Calix[6] arene-bismetalloporphyrins as enzyme models for cytochrome P450II. Influence of axail ligand on the epoxidation of cyclohexene and kinetic law of the reaction

The influence of axial ligand on the epoxidation of cyclohexene under the c atalysis of manganito-calix[6] arene-bisporphyrin P450 enzyme model compoun d was investigated. It was found that the stronger the electron donating ab ility of the axial ligand the more effective is the auxocatalysis. The resu lt also shows that the larger the cavity, the higher the catalytic activity . The OH on the calixarene also plays a certain role, Such findings strongl y suggest that there are two parameters that enhance the catalysis, namely electronic factor and the steric interaction of the axial ligand. Kinetic study indicated that with the concentration of catalyst varying wit hin the range of 5.0 x 10(-5) similar to 2.5 x 10(-4)mol.L-1, the rate of e poxidation increased first, then approached a constant value as the concent ration reached 2.0 x 10(-4)mol.L-1, indicating of a less reactive mu-OXOMn( IV) dimer which lower the effective concentration while the reaction order varies from 1 to zero. The optimum concentration is 2.0 x 10(-4)mol.L-1. Va riation of the concentration of the cyclohexene causes the order of epoxida tion changing from 1 to zero, indicating that cyclohexene is involved in th e rate determining step of the catalytic reaction only at low concentration . It was found that the epoxidation under the catalysis of Mn(III)Por. (CI) a bides Michaelis-Menten behavior with, v(max) = 2.58 x 10(-2).mol.L-1.min(-1 ), Michaelis-Menten constant = 0.0279 and K-cat = 206.4min(-1). On the basi s of all the experimantal results, a mechanism was proposed.