EPR studies on understanding the intricacy of HbNO complexes

Hemoglobin is a representative for proteins of quaternary structure and allosteric regulation. In reaction with natural metabolite, nitric oxide forms the paramagnetic complex, nitrosyl hemoglobin (HbNO). Electron paramagnetic resonance is the method of choice to investigate nitrosyl species in biological systems. The interpretation of HbNO EPR spectra belongs to the biggest challenges in biologically oriented EPR spectroscopy. The recorded EPR spectrum is sensitive to geometric and electronic structure of the essential moiety, heme-NO unit. The composite character of the HbNO spectrum is apparent. The contributions from the \alpha and \beta subunits of the tetramer, as well as the two possible heme coordination states, are recognized. The magnetic signatures of these structural variants are determined from EPR signals. The chapter presents the intuitive explanation of the basic EPR parameters, g- and A-tensors, as the structural fingerprints of HbNO. The overview of the temperature and pH-dependent effects on the spectral shape is given. The application of EPR as a tool for quantitation of different HbNO levels in biological samples is also discussed.