Identification of Protein-Bound Dinitrosyl Iron Complexes by Nuclear Resonance Vibrational Spectroscopy

Among various sites of nitric oxide reactivity in biology,1 iron-sulfur clusters have emerged as an important class of metallocofactors targeted by NO.2 Proteins containing these clusters perform a diverse array of functions from electron transfer to small molecule sensing and substrate binding.3 Consequently, iron-sulfur clusters are potential sites for both NO-derived physiological signal transduction4 and toxicity.2a,5 The primary products of NO reactivity at iron-sulfur clusters are dinitrosyl iron complexes (DNICs).6 Identification of DNICs in proteins and tissue samples has traditionally relied upon the diagnostic g) 2.03 EPR signal,7 which is characteristic of these S) 1/2 species. Because iron dinitrosyl complexes can exist in forms other than the paramagnetic mononuclear species (Chart 1), however, EPR characterization is not definitive.8 Other means of characterization include 57Fe Mössbauer,9 UV-vis,10 Raman,11 an