Resonance Raman studies of dioxygen and carbon monoxide binding to imidazole-appended hemes

Abstract

Resonance Raman spectroscopy has been employed to probe the effects of proximal base strain on the bonding of O2 and CO in three synthetic hemins with covalently linked imidazole ligands. The strain is introduced by varying the length of the imidazole-containing side chain and by restricting the side chain flexibility with a phenyl ring. These hemins are abbreviated as "long," "short," and "stiff" hemins, respectively. In the deoxy state, the iron-imidazole stretching frequencies [nu(Fe--N epsilon)] for long, short, and stiff hemins are detected at 200, 207, and 204 cm-1, respectively. The strain induced in the iron-imidazole bond by the short hemin results in a higher nu(Fe--N epsilon) frequency, in contrast to the strain induced by sterically hindered 2-methylimidazole or 1,2-dimethylimidazole complexes in which the Fe--N epsilon bond is tilted and lengthened, but the imidazole ring remains perpendicular to the heme plane. However, in the short hemin, the plane of the imidazole ring may not be perpendicular to the plane of the porphyrin, altering the amount of pi-interaction (hence the strength of Fe--N epsilon bond) and the nature of normal mode containing Fe--N epsilon bond stretching. Upon CO binding, we have observed the nu(Fe--CO) stretching frequencies at 497 (long), 499 (short), and 496 cm-1 (stiff), somewhat lower than those reported by Mitchell et al. (Inorg. Chem., 1985, 24:967) for the chelated-heme X CO complexes (i.e., 501-506 cm-1). This is the first report of an iron-oxygen-associated vibration observed in solution for an unprotected heme. The oxy complexes were formed by introducing dioxygen to the deoxy complexes at -700C. The isotope-sensitive line was detected at 576 cm- (1602) in oxy stiff hemin, which was shifted to 545 cm-1 upon 1802 substitution. This is perhaps the largest isotope shift (31 cm-') observed to date, compared with the usual 22-24 cm-'.For the long and short hemins, the iron-oxygen-associated vibration was detected at 574 and 573 cm-', respectively.These values are very similar to those observed(N-methylimidazole) and myoglobin/hemoglobin.

Effect of Distal Interactions on O2 Binding to Heme

“…For comparison, the Fe–O vibration of unprotected oxyheme produced from hemins was estimated at 573–574 cm –1 , with a characteristic iron–porphyrin band occurring also at 396 cm –1 , consistent with picket-fence [Fe(TpivPP)(1-methylimidazolyl)O 2 ] showing an Fe–O band at 571 cm –1 and two bands near 400 cm –1 . The O–O stretch ν(O–O) has been estimated at 1159 cm –1 from picket-fence [Fe(TpivPP)(1-methylimidazolyl)O 2 ] and 1103 cm –1 for oxyMb, very similar to free superoxide (1100–1150 cm –1 ) .…”

Synthetic Heme-Dioxygen Complexes

“…Similarly, an analysis of imidazole-appended heme spectra showed an insensitivity of v(Fe-O) to a constraint introduced into the length of the chain bearing the imidazole. 332 The electron-donating capabilities of the substituents of the porphyrin ring exert a significant effect on the O2 binding constant.224 However, this thermodynamic trend is not manifest in measurable changes in the ironoxygen bond strength.321 '324'327-329 Thus, the relationship between the binding affinity and the iron-ligand stretching frequency is not necessarily straightforward. allow some suggestions to be made about the factors which affect ligand binding.262 '263 Relatively few 7H NMR studies have concentrated on the oxy forms of hemoproteins because of the limited lifetime of oxygenated hemoproteins and the complications that arise from paramagnetic impurities.260 Oxygenated heme complexes are perhaps somewhat better targets for NMR study because even though they can be less stable than oxygenated hemoproteins at room temperature, their organic solvent solubility allows measurement at stabilizing low temperatures.…”