Abstract:

The oxidative transformations mediated by heme-containing metalloenzymes, such as peroxidases, cytochrome P450 and nitric oxide synthase (NOS), have attracted sustained attention. In this study, various synthetic iron porphyrins were used as enzyme models to study the reactive intermediates and the mechanism of these enzymatic reactions.

In Chapter 1, an oxoiron(IV) species, (TPFPP)Fe IV =O, as model of compound II of peroxidases and cytochrome P450, was prepared and its reactivity toward epoxidation and hydroxylation was investigated. A competitive oxidation of cyclohexene, cyclooctene and adamantane by (TPFPP)Fe IV =O showed significant difference from the product distribution pattern of (TPFPP +* )Fe IV =O. The kinetic analysis indicated that cyclooctene and adamantane had similar reactivity toward (TPFPP)Fe IV =O, and therefore ruled against the hypothesis that (TPFPP +* )Fe IV =O was the reactive species. Significant solvent and acidity effects in the reaction were not compatible with simple direct reactions between (TPFPP)Fe IV =O and substrate, however, a disproportionation mechanism could explain the results qualitatively.

Chapter 2 and 3 focused on mechanistic study of NOS. In chapter 2, the aerobic oxidation of N ? -hydroxy-L-arginine (NHA) catalyzed by water-soluble iron(III) porphyrins (FeP) was reported. The product distribution was pH-dependent. A novel product, N -nitrosoarginine, was found at pH 7.4, 9.2 and 11. A new species was observed on UV-Vis upon mixing NHA and FeP at basic pH and was tentatively assigned to NHA-Fe(III)porphyrin complex. The logarithm of rate constants at pH 7.4 were linearly related to the redox potentials for the M(III)/M(II) couple, which suggests that the rate-determining step is the one-electron oxidation of NHA by the metal. It was then proposed that this NHA oxidation is initiated by a homolytic cleavage of the Fe-O bond to form the NHA iminoxyl radical and ferrous porphyrin. In Chapter 3, a series of NHA analogs bearing norcaranyl or (1-methyl cyclopropyl)methyl group were designed as diagnostic probes for NOS reaction. Two probes and several important synthetic precursors toward the diagnostic probes were prepared.

In Chapter 4, we described chiral recognition of amino acids in water by synthetic chiral porphyrin, ????-Zn-2-TMBzPyP. The results showed chiral selectivity favoring D or L isomer by a factor of 2-3. The structural basis of the observed selectivity was discussed.