PubMed2004

Crystallography and magnetism of radicals with hindered hydroxyl groups: 2-(3,5-di-tert-butyl-4-hydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and 2-(3,5-di-tert-butyl-4-hydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl.

Radicals form networks of OH... ON and OH...Me(nitroxide) interactions. In 2, a frustrated network forms with insufficient N-O units to form extended chain interactions. The magnetism of 1 fits a 1-D Heisenberg model with J/k=-25 J mol(-1), while 2 shows more complex exchange behaviour consistent with its disordered crystal lattice.

Journal Article↗
PubMed2005

C-halogen...pi dimer and C-H...pi interactions in 1-(2-bromo-4,5-dimethoxybenzyl)-2-butyl-4-chloro-1H-imidazole-5-carbaldehyde and 2-butyl-4-chloro-1-(6-methyl-1,3-benzodioxol-5-ylmethyl)-1H-imidazole-5-carbaldehyde.

The structures of the title compounds, C17H20BrClN2O3, (I), and C17H19ClN2O3, (II), are stabilized by intramolecular C-H...O and C-H...pi interactions. The stability of the molecular packing in (I) and (II) arises from a diverse set of weak intermolecular C-H...O, C-H...pi and C-halogen...pi interactions. In the crystal structure of (I), molecules aggregate in dimeric subunits via C-Br...pi interactions. The dimers are interlinked by C-H...O hydrogen bonds. The halogens cluster together and form a channel along the b axis. In (II), the packing is mainly governed by intermolecular C-H...O and C-H...pi interactions.

Journal Article↗
PubMed2001

Factors determining the orientation of axially coordinated imidazoles in heme proteins.

Factors determining conformations of imidazole axially coordinated to heme in heme proteins were investigated by analyzing 693 hemes in 432 different crystal structures of heme proteins from the Protein Data Bank (PDB), where at least one histidine is ligated to heme. The results from a search of the PDB for protein structures were interpreted with molecular force field computations. Analysis of data from these crystal structures indicated that there are two main factors that determine the orientations of imidazole ligated to heme. These are the interactions of imidazole with the propionic acid side chains of heme and with the histidine backbone. From the analysis of the crystal structures of heme proteins, it turned out that the hydrogen bonding pattern is often not decisive, though it is probably used by nature to fine-tune the orientation of imidazole axially ligated to heme. We found that in many heme proteins the NdeltaH group of imidazole ligated to heme can assume a number of different hydrogen bonds and that in mutant structures the orientation of the ligated imidazole often does not change significantly, although the mutant altered the hydrogen bonding scheme involving the imidazole. Data from crystal structures of heme proteins show that there are preferred orientations of imidazoles with respect to heme. Generally, the NdeltaH group of imidazole is oriented toward the propionic acid groups of the heme. In some cases, the NdeltaH group of imidazole is close to only one of the propionic acid groups, but it is practically never oriented in the opposite direction. The imidazole also adopts a preferred orientation with respect to its histidine backbone such that the plane of the imidazole ring is practically never parallel to the Calpha-Cbeta bond of its histidine backbone. For a given conformation of histidine backbone with respect to heme, as well as imidazole with respect to histidine backbone, the orientation of the imidazole with respect to heme is uniquely determined, since the three orientations depend on each other. Hence, the interaction of the imidazole with the backbone also influences the orientation of the imidazole with respect to the heme. Force field computations are in agreement with experimental data. With this method, we showed that there is an energy minimum when the NdeltaH group of the imidazole is oriented toward the propionic acid groups and that there are minima of energy for orientations where the imidazole ring is orthogonal to the plane defined by the Calpha-Cbeta and Cbeta-Cgamma bonds of the histidine. The computations also demonstrated that these interactions are mainly of electrostatic origin. By taking into account these two major factors, we were able to understand the orientations of axially coordinated imidazoles for all groups of heme proteins, except for the group of cytochrome c peroxidase. In this group, the orientation of the imidazole is determined by a strong hydrogen bond of the NdeltaH group with Asp235.

Crystallography, X-Ray↗
PubMed2001

Formation of mu-oxo dimer by the reaction of (meso-tetraphenylporphyrinato)iron(III) with various imidazoles.

Reactions between (meso-tetraphenylporphyrinato)iron(III) perchlorate [Fe(tpp)]ClO4 and various imidazoles have been examined in CD2Cl2 solutions. 1H NMR analysis revealed the formation of three kinds of complex; mu-oxo dimer, mono-imidazole adduct, and bis-imidazole adduct. The product ratios changed to a great extent depending on the amount and nature of imidazoles. In general, addition of less than 1.0 equiv of imidazole relative to [Fe(tpp)]ClO4 led to the formation of both mu-oxo dimer and mono-imidazole adduct. However, by the addition of excess amount of imidazole, either the mu-oxo dimer or bis-imidazole adduct was formed exclusively depending on the bulkiness of the imidazole used. In the case of bulky imidazole such as 2-methylbenzimidazole or 2-isopropyl-1-methylimidazole, the mu-oxo dimer was formed quantitatively. In the case of less bulky imidazole such as parent imidazole or 1-methylimidazole, bis-imidazole adduct became the sole product. The results have been explained in terms of the difference in steric interactions between the axial ligands and porphyrin core; the severe steric repulsion prohibits the formation of bis-adduct in the case of bulky imidazoles. As a result, bulky imidazoles prefer to behave as a base; they abstract a proton from coordinated water, and lead to the formation of mu-oxo dimer. Thus, the role of bulky imidazoles in these reactions has some relevance to that of distal histidine in hemoglobin and peroxidase.

Journal Article↗