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

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 14 N of 1-MeIm decrease ( A iso, from 3.54 to 3.04 MHz; e 2 qQ, 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) 1 TPP][1-MeIm] (where R = −H, −NHCOC(CH 3 ) 3, −NHCOCH 3, or −NHCONHC 6 H 5, an ortho substitutent on one of the four meso phenyls of TPP), couplings to the axial nitrogen decrease ( A iso, from 3.54 to 3.07 MHz; e 2 qQ, from 2.39 to 2.09 MHz) with increased electron-withdrawing strength of R, i.e., with increased acidity of the amide proton of R that may interact with the bound dioxygen. EXAFS measurements, and analysis using ab initio EXAFS codes and global mapping, find that the cobalt−axial nitrogen (N ax ) bond of oxyCoTPP-1-MeIm shortens by 0.18 ± 0.06 Å when the solvent is changed from 100% toluene (Co−N ax = 2.12 Å) to 50% toluene/50% dichloromethane (Co−N ax = 1.94 Å). The average cobalt−equatorial nitrogens (1.94−1.96 Å) and cobalt−oxygen (1.95−1.98 Å) distances are unchanged within the error. Similar results were obtained when oxyCoTPP-1-MeIm was compared (in 100% toluene) with its ( o -NHCONHC 6 H 5 ) 1 TPP counterpart, where the cobalt−ligand bond lengths are indistinguishable from those of oxyCoTPP-1-MeIm in 50% toluene/50% dichloromethane. Increasing the polarity of the solvent and of the vicinity of the bound dioxygen increases oxygen affinity of the metal due to an increase in the ionicity of the cobalt−dioxygen bond that is manifested in reduction in electron-nuclear couplings to the axial nitrogen [Lee et al. Biochemistry 1994, 33, 7609] and shortening of the cobalt−axial nitrogen bond. These ESEEM and EXAFS characterizations of metal−ligand interactions demonstrate the correlation of electron-nuclear coupling and metal−ligand bond lengths with oxygen affinity of hemoprotein model complexes.