| Abstract
| - Hydrothermally stable and structrurally ordered mesoporous and microporous aluminosilicates with differentpore sizes have been synthesized to immobilize cytochrome c (cyt c): MAS-9 (pore size 90 Å), MCM-48-S(27 Å), MCM-41-S (25 Å), and Y zeolites (7.4 Å). The amount of cyt c adsorption could be increased by theintroduction of aluminum into the framework of pure silica materials. Among these mesoprous silicas (MPS),MAS-9 showed the highest loading capacity due to its large pore size. However, cyt c immobilized in MAS-9could undergo facile unfolding during hydrothermal treatments. MCM-41-S and MCM-48-S have the poresizes that match well the size of cyt c (25 × 25 × 37 Å). Hence the adsorbed cyt c in these two medium poresize MPS have the highest hydrothermal stability and overall catalytic activity. On the other hand, the poresize of NaY zeolite is so small that cyt c is mostly adsorbed only on the outer surface and loses its enzymaticactivity rapidly. The improved stability and high catalytic activity of cyt c immobilized in MPS are attributedto the electrostatic attraction between the pore surface and cyt c and the confinement provided by nanochannels.We further observed that cyt c immobilized in MPS exists in both high and low spin states, as inferred fromthe ESR and UV−vis studies. This is different from the native cyt c, which shows primarily the low spinstate. The high spin state arises from the replacement of Met-80 ligands of heme Fe (III) by water or silanolgroup on silica surface, which could open up the heme groove for easy access of oxidants and substrates toiron center and facilitate the catalytic activity. In the catalytic study, MAS-9-cyt c showed the highest specificactivity toward the oxidation of polycyclic aromatic hydrocarbons (PAHs), which arises from the fast masstransfer rate of reaction substrate due to its large pore size. For pinacyanol (a hydrophilic substrate), MCM-41-S-cyt c and MCM-48-S-cyt c showed higher specific activity than NaY-cyt c and MAS-9-cyt c. The resultindicated that cyt c embedded in the channels of MCM-41-S and MCM-48-S was protected against unfoldingand loss of activity. By increasing the concentration of the spin trapping agent, 5,5-dimethyl-1-pyrrolineN-oxide (DMPO) in ESR experiments, we showed that cyt c catalyzes a homolytic cleavage of the O−Obond of hydroperoxide and generates a protein cation radical (g = 2.00). Possible mechanisms for MPS-cytc catalytic oxidation of hydroperoxides and PAHs are proposed based on the spectroscopic characterizationsof the systems.
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