Freezing induced change in ligand orientation in oxycobalt-myoglobin

Single crystals of oxycobalt-myoglobin were examined by electron paramagnetic resonance (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 - O - O bond angle was estimated to be 125° at ambient temperature. The single crystal EPR data of oxycobalt myoglobin at 77 K showed two sets of the principal values for g and hyperfine coupling tensors and eigenvectors, indicating that the bound oxygen molecule takes two distinct orientations. The result has demonstrated for the first time that the well defined change in the molecular orientation is induced upon freezing the biological macromolecule.