| Summary: | 博士 === 國立臺灣大學 === 化學研究所 === 93 === The better separability is a good advantage for immobilization of molecular catalysts. The immobilized systems have an improved activity and regioselectivity compared to the homogeneous system. The enhancement of selectivity might be explained by the steric constraints which the support with the uniform channels. The confinement of the substrate in the mesoporous channels could then lead to a larger influence of the directing group on the orientation of the substrate relative to the reactive catalytic center when compared to the homogeneous reaction.
Dicupric complex (HPC) can be immobilized in MCM-41 and Y zeolite. We use this system to study the catalytic activities of HPC in the oxidation of 3,5-di-tert-butylcatechol (DTBC) to the corresponding quinone to mimic the functionality of catechol oxidases. HPC complexes can adsorb only on the outside surface of the Y zeolite due to its smaller pore size. The EXAFS spectrum gives 3.51 Å for the Cu--Cu distance in HPC encapsulated in the nanochannels of Al-MCM-41, which is comparable to the O--O distance of the two hydroxyl groups of DTBC, and this made a simultaneous coordination of the diol group to the dicupric center possible. The nanochannels of Al-MCM-41 provide stability, due to confined space and surface charge, which could prevent excessive separation of the dinuclear cupric centers after removal of the hydroxo bridge in the catalytic process. The study demonstrates that HPC encapsulated in the nanochannels of Al-MCM-41 mesoporous materials could be a viable system for a broad range of catalytic oxidation to mimic natural occurring enzymes.
Enzyme cyt c can also be immobilized in the nanochannels of mesoporous silica by electrostatic attraction. The amount of cyt c adsorption could be increased by the introduction of aluminum into the framework of pure silica materials. Among these, MAS-9 showed the highest loading capacity due to its large pore size. However, cyt c immobilized in MAS-9 could undergo facile unfolding during hydrothermal treatments. MCM-41-S and MCM-48-S have the pore sizes that match well the size of cyt c. Hence the adsorbed cyt c in these two medium pore size have the highest hydrothermal stability and overall catalytic activity. On the other hand, the pore size of NaY zeolite is so small that cyt c is mostly adsorbed only on the outer surface and loses its enzymatic activity rapidly. The improved stability and high catalytic activity of cyt c immobilized in mesoporous silica are attributed to the electrostatic attraction between the pore surface and cyt c and the confinement provided by nanochannels. We further observed that cyt c immobilized in mesoporous exists in both high and low spin states. The high spin state arises from the replacement of Met-80 ligands of heme Fe (III) by water or silanol group on silica surface, which could open up the heme groove for easy access of oxidants and substrates to iron center and facilitate the catalytic activity. By the study of the EPR spin trapping experiments, we showed that cyt c catalyzes a homolytic cleavage of the O-O bond of hydroperoxide and generates a protein cation radical. Possible mechanisms for MPS-cyt c catalytic oxidation of hydroperoxides and PAHs are proposed based on the spectroscopic characterizations.
Functionalised mesoporous silica which has its pore size in the range of 30-300 Å has been prepared and used for immobilization of the enzyme cyt c. Thio, carboxyl ester and phosphatyl ester functional groups were attached in the nanochannels of mesoporous silica surface by organo silane co-condensation with silanol groups. Coming from the the coordination of sulfur to Heme Fe(Ⅲ) center, the immobilization of cyt c in the thio-functionalised surface showed low activity . The strong nucleophiles of thio groups will cause the destruction of active site, and Fe(Ⅲ) center will leach from the Heme group.
The immobilization of cyt c in the ester-functionalized mesoporous silica showed the increasing activity of pyrene oxidation. The high activity primarily came from the formation of good covalent bond. The strong covalent bond will prevent cyt c from unfolding in the catalytic process.
The hydrolysis of ester bonds produced the corresponding carboxylic and phosphatic acid. The acidic groups provide the negative charge that could immobilize cyt c by the electrostatic attraction. We further observed that the immobilization of cyt c in the acidic functionalized mesoporous have 3-4 fold increase in activity than aluminosilicate as a support. The high activity comes from the support with organo silane in the surface that could prevent cyt c from unfolding by exposing the protein skeleton in the surface of silica.
|
|---|
