Direct evidence for the role of imidazole in disproportionation of hydrogen peroxide by a mononuclear manganese salen complex

It has long been known that imidazole can enhance the catalytic activity of catalase model compounds; however, the role of imidazole is still not well understood. In an attempt to elucidate the role of imidazole in promoting the disproportionation of hydrogen peroxide by model compounds, four mononuclear manganese salen (Mn-Salen) complexes with and without axial imidazole ligands were synthesized and characterized by single-crystal X-ray diffraction, UV–vis spectroscopy, electrochemical and HPLC measurements. By comparing the Mn-Salen compounds with and without imidazole ligands, we demonstrated that the activity enhancement of imidazole originated from coordination of imidazole to the manganese center when less than one equivalent of imidazole was present, and from assisted deprotonation of the substrate when excess imidazole was present. These results provide direct evidence for the mechanism of activity enhancement of imidazole in the catalysis of enzyme model compounds.     

Spectroelectrochemistry of a photochromic [2.2]paracyclophane-bridged imidazole dimer: clarification of the electrochemical behavior of HABI

The photochromic behavior of the imidazole dimers can be attributable to the photoinduced homolytic cleavage of the C-N bond between the two imidazole rings. On the other hand, although the simultaneous formation of the imidazolyl radical and imidazole anion by the one-electron reduction of an imidazole dimer was reported, no definitive evidence for this electrochemical reaction has been demonstrated. We report the first direct evidence for the electrochemical generation of the imidazolyl radical from the radical anion of the imidazole dimer by conducting the UV-vis-NIR spectroelectrochemical analysis of the [2.2]paracyclophane-bridged imidazole dimer.     

Computational calculations of pKa values of imidazole in Cu(II) complexes of biological relevance

The imidazole ring is part of the lateral chain of histidine. One of the main features of this amino acid is the ability to coordinate copper, especially Cu(2+), because of the intermediate base nature of its imidazole ring, which has a great biological relevance. Proteins such as cytochrome c oxidase, a crucial enzyme in the respiratory chain, and β-amyloid peptide, implicated in the pathology of Alzheimer's disease, are examples of proteins containing histidines in their coordination sphere. Several studies indicate that the presence of this metal ion produces a decrease in the pK(a) of the imidazole ring of histidine. However, there are no reports of systematic studies of pK(a) variation in these types of metal cation complexes. In this work we use density functional theory to study the dependence of imidazole pK(a) with the number of imidazole rings in Cu(2+) coordination environments. The pK(a) of isolated imidazole (ImH), and the pK(a) of imidazole in Cu(2+)(ImH)(m)(H(2)O)(4-m) (m=1-3) complexes have been studied using two different functionals, B3LYP and MPWB1K, which have different percentage of exact exchange, and the highly-correlated CCSD(T) method. Results show that imidazole pK(a) decreases between 2 and 7 units depending on the method employed and the number of imidazole rings coordinating the metal cation. Taking into account that the pK(a) of imidazole is 14, this decrease could be relevant in biological processes.     

Stepwise fusion of porphyrin β,β'-pyrrolic positions to imidazole rings

A strategy for the stepwise annulation of pyrrolic rings of a porphyrin to imidazole rings is presented. Mono(imidazole), Janus and corner bis(imidazole), T-shaped tris(imidazole), and tetrakis(imidazole) porphyrins have been synthesized and characterized.     

Luminescent study on the binding interaction of bioactive imidazole with bovine serum albumin-A static quenching mechanism

Novel bioactive imidazole derivatives were synthesized and characterized by NMR spectra, mass and CHN analysis. The interaction between the imidazole derivative and bovine serum albumin (BSA) was investigated by fluorescence and UV-vis absorption spectroscopy. The fluorescence quenching of BSA by the imidazole derivatives may be due to the formation of imidazole-BSA complex. The fluorescence quenching mechanism of BSA by imidazole was analyzed and the binding constant has been calculated. The binding distance between imidazole and BSA was obtained based on Forester's non-radiation energy transfer (FRET). The effect of some common ions on the binding constant between imidazole and BSA was also examined.     

Synthesis of Bi- and tridentate imidazole ligands

A number of imidazole derivatives were prepared in the course of studying the limits of carbon-metal bond formation in complexes of imidazole with first-row transition metals. Seven of the compounds were new. The 4- and 5-methyl and 4,5-dimethyl derivatives of l-(2-pyridyl)-imidazole were prepared by reaction of the appropriate imidazole with 2-bromopyridine. Alkylation of imidazole, 4,5-dimethylimidazole, and benzimidazole with 2-chloromethylpyridine gave a series of 1-(2-pyridylmethyl)imidazoles. 1-(2,3-Diaminopropyl)imidazole was prepared in six steps via a Gabriel sequence.     

