Protein relaxation in the photodissociation of myoglobin–CO complexes

Abstract

Laser-induced optoacoustic spectroscopy has been applied to the study of the photodissociation of myoglobin–CO complexes. Time-resolved optoacoustic signals have been measured from aqueous solutions of horse myoglobin–CO complex (hMbCO) at pH 3.5 and 8, and of sperm whale myoglobin–CO complex (swMbCO) at pH 8, in the temperature range 273–300 K. The signal of hMbCO at pH 8 exhibits three components. The first, which is faster than 20 ns and is associated with a reaction enthalpy of 61 kJ mol⁻¹, corresponds to Fe–CO bond breakage. The second component has a decay time of 80 ns at 293 K and is associated with an exothermic protein relaxation (−13 kJ mol⁻¹) and a volume change of −3 ml mol⁻¹. The relaxation, which involves a state where the photodissociated CO is still in a protein docking site, is thermally activated, with an activation enthalpy of 51 kJ mol⁻¹. The third component has a decay time of 800 ns at 293 K and an activation enthalpy of 39 kJ mol⁻¹, and is associated with an endothermic process (26 kJ mol⁻¹) and an expansion of 19 ml mol⁻¹. This process is ascribed to the migration of the photodissociated CO to the bulk solvent. At acidic pH, the latter process becomes faster (230 ns) and the volume change decreases. These features are correlated with the presence of an open form of the protein. swMbCO exhibits two components only, due to the overlap of the two fastest processes. The first involves a reaction enthalpy of 49 kJ mol⁻¹ and a volume contraction of −4.9 ml mol⁻¹. The second component (900 ns at 293 K, activation enthalpy 45 kJ mol⁻¹) is associated with a reaction enthalpy of 38 kJ mol⁻¹and a volume expansion of 15.3 ml mol⁻¹. These experimental findings have been interpreted by means of a new model, which also takes into account both laser flash photolysis results and structural information. The model is based on a two-dimensional scheme which describes both protein relaxation and the CO pathway following photodissociation.