Effect of spin configuration on the reactivity of cytochrome c immobilized in mesoporous silica

Cytochrome c (cytc), a heme protein with a positive electric charge, is immobilized in the nanochannels of mesoporous silica (MPS) by either electrostatic forces or covalent bonding. The electrostatic interaction between cytc and MPS arises from the introduction of aluminum into the framework of MPS to produce a negative charge on the porous surface (Al-MPS). The covalent bonding arises from the binding between the heme iron center and the -SH group of mercaptotriethoxysilane in thiol-functionalized MPS (MPS-SH). The nanochannels of MPS provide a confining space that can prevent cytc from protein unfolding and preserve its activity. Cytc immobilized in Al-MPS exists in a high-spin state, as inferred from ESR and UV-Vis studies. This is different from native cytc, which shows primarily the low-spin state. The high-spin state arises from the replacement of the Met-80 ligands of heme Fe(III) by water or silanol groups on the silica surface, which can open up the heme groove for ready access of oxidants to the iron center and facilitate catalytic activity. MPS-SH-supported cytc can exist in both high- and low-spin states. The low-spin state arises from the replacement of the axial ligands of heme Fe(III) by the -SH group, which can result in poisoning of the active site and reduce the catalytic activity with respect to the decomposition of hydroperoxides. In ESR spin trapping experiments, we show that cytc catalyzes homolytic cleavage of the O-O bond of hydroperoxide and generates a protein cation radical ( g =2.00). A possible mechanism for the MPS-cytc catalytic oxidation of hydroperoxide is proposed based on a spectroscopic characterization of the system.