A Pulse EPR and ENDOR Investigation of the Electronic and Geometric Structure of Cobaltous Tetraphenylporphyrin(Pyridine)

Abstract

Different pulse Electron Paramagnetic Resonance (EPR) and Electron Nuclear DOuble Resonance (ENDOR) techniques are used to study the electronic and geometric structure of (tetraphenylporphyrinato)cobaltate(II) with pyridine as an axial ligand (cobaltous tetraphenylporphyrin(pyridine), CoTPP(py)). This complex is considered as a model for the heme group in deoxygenated hemoglobin and myoglobin. For the first time, the small hyperfine and nuclear quadrupole interactions of the porphyrin nitrogens of CoTPP(py) are determined using three-pulse electron spin−echo envelope modulation (ESEEM), hyperfine sublevel correlation (HYSCORE), and double nuclear coherence transfer (DONUT)−HYSCORE spectroscopy as well as Davies-ENDOR, and hyperfine correlated ENDOR (HYEND). The assignment of the different cross-peaks in the HYSCORE spectra is shown to be considerably facilitated by the DONUT−HYSCORE experiment. Furthermore, the hyperfine interactions of the surrounding protons are investigated by Davies- and Mims-ENDOR an...