Electron-nuclear coupling in nitrosyl heme proteins and in nitrosyl ferrous and oxy cobaltous tetraphenylporphyrin complexes

Abstract

Electron spin echo envelope modulation (ESEEM) spectroscopy has been used to study electron-nuclear interactions in the following isoelectronic S = 1/2 complexes: NO-FeII(TPP) (TPP = tetraphenylporphyrin) with and without axial nitrogenous base, nitrosylhemoglobin in R and T states, and O2-CoII(TPP) with and without axial base. Only the porphyrin pyrrole nitrogens contribute to the ESEEM of the 6-coordinate nitrosyl FeII(TPP) complexes, nitrosylhemoglobin (R-state), and the nitrosyl complexes of alpha and beta chains. Pyrrole nitrogens in the 5-coordinate complex NO-FeII(TPP) are coupled too weakly to unpaired spin and therefore do not contribute to the ESEEM. A partially saturated T-state nitrosylhemoglobin does not exhibit echo envelope modulations characteristic of 6-coordinate nitrosyl species, which confirms that the proximal imidazole bond to heme iron is disrupted. Study of 6-coordinate O2-CoII(TPP)(L) complexes (L = nitrogenous base) using 14N- and 15N-labeled ligands and porphyrins enabled a detailed analysis of coupling parameters for both pyrrole and axial nitrogens. The pyrrole 14N coupling frequencies are similar to those in NO-FeII(TPP)(L). The Fermi contact couplings for axially bound nitrogen, calculated from simulation of ESEEM spectra for a series of O2-CoII(TPP)(L) complexes (L = pyridine, 4-picoline, 4-cyanopyridine, 4-carboxypyridine, and 1-, 2-, and 4-methylimidazole) illustrate a trend toward stronger hyperfine interactions with weaker bases.

Hydrogen bonding to the bound dioxygen in oxy cobaltous myoglobin reduces the superhyperfine coupling to the proximal histidine

“…(Poorly-resolved spectral components arising from coupling to pyrrole 14N were also identified.) The ESEEM spectral features of oxyCoMb, however, appear at lower frequencies than those of the model compounds recorded at similar magnetic field settings, suggesting smaller hyperfine and/or nuclear quadrupole coupling to the directly coordinated 14N of the proximal histidyl imidazole (Magliozzo & Peisach, 1992). A computer simulation (discussed in detail below) of the spectrum yielded an isotropic hyperfine coupling (4,0) of 2.46 MHz (Figure IB).…”

“…The ESEEM spectrum of oxyCoMb is analogous to the spectra of hexacoordinated oxyCoTPP model compounds containing substituted imidazole as an axial ligand (Magliozzo et al, 1987). The study of models containing pyridine or imidazole demonstrated that the spectral components from axial bases arose from the coupling to the directly coordinated 14N.…”

“…This method has been used to study electron-nuclear coupling in cupric proteins McCracken et al, 1987McCracken et al, ,1988, in high-and low-spin ferric hemoproteins (Peisach et al, 1979(Peisach et al, , 1984; Magliozzo & Peisach, 1992), and in low-spin ferrous nitrosylhemoglobin (Magliozzo et al, 1987), as well as in other paramagnetic biomolecules (Mims & Peisach, 1989). ESEEM spectroscopy of proteins in D2O provides a rapid and direct method for demonstrating the presence of exchangeable deuterons in the vicinity of the paramagnetic center, such as that in oxyCoMb, and for investigating the magnetic coupling between specific protons/deuterons and the electron spin (Mims et al, 1977(Mims et al, , 1984Peisach et al, 1984;McCracken et al, 1987).…”

“…Some background for the application of ESEEM spectroscopy to the current investigation comes from ESEEM studies of oxy cobaltous tetraphenylporphyrin (CoTPP) complexes (Magliozzo et al, 1987) in which the coupling to the directly coordinated axial nitrogens and to the pyrrole nitrogens was investigated. Computer simulations of spectra were used to calculate the ligand hyperfine and nuclear quadrupole interaction (NQI) parameters, and a similar approach is used here.…”

Nitric oxide interaction with insect nitrophorins and thoughts on the electron configuration of the FeNO complex

“…Spectroscopic techniques that have been utilized to characterize nitrosylheme proteins include magnetic circular dichroism (MCD) [168,169], infrared (IR) [134,[170][171][172][173][174], resonance Raman (RR) [43,[175][176][177][178][179][180], Mössbauer [181][182][183][184][185], and EPR spectroscopies [61,134,150,[186][187][188][189][190][191][192][193][194][195][196][197][198][199][200]. MCD has been used to characterize the protein-provided axial ligand bound to both Fe(II) and Fe(III) forms of various heme proteins, using the NO, CO, phosphine, and other adducts to develop MCD spectra that are unique for the fifth (protein-provided) ligand [168,169].…”