Spin-label study of hemoglobin conformations in solution.

There is now a great deal of physical and chemical evidence that horse and human hemoglobins undergo a change of protein conformation on oxygenation.' 9 The molecular mechanism whereby these hemoglobins show cooperative or sigmoidal oxygen binding has remained obscure, however. In order to understand this "heme-heme" interaction, one must obtain structural information on partially oxygenated hemoglobin molecules in solution. The present paper is a preliminary report of our study of oxygen-linked conformational changes in hemoglobin, using the spin-label technique. We also discuss the relation between conformational changes and allosteric interactions such as the cooperative binding of oxygen to hemoglobin. Materials and Methods.-Horse and human hemoglobins were prepared according to the method of Benesch and Benesch'0 from fresh defibrinated horse blood (W. T. Bennett Ranch Laboratory) and from samples obtained through the courtesy of Dr. Marcus Krupp, Palo Alto Medical Clinic. The spin label, N-(1-oxyl-2,2,5,5-tetramethyl-3-pyrrolidinyl)iodoacetamide (I),