Models of Nitric Oxide Synthase: Iron(III) Porphyrin-Catalyzed Oxidation of Fluorenone Oxime to Nitric Oxide and Fluorenone

Nitric oxide synthase (NOS) is a heme-containing monoxygenase that catalyzes the oxidation of
l-arginine to l-citrulline and NO in two steps. In the second step of the NOS reaction, citrulline and NO are
generated from the heme-catalyzed 3-electron oxidation of l-N-hydroxyarginine. To model this unusual reaction,
iron porphyrin-catalyzed oxygenations of oximes with O2 were investigated. The oxidation of fluorenone oxime
and a stoichiometric amount of hydroxoiron(III) porphyrin (Fe(OH)P, P = TMP and TPFPP) with O2 in benzene
generated Fe(NO)P, fluorenone, and O-(9-nitro-9-fluorenyl)fluorenone oxime. The X-ray crystal structure of
the oxime ether product suggests that it originated from the dimerization of the fluorenyl iminoxy radicals.
Detailed analysis of this reaction showed that the oxime reacted first with Fe(OH)P to generate a 5-coordinate,
high-spin oximatoiron(III) porphyrin species [Fe(oximate)P]. The X-ray crystal structure of oximatoiron(III)
tetrakis(2,6-dichlorophenyl)porphyrin [Fe(oximate)TDCPP] showed that the oximate ligand was monodentate,
O-bound to Fe(III)P. The aerobic oxidation of Fe(oximate)P followed the characteristic kinetics of a
metalloporphyrin-catalyzed radical-type autoxidation. O2 surrogates, the π-acids NO and CO, induced the
homolysis of Fe(oximate)P to generate Fe(NO)P or Fe(CO)P and the iminoxy radical, implicating a similar
reaction mode for O2 with Fe(oximate)P. Fe(oximate)TMP reacted with 18O2 to generate predominantly 18O-labeled fluorenone (75% yield), while the reaction conducted under 16O2 and H218O generated only 16O-labeled
fluorenone. This reaction is proposed to proceed via an Fe−O bond homolysis of Fe(oximate)TMP followed
by O2 insertion to generate 9-nitroso-9-fluorenylperoxyFe(III)TMP, which decomposes via an O−O bond
homolysis to generate NO, fluorenone, and oxoFe(IV)P. The implications of this system for the NOS reaction
mechanism are discussed.