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

Publisher Website
Google Scholar
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 axillary 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-methylmiidazole) illustrate a trend toward stronger hyperfine interactions with weaker bases.