STEREODYNAMIC PROPERTIES OF THE COOPERATIVE HOMODIMERIC SCAPHARCA INAEQUIVALVIS HEMOGLOBIN STUDIED THROUGH OPTICAL-ABSORPTION SPECTROSCOPY AND LIGAND REBINDING KINETICS

The study of the thermal evolution of the Soret band in heme proteins has proved to be a useful tool to understand their stereodynamic properties; moreover, it enables one to relate protein matrix fluctuations and functional behavior when carried out in combination with kinetic experiments on carbonmonoxide rebinding after flash photolysis. In this work, we report the thermal evolution of the Soret band of deoxy, carbonmonoxy, and nitric oxide derivatives of the cooperative homodimeric Scapharca inaequivalvis hemoglobin in the temperature range 10-300 K and the carbonmonoxide rebinding kinetics after flash photolysis in the temperature range 60-200 K. The two sets of results indicate that Scapharca hemoglobin has a very rigid protein structure compared with other hemeproteins. This feature is brought out i) by the absence of nonharmonic contributions to the soft modes coupled to the Soret band in the liganded derivatives, and ii) by the almost ''in plane'' position of the iron atom in the photoproduct obtained approximate to 10(-8) s after dissociating the bound carbonmonoxide molecule at 15 K.