An electron spin echo envelope modulation (ESEEM) study of nitric oxide synthase model compounds: Nitrosyl tetraphenylporphyrinato iron(II) and nitrosyl pyridyl tetraphenylporphyrinato iron(II)

Nitric oxide (NO) has been shown to be of vital physiological importance in recent years. It is synthesized in vivo by heme containing enzymes known as nitric oxide synthases (NOS). While the mechanism has yet to be fully understood, it is generally accepted that the heme center in NOS binds to O2 before reacting with substrate to produce NO, making nitrosylated iron porphyrins good models for these enzymes.;Electron paramagnetic resonance (EPR) would seem to be an ideal technique to examine these systems, however the EPR spectrum is dominated by the axial ligands with limited information about the pyrrole nitrogen interactions with the iron available. ESEEM, with its selectivity for small nitrogen hyperfine couplings, is therefore the ideal tool to examine these models. The ESEEM spectra of these systems with the natural isotopic abundance of nitrogen leads to a complicated spectrum with nine possible features for each pyrrole nitrogen. A building block approach to this problem was therefore undertaken.;The first step in these studies was to examine the six coordinate nitrosyl pyridyl tetraphenylporphyinato iron(II) species with 15N isotopic substitution at the pyrrole positions. This simplified the spectrum to four possible features. This analysis showed three magnetically inequivalent nitrogen nuclei present. A point dipole analysis finds the dihedral angle between O-N NO-Fe and NNO-Fe-NTPP to be ∼37° with the spin density primarily in the iron dz 2 orbital.;The five coordinate nitrosyl tetraphenylporphyinato iron(II) system with the natural isotopic abundance was then examined. In this case the ESEEM spectra show two sets of magnetically equivalent nitrogen nuclei. A more complete dipolar analysis shows that not only must the iron 3dz 2 orbital be included in the hyperfine interaction, but also the nitrogen 2p orbitals. The N 2pz quantization axis points at the iron with negative spin density on the 2p y and positive spin density on the 2pz orbitals. The positive spin density on the 2pz orbital is consistent with a partially covalent interaction between the pyrrole nitrogen and the iron.;Finally, the viability of the ESEEM technique to examine the six coordinate model with the natural isotopic abundance and the NOS systems is examined.