Electron Spin Echo Envelope Modulation and Extended X-ray Absorption Fine Structure Studies of Active Site Models of Oxygenated Cobalt-Substituted Hemoproteins: Correlating Electron-Nuclear Couplings and Metal−Ligand Bond Lengths

Abstract

Electron spin echo envelope modulation (ESEEM) and extended X-ray absorption fine structure (EXAFS) spectroscopic studies of oxygenated cobalt (oxyCo) [tetraphenylporphyrin(TPP)][1-methylimidazole (1-MeIm)], an active site model of oxyCo-substituted globins (functional and EPR-active [S = 1/2] analogues of oxygen carrying hemoproteins), are carried out in order to examine the correlation of oxygen affinity with electron-nuclear coupling parameters and metal−ligand bond lengths. ESEEM demonstrates that the magnitude of the electron-nuclear hyperfine and nuclear quadrupole couplings to the directly-coordinated 14N of 1-MeIm decrease (Aiso, from 3.54 to 3.04 MHz; e2qQ, from 2.39 to 2.08 MHz) as the solvent compositon is varied from 0 to 50% (v/v) dichloromethane in toluene. For oxyCo[(o-R)1TPP][1-MeIm] (where R = −H, −NHCOC(CH3)3, −NHCOCH3, or −NHCONHC6H5, an ortho substitutent on one of the four meso phenyls of TPP), couplings to the axial nitrogen decrease (Aiso, from 3.54 to 3.07 MHz; e2qQ, from 2.39 to 2...