| As the proteins with high substrate specificities and large rate accelerations, enzymes play very significant role in maintaining metabolizing balance for all living things. To the scientists'surprise, it can catalyze the recations with highly stereoselectivities and specificities under mild conditions. It is always their research purpose to design and synthesize catalysts which could act like those enzymes and understand their catalytic mechanisms. For the sake of obtaining an effectivie enzyme mimic, we must consider that the artificial enzyme has a good binding ability with transition state compounds, and that the enzyme is beneficial to release the products in thermodynamics.As one of a series of antioxidative selenoenzymes in living organisms, Glutathione Peroxidase(GPx) catalyzes the reduction of harmful hydroperoxide by glutathione to protect biological molecules from oxidative stress both inside and outside the cells. GPx has a promising future for applications in antioxidant drugs. Ebselen, the first reported molecular compound with GPx activity, has became the antioxidant drug in the market. Its structure and catalytic mechanisms have been extensively and thoroughly studied. Because of its low GPx activity and poor dissolvability in aqueous solution, scientists pay attentions to developing highly effecient and well water-soluble GPx mimics.The active site of GPx includes a selenocysteine residue which forms a catalytic triad with glutamine and tryptophan residues in a depression at the protein's surface, and some charged and hydrophobic amino acid residues form a hydrophobic cavity. The enzyme's intrinsic property is substrate binding and catalyzing inside itself. The ideal enzyme model should have both substrate-binding site and catalytical site. And its active site should be made up of substrate-binding site and catalytic group. Some reported GPx mimics have low activity. Because the contribution to GPx activity from identification and binding subtrates by the enzymes were not considered, then the mimics cannot provide proper microenvironment for the Sec which is the catalytical site. We begin with catalytic intrisic propertise of enzymes, take advantage of supramolecular chemistry principles, and constructe a mimic with GPx activity based on molecular recognition. Since a series of research work on cyclodextrin-based GPx mimics have been done in our group, we construct a new GPx mimic based on cylcophane, in which we choose the cyclophane's hydrophobic cavity as the substrate binding site and introduce the catalytic site on it.There are several reasons for which we choose cyclophane as the substrate binding sites. It can provide not only a hydrophobic environment for the substrate, but also a stable binding site for the guest. Since its cavity is seldom affected by some changes outsides such as temperature, pH and ionic strength. Besides, a wide synthetic variation of the macrocycless can be achieved, so an appropriate recognition site with regard to size, shape, and microenvironmental property is provided for the guest molecule. What's more, distinct molecular discrimination can become operative by the introduction of functional groups into appropriate sites of cyclophanes giving additional noncovalent interactions, such as electrostatic, hydrogen-bonding, charge-transfer, and metal-coordination interactions. Kenji Koga et al. first reported the synthesis of 1,6,20,25-tetraaza[6.1.6.1]paracyclophane(CP44) and the X-ray crystal structure of the complex of CP44 with durene in 1980. It is the first direct evidence of guest inclusion by a water-soluble cyclophane. We conceive the cyclophane can be used as the binding site for phenthiol substrate as the size of its hydrophobic cavity match benzene ring very well. Meanwhile, we successfully introduce Se/Te as the catalytical site of the GPx mimic near the hydrophobic cavity. Thus, we have finished the basic structure of the artificial enzyme. There is not any report on construction of GPx mimic based on cyclophane.For the purpose of obtaining a mimic with higher effency, we could modify the side chains to enhance its binding ability to phenthiol substrate. On the other hand, we could make some changes to the catalytic site to speed up the catalytical reaction. Se and Te are in the same main group. They have similar redox properties. But tellunol can reduce the hydroperoxides much easier than selenol, and its catalytical effeciency is higher. Considering that the ditellurides have the enzymetic behaviors of acting as a GPx catalytic group, and that the cyclophane's hydrophobic cavity can identifing and binding the substrates, we successfully synthesize a GPx mimic with higher effeciency by introducing ditelluride into cyclophane after we introduced diselenide into it.The results of the cyclophane-based mimic we synthesized are good, which support that our designing before is totally right. We test the cyclophane-based mimics'binding ability with the substrate in PBS-DMSO solution with a 1:10 volume ratio, as their solubility is not well. The binding constants are on the scale of 102 M-1. We also test the mimics'catalytic effeciency of catalyzing the reduction of hydroperoxide with p-Nitrobenzenethiol as the substrate in the same solution. It turned out to be that the GPx mimics do have catalytic ability, and the effeciency of ditelluride mimic is higher than that of diselenide mimic.The cyclophane mimics we synthesized are just preliminary attemptation apparently, and more thorough research is undergoing. We can have more try on the mimics'water solubility next step. Then, we could make a good use of cyclophane's hydrophobic cavity for binding substrates, and construct GPx mimic models with higher effeciency. At the same time, as some researches reported, we could use supramolecular interactions to construct and assembly aggregates based on cyclophane GPx mimics. And a series of research will be carried out for more useful information about mimic enzymes. |
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