ELECTRON PARAMAGNETIC RESONANCE OF NITRIC OXIDE-PROTOHEME COMPLEXES WITH SOME NITROGENOUS BASE . MODEL SYSTEMS OF NITRIC OXIDE HEMOPROTEINS

In order to help interpret the electron paramagnetic resonance spectra of the nitric oxide complex of ferrohemoglobin and ferrocytochrome c, more than 20 model systems were studied which consist of NO, heme (Fe2+), and some nitrogenous base which coordinates the heme Fe. The observed paramagnetic resonance spectra were grouped into four types according to their shape. An assignment of the absorption peaks was proposed by which the relation between the four spectral types can be explained and the interpretation of the resonance spectra of NO-ferrohemoglobin and NO-ferrocytochrome c became possible. The 14NO hyperfine splitting in each is 26.8 and 23.7 gauss, respectively, along the heme normal. Also, the hyperfine splittings of 6.4-6.S gauss which are due to the 14N in the imidazole ring of the proximal histidine residue are now resolved. Evidence for the strong interaction between the base π and the Fe dπ. electrons was presented in these hemoproteins. At least 50% delocalization of the unpaired electron to the Fe dz2 orbital was demonstrated by 57Fe experiments with two of the model systems. © 1969, American Chemical Society. All rights reserved.