Nitric oxide (NO), a simple diatomic free radical, is known to play a critical physiological role in diverse organisms. An iron complex, with N-(dithiocarboxy)sarcosine (Fe-DTCS), has a high affinity for endogenous NO and can trap, stabilize, and accumulate it. The stable NO adduct thus formed is detectable at room temperature with electron paramagnetic resonance (EPR) spectrometry. We report in vivo EPR imaging of endogenous NO, trapped by an Fe-DTCS complex, in the abdomen of a live mouse. To our knowledge, this is the first report on EPR imaging of endogenous free radicals produced in vivo. This EPR imaging method will be useful for the noninvasive investigation of the spatial distribution of NO in pathologic organs or tissues.

This study shows the ESR spectra of oxoiron(IV) porphyrin π-cation radicals of 1−8 in dichloromethane−methanol (5:1) mixture. We reported in a previous paper that oxoiron(IV) porphyrin π-cation radicals of 1−4 are in an a1u radical state while those of 5−8 are in an a2u radical. The ESR spectra (g ⊥ eff ∼ 3.1 and g ∥ eff ∼ 2.0) for the a1u radical complexes, 1−4, appear quite different from those reported previously for the oxoiron(IV) porphyrin π-cation radical of 5 (gy = 4.5, gx = 3.6, and gz = 1.99). The unique ESR spectra of the a1u radical complexes rather resemble those of compound I from Micrococcus lysodeikticus catalase (CAT) and ascorbate peroxidase (ASP). This is the first examples to mimic the ESR spectra of compound I in the enzymes. From spectral analysis based on a spin Hamiltonian containing an exchange interaction, the ESR spectra of 1−4 can be explained as a moderate ferromagnetic state (J/D ∼ 0.3) between ferryl S = 1 and the porphyrin π-cation radical S‘ = 1/2. The magnitudes of zero-field splitting (D) for ferryl iron and isotropic J value, estimated from the temperature-dependence of the half-saturation power of the ESR signals, are ∼28 and ∼+8 cm-1, respectively. A change in the electronegativity of the β-pyrrole substituent hardly changes the ESR spectral feature while that of the meso-substituent slightly does owing to the change in the E/D value. On the basis of the present ESR results, we propose the a1u radical state for compound I of CAT and ASP.

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