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 catalyze s the oxidation of L-arginine to L-citrulline and NO in two steps. In the s econd step of the NOS reaction, citrulline and NO are generated from the he me-catalyzed 3-electron oxidation of L-N-hydroxyarginine. To model this unu sual reaction, iron porphyrin-catalyzed oxygenations of oximes with O-2 wer e investigated. The oxidation of fluorenone oxime and a stoichiometric amou nt of hydroxoiron(III) porphyrin (Fe(OH)P, P = TMP and TPFPP) with Oz in be nzene 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. Det ailed analysis of this reaction showed that the oxime reacted first with Fe (OH)P to generate a 5-coordinate, high-spin oximatoiron(III) porphyrin spec ies [Fe(oximate)P]. The X-ray crystal structure of oximatoiron(III) tetraki s(2,6-dichlorophenyl)porphyrin [Fe(oximate)TDCPP] showed that the oximate l igand was monodentate, O-bound to Fe(III)P. The aerobic oxidation of Fe(oxi mate)P followed the characteristic kinetics of a metalloporphyrin-catalyzed radical-type autoxidation. O-2 surrogates, the pi-acids NO and CO,induced the homolysis of Fe(oximate)P to generate Fe(NO)P or Fe(CO)P and the iminox y radical, implicating a similar reaction mode for O-2 with Fe(oximate)P. F e(oximate)TMP reacted with O-18(2) to generate predominantly O-18-labeled f luorenone (75% yield), while the reaction conducted under O-16(2) and (H2O) -O-18 generated only O-16-labeled fluorenone; This reaction is proposed to proceed via an Fe-O bond homolysis of Fe(oximate)TMP followed by O-2 insert ion to generate 9-nitsoso-9-fluorenylperoxyFe(III)TMP, which decomposes via an O-O bond homolysis to generate NO, fluorenone, and oxoFe(IV)P. The impl ications of this system for the NOS reaction mechanism are discussed.