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 (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)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 (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 (Nax) bond of oxyCoTPP-1-MeIm shortens by 0.18 ± 0.06 Å when the solvent is changed from 100% toluene (Co−Nax = 2.12 Å) to 50% toluene/50% dichloromethane (Co−Nax = 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-NHCONHC6H5)1TPP 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.

A Continuous Wave and Pulse EPR and ENDOR Investigation of Oxygenated Co(II) Corrin Complexes

“…This is illustrated here, where [Cob(II)ester‚O 2 ] + acts as the Lewis acid. Comparison with our previous data for the base-on Co(II) porphyrin complexes ( 51 showed a relation between |e 2 qQ/h| and the Co-N distance. For the same nitrogen base, a lower value of |e 2 qQ/h| of the directly binding nitrogen corresponds to a shorter Co-N distance.…”

Effects of the Dendrimer Cage on O2 Binding of Dendritic Cobalt(II) Porphyrins

“…Indeed, the present studies show that the g values, the cobalt hyperfine interaction, and the hyperfine and nuclear quadrupole couplings of the imino nitrogen of 1,2-DiMeIm differ for [1 ¥ Co II ¥ 1,2-DiMeIm] ± O 2 and [2 ¥ Co II ¥ 1,2-DiMeIm] ± O 2 . Comparisons with earlier studies [15,20] indicate that the CoÀN ax distance is shortest in [1 ¥ Co II ¥ 1,2-DiMeIm] ± O 2 . A decrease in the CoÀN ax distance has been related to a higher ionicity in the cobalt ± dioxygen bond and a larger O 2 affinity.…”

“…Subsequently a surplus of a nitrogen base was added to form the base-on complexes. The used axial nitrogen bases were pyridine (py), 1-methyl imidazole (1-MeIm), and 1,2-dimethylimidazole (1,2-DiMeIm) (Fluka analysis), 15 (> 98 %, Cambridge Isotope Laboratories), and 15 N-labeled 1,2-dimethylimidazole (1-Me[ 15 N] 2 Im). The latter was obtained in 73 % yield (17 mg) by stirring a solution of 15 N-labeled imidazole ([ 15 N] 2 Im, Cambridge Isotope Laboratories; 20 mg, 0.28 mmol), NaH (10.8 mg, 0.29 mmol), and Me 2 SO 4 (28 mL, 0.3 mmol) in THF (2 mL) at 20 8C for 3 h, followed by evaporation in vacuo and chromatography (SiO 2 , CH 2 Cl 2 :MeOH 100:6).…”

“…In order to differentiate between the two hypotheses, control experiments were performed with 15 Figure 4 c. The signals overlap with the signals of the porphyrin nitrogen atoms. The observation that the 15 N signals are so weak agrees with earlier findings [17] and results from the fact that the observed hyperfine interaction is largely isotropic.…”

“…A combined three-pulse ESEEM (electron spin envelope modulation) and EXAFS (extended X-ray absorption fine structure) spectroscopic study of [(o-R)TPP ¥ Co II ¥ 1-MeIm] ± O 2 (where R -H, -NHCOC(CH 3 ) 3 , -NHCOCH 3 , -NHCONHC 6 H 5 , an orthosubstitutent on one of the four meso-phenyl rings of TPP) in different solvents [15] showed a relation between j e 2 qQ=hj and the CoÀN distance. For the same nitrogen base, a lower value of j e 2 qQ=h j for the directly binding nitrogen corresponded to a shorter CoÀN distance and larger ionicity of the cobalt ± dioxygen bond.…”