EPR Studies of the Dynamics of Rotation of Dioxygen in Model Cobalt(II) Hemes and Cobalt-Containing Hybrid Hemoglobins

Abstract

Earlier we showed that the shapes of the EPR spectra of cobalt(II) porphyrinate(nitrogen base)(dioxygen) complexes in fluid solution were sensitive to the rate of rotation about the Co−O bond (Walker, F. A.; Bowen, J. H. J. Am. Chem. Soc. 1985, 107, 7632). We have now extended these studies to four metal-substituted hybrid hemoglobins in an attempt to determine whether EPR spectroscopy is sensitive to differences in the mobility of dioxygen in the α and β subunits of the T and R quaternary states. For purposes of this study, [α2(CoO2)β2(FeO2)] and [α2(FeO2)β2(CoO2)] were used as R-state models and [α2(CoO2)β2(Zn)] and [α2(Zn)β2(CoO2)] were used as T-state models. EPR spectra were recorded for samples of each of the above hybrids, equilibrated with 1 atm of O2 gas, as a function of temperature. The “progress toward averaging” of the EPR signals of the Co−O2-containing subunits was measured as the difference in field positions, ΔH, for the midpoint of the low- and high-field extrema of the derivative EPR spectra. A plot of ΔH vs temperature for each hybrid shows that the [α2(Zn)β2(CoO2)] hybrid is unique in averaging more slowly than the other three (all of which behave similarly), indicating more restricted rotation of dioxygen in T-state β-chain pockets than in the heme distal O2-binding pockets of any other form. This finding is consistent with X-ray crystallographic data which show that valine E11 on the distal side of the T-state β-chain pocket partially blocks the dioxygen binding site (Perutz, M. F.; Fermi, G.; Luisi, B.; Shaanan, B.; Liddington, R. C. Acc. Chem. Res. 1987, 20, 309). Simulation of EPR spectra as a function of jump time provides semiquantitative estimates of the rate of dioxygen rotation in these mixed-metal hemoglobin−dioxygen samples; these rates are in the 1 × 108 s-1 range for three of the hybrids at 35−37 °C, and about one-third that value for T-state β(CoO2) centers. These results provide new insight into the highly dynamic nature of dioxygen bound to the metal centers of hemoglobin at physiological temperatures.

Probing the surrounding of a cobalt(II) porphyrin and its superoxo complex by EPR techniques

“…The g and 59 Co hf-tensor values of the Co(III)OEP • py • Oz' complex and its deoxy precursor have been obtained from the spectra (see Table 4). The values are in good agreement with those found by van Doorslaer and Schweiger [10] and also Bowen et al [11] for the oxygenated CoTPP complex, who also pointed out temperature-dependent changes of the g anisotropy due to oxygen rotation at liquid-nitrogen temperature (100 -200 K).…”

“…From this information one can, in principle, obtain the location of the nuclei around the spin-carrying metal center (distance greater than 0.2 nm) [34,38] by use of the point-dipole approximation as demonstrated earlier for axially ligated Cu(II) and Co(II) complexes [39][40][41][42][43]. More recently cw and pulsed multifrequency EPR and ENDOR studies of Co(II) tetraphenylporphyrin complexes (CoTPP) with imidazol and pyridine as axial ligands as well as their superoxo complexes have been published [8][9][10][11]. In this paper we use multinuclear (cw) ENDOR performed on Co(II) octaethylporphyrin (CoOEP) with various ligands as a model (Fig.…”

“…Studies of square-planar cobalt(II) complexes have been extensively reported in the literature [1][2][3][4][5][6][7][8][9][10][11] both for their interesting prospects in metal catalysis and as model systems for biologically active species, e.g., vitamin B 12 [12][13][14][15][16]. Probably the most outstanding property of many cobalt(II) complexes is their ability to form adducts with molecular oxygen [17,18], which has many implications in chemistry and biology, including catalytic oxidation processes [19][20][21][22][23][24][25] and the uptake, transport, storage, and activation of oxygen by heme proteins [26][27][28].…”

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

“…The EPR measurements show however that this complex is also already fully oxygenated at 240 K in contrast to earlier observations for ™simple∫ Co II porphyrins. [25,27,28] Finally, the present study clearly shows that high O 2 affinity and the presence of functional groups capable of H-bond donation in synthetic iron(II) and cobalt(II) porphyrins does not guarantee the stabilization of dioxygen by an H-bond. It forms a challenge for metalloporphyrin synthetic chemistry to design new classes of iron(II) porphyrins where the H-bond is ™forced∫ upon the dioxygen fragment by steric constraints to truly mimic the heme in its natural protein environment.…”

“…Earlier studies on toluene solutions of oxygenated cobalt(II) complexes showed that the EPR spectrum changes continuously with increasing temperature from 140 to 240 K. [27,28] This reflects the increased motion of both the dioxygen fragment and the entire molecule, which averages the anisotropic part of the spectrum. In the limit of fast isotropic motion, the EPR spectrum consists of eight narrow lines, centered around g iso ( 1…”