Enhancement of Acid‐Catalyzed Esterification by the Addition of Base

General acid‐catalyzed reaction can be enhanced by the addition of base. Self‐catalyzed esterification of benzoic acid and octan‐1‐ol was enhanced by the addition of certain base such as imidazole. The rate of the esterification was accelerated as the concentration of imidazole increased. Trans‐esterification of 4‐nitrophenyl acetate was promoted in chloroform by the mixture of benzoic acid and imidazole, but not by benzoic acid or imidazole alone.     

Synthesis of cytochrome c oxidase models bearing a Tyr244 mimic

A close structural analogue of the metal-free cytochrome c oxidase active site has been synthesized. This model has a proximal imidazole tail and three distal imidazole pickets attached to a porphyrin. One distal imidazole is cross-linked to a phenol, mimicking Tyr(244). The strategy behind the successful synthesis of this regioisomerically pure model involved discovering the best sequence to introduce the phenol-substituted imidazole and employing a fluorinated substituent.     

香豆素-咪唑类荧光染料的设计与研究

合成了不同给电子取代基(羟基、丁氧基、二乙基氨基等)的菲并[9,10-d]咪唑(CA1~CA6)或4,5-二苯基咪唑(CB1~CB6)修饰的香豆素衍生物,初步考察了它们的溶液发光和固体发光现象.研究表明,当香豆素取代基为氨基时,化合物在二氯甲烷中的荧光较强,而羟基取代、丁氧基取代或者无取代的衍生物在二氯甲烷中的荧光都很弱,而菲并[9,10-d]咪唑修饰的衍生物CA1~CA5的溶液荧光要比4,5-二苯基取代咪唑修饰的衍生物CB1~CB5的溶液强.另外,染料分子的分子内氢键强度及咪唑基-香豆素环间二面角大小都会对染料分子的发光性能产生影响.     

A practical and green approach towards synthesis of multisubstituted imidazoles using boric acid as efficient catalyst

An efficient one-pot, cyclocondensation of ammonium acetate, 1,2-diphenyl ethanedione, aromatic aldehyde or arylamine, and catalyzed by boric acid is achieved to form multisubstituted imidazole derivatives. Boric acid as green property is harmless to the environment and has improved the yield of multisubstituted imidazole derivatives. Moreover, we have tested the biological activities of imidazole derivatives by the four fungi. And the investigations showed most of imidazole derivatives have antifungal action on four fungus.     

某些1-酰基苯并咪唑及1-酰基咪唑衍生物的合成

2-取代的苯氧甲基苯并咪唑、咪唑分别与酰氯在缚酸剂存在下反应,制备了24个新的1-酰基2-苯氧甲基苯并咪唑及1-苯氧乙酰咪唑衍生物.后者容易吸水,转变成相应的铵盐.所制得的化合物中某些经小麦垂直生长法测定表现出一定的促进或抑制生长活性.     

Imidazole facilitates electron transfer from organic reductants

In cyclic voltammetry studies at pH 8, imidazole facilitates oxidation of organic compounds that normally lose hydrogen atoms. High concentrations of imidazole shift the oxidizing wave of ascorbic acid, 2,3-dimethoxy-5-methyl-1,4-hydroquinone, and the vitamin E analogue Trolox toward lower potentials. By contrast, imidazole has no effect on the cyclic voltammogram of methyl viologen, which undergoes electron rather than hydrogen-atom transfer. The effect of imidazole is observed at pH 8.0 but only to a lesser extent at pH 5.5 indicating that imidazole must be unprotonated to facilitate oxidation. Digital simulation shows that these results are consistent with a mechanism in which imidazole acts as a proton acceptor permitting concerted proton/electron transfer by the organic reductant.     

Masked imidazolyl-dipyrromethanes in the synthesis of imidazole-substituted porphyrins

Imidazole-substituted metalloporphyrins are valuable for studies of self-assembly and for applications where water solubility is required. Rational syntheses of porphyrins bearing one or two imidazol-2-yl or imidazol-4-yl groups at the meso positions have been developed. The syntheses employ dipyrromethanes, 1-acyldipyrromethanes, and 1,9-diacyldipyrromethanes bearing an imidazole group at the 5-position. The polar, reactive imidazole unit was successfully masked by use of (1) the 2-(trimethylsilyl)ethoxymethyl (SEM) group at the imidazole pyrrolic nitrogen, and (2) a dialkylboron motif bound to the pyrrole of the dipyrromethane and coordinated to the imidazole imino nitrogen. The nonpolar nature of such doubly masked imidazolyl-dipyrromethanes facilitated handling. Selected masked dipyrromethanes were characterized by 11B and 15N NMR spectroscopy. Five distinct methods were examined to obtain trans-A2B2-, trans-AB2C-, and trans-AB-porphyrins. Each porphyrin contained one or two SEM-protected imidazole units. The SEM group could be removed with TBAF or HCl. Two zinc(II) porphyrins and a palladium(II) porphyrin bearing a single imidazole moiety were prepared and subjected to alkylation (with ethyl iodide, 1,3-propane sultone, or 1,4-butane sultone) to give water-soluble imidazolium- porphyrins. This work establishes the foundation for the rational synthesis of a variety of porphyrins containing imidazole units.     

