Freezing induced change in ligand orientation in oxycobalt-myoglobin

Abstract
Single crystals of oxycobalt-[Sperm Whale] myoglobin were examined by EPR spectroscopy at ambient and cryogenic temperatures. The principal values and eigenvectors of the g-tensor and the hyperfine coupling tensor were determined. The Co.sbd.O.sbd.O bond angle was estimated to be 125.degree. at ambient temperature. The single crystal EPR data of oxycobalt myoglobin at 77.degree. K showed 2 sets of the prinicipal values for g and hyperfine coupling tensors and eigenvectors, indicating that the bound oxygen molecule takes 2 distinct orientations. The well defined change in the molecular orientation is induced upon freezing the biological macromolecule.