Kinetics and mechanisms of oxidation of bemoprotein model compounds

+(Cf. C. A. 83:202905y) Under 1 atm O2 at -45degC in methylene chloride (MC) the monoimidazole heme (I) forms an oxygen complex which is stable against oxidation for several hours, but at pO2 down to 2 Torr the oxidation half time is about 16 min for 4.4x10-5 M heme. The oxidation is slower in dimethylformamide (DMF) and second-order rate constants were derived spectrophotometrically. The reaction rate is inversely proportional to pO2 350 Torr, and is expected to go through a maximum at about 0.5 Torr. Comp. I, oxygenated at -45 deg C is oxidized in about 10 min in a 60% MC, 35% methanol and 5% H2O. In the DMF soln. it showed no appreciable oxidation in 5 min at 0 deg C, and addn. of 20% H2O did not accelerate the autoxidn. significantly. The oxidn. is retarded by solvents as DMF or DMF-H2O. The dipyridine heme compd.(II) is not oxygenated in benzene, but is oxidized readily, the reaction being accurately second-order in heme and is accelerated by increasing oxygen pressure. Because the rates of autooxidn. of oxyheme complexes were inversely proportional to oxygen pressure and proportional to the square of the total heme concn., it was concluded that the reaction proceeds through heme-OO-heme as suggested by Cohen and Caughey (cf.C.A. 68: c). By using high pressures (1 atm) of CO and O2 it should be possible to study the equilibrium oxygenation of simple hemes in soln. Several reaction mechanisms for different species concns. in various solvents are discussed and related to the hemoprotein autooxidn. The direct reaction of dithionite ion with oxyheme complexes accords with the Fe+O2-H formulation of the iron-oxygen bond