咪唑键合细胞色素C的核磁共振共振研究

我们用^1HNMR双共振技术测定了在pH=7.04,温度303-319K范围内咪唑键合氧化型细胞色素C的平衡常数,从Van'tHoff和Arrhenius方程得到键合的热力学常数△H=48.5kj·mol^-1,△S°=184J·mol^-1·K^-1和活化能E~a=159kj·mol^-1,并与其它血红素蛋白和模型化合物的热力学常数作了比较和讨论.用饱和转移法归属了cytc·Im的血红素环上甲基峰,首次用NMR方法确证咪唑在与氧化型cytc反应中取代了轴向Met80,形成新的Fe-N键.     

Acid Dissociation Constants of the Benzimidazole Unit in the Polybenzimidazole Chain: Configuration Effects

The acid dissociation constant of three benzimidazoles, namely 2,2′-bibenzo[d]imidazole, 2,5′-bibenzo[d]imidazole, and 5,5′-bibenzo[d]imidazole, have been investigated by means of density functional theory calculations in gas phase and in aqueous solution. The theoretical approach was validated by the comparing of predicted and experimentally determined pK_a values in imidazole, benzimidazole, and 2-phenylbenzimidazole. From the studied compounds, 2,2′-bibenzo[d]imidazole was found to be the most acidic, which made it a valuable candidate as a material for polymer electrolyte membrane fuel cells.     

磷酰化组氨酸形成六配位磷中间体的理论研究

用MNDO方法对磷酰化组氨酸磷上酯交换反应侧链咪唑基所参与的六配位磷机理进行了研究。六配位磷中间体形成后,使咪唑基对面的异丙氧基反应活性提高。当磷上酯交换反应发生时,异丙氧基离去和另一分子醇进攻磷,从咪唑基的对面发生,在能量上和空间上都是有利的。六配位磷机理比较好地解释了咪唑的催化作用。     

Synthesis of new imidazole derivatives as potential inhibitors of thromboxane synthetase. II

The preparation of new imidazole derivatives containing ether or amide functions into the side chain, is reported starting from the suitable imidazole intermediates.     

Sequential Nucleophilic Substitution of Phosphorus Trichloride with Alcohols in a Continuous-Flow Reactor and Consideration of a Mechanism for Reduced Over-reaction through the Addition of Imidazole

We demonstrate a sequential nucleophilic substitution of highly electrophilic and inexpensive phosphorus trichloride with three different alcohols in a continuous-flow reactor. A variety of alcohols including ones that contained acid- and/or basic-labile functionalities were rapidly reacted. A over nucleophilic substitution that occurred during reaction of the second alcohol was suppressed by the addition of imidazole. Density functional theory calculations of the sequential nucleophilic substitutions of alcohols were performed both with and without imidazole, and Berry pseudorotation was suggested as a rate-limiting step in both cases. Herein, we discuss the reasons for the decreased selectivity in the absence of imidazole as well as those for improved selectivity in the presence of imidazole during the second nucleophilic substitution.     

Comprehensive understanding of structure- photosensitivity relationships of photochromic [2.2]paracyclophane-bridged imidazole dimers

The photochromic [2.2]paracyclophane-bridged imidazole dimers show instantaneous coloration upon exposure to UV light and rapid fading in the dark. Experimental details for the enhancement of the photosensitivity and the unique photoisomerization of newly designed [2.2]paracyclophane-bridged imidazole dimers are demonstrated. We explored the structure-property relationships and demonstrated an efficient strategy for designing high-performance fast-photochromic molecules with increased photosensitivity to solar UVA radiation. The [2.2]paracyclophane-bridged imidazole dimer consists of two types of imidazole rings, Im1 and Im2. Im1 is characterized by a 6π electron system with an electron-donating characteristic, whereas Im2 is distinguished by a 4π electron system with an electron-withdrawing characteristic. The introduction of electron-donating substituents into the phenyl rings attached to the electron-withdrawing Im2 was proved to enhance the photosensitivity with the aid of the intramolecular charge transfer transitions. The unique photoisomerization resulting from the changes in the bonding manner between two imidazole rings was also investigated in detail.     

Individual and joint kinetic fluorometric determination of imidazole and 4-methylimidazole

The oxidation of 1,1,3-tricyano-2-amino-1-propene in the presence of hydrogen peroxide and copper is favored by imidazole, 4-methylimidazole, or any other compound possessing the imidazole ring. This fluorescent reaction has been used for the individual determination of imidazole and 4-methylimidazole at 10⁻⁵M level by application of several kinetic methods (tangent, fixed-time, and maximum fluorescence intensity) with a precision (%RSD) of about ±1%. A differential-rate principle allows the determination of binary mixtures of both compounds, which is subject to synergistic effects. Mixtures of imidazole and 4-methylimidazole in ratios between 1:1 and 1:9 have been resolved with a %RSD of ±7.1% for imidazole and ±3.1% for 4-methylimidazole.