Continuous Wave and Pulse EPR and ENDOR Study of Oxygenated Cobalt(II) Heme Model Systems

Abstract

Continuous wave (CW) and pulse electron paramagnetic resonance (EPR), and electron nuclear double resonance (ENDOR) techniques were used to study frozen solutions of (oxyCo)TPP(L) (TPP = tetraphenylporphyrin, L = pyridine, 1-methylimidazole). By using a combination of CW EPR at X-, Q-, and W-band and Davies-ENDOR at Q-band, the g and cobalt hyperfine matrices and their corresponding principal axes were, for the first time, determined in detail. The variation of the g values as a function of temperature was followed by Q-band CW EPR. The existing ambiguity about the assignment of one of the g principal axes to the O−O bound direction was analyzed in detail. By studying the different proton interactions with Mims-ENDOR at X-band, a considerable solvent effect was found. With hyperfine sublevel correlation (HYSCORE) at X-band and two-pulse ESEEM (electron spin-echo envelope modulation) at S-band, the hyperfine and nuclear quadrupole interactions of the nitrogen nuclei of the axial ligand and the TPP ligand w...