to stand for minutes to hours, then deoxygenated by sparging with argon. Samples were withdrawn just before or just after deoxygenation for product characterization. Hb(NO)4 oxidation was conducted in 2 mM solutions with an excess of potassium ferricyanide; the formation of products was followed by UV VIS spectroscopy. Samples were cleansed of low molecular weight species by centrifugation through a G-25 column, then analyzed for products.