Photolysis-induced structural changes in single crystals of carbonmonoxy myoglobin at 40 K.

Myoglobin's reversible binding of oxygen is a model for studies of protein control of ligand binding and discrimination. Protein relaxation and geminate ligand rebinding subsequent to ligand photodissociation have been studied extensively by a variety of techniques. The ps to ns time scales for these processes are still much shorter than the ms time resolution of X-ray diffraction experiments, but it may be possible to trap these intermediates at low temperatures. We report here an X-ray diffraction investigation of structural changes induced by photolysis of carbonmonoxy myoglobin crystals at 40 K. Our results provide a structural basis for the interpretation of ambient and low temperature spectroscopic observations and molecular dynamics simulations of the ligand photodissociation and binding processes in haem proteins.