Effects of Solvent Viscosity on the Microsecond Protein Motions of Myoglobin Determined by Pulsed-Laser Photoacoustics

Pulsed-laser photoacoustics offers a means for measuring the Fe-CO bond strength in carboxymyoglobin, as well as the dynamics of the protein following cleavage of the Fe-CO bond. The technique is sensitive to processes which occur on the nsec and psec time scales. Preliminary results from our laboratory support a positive value for the enthalpy of formation of the Fe:CO geminate pair, formed within 10 nsec of photolysis of carboxymyoglobin, with little volumetric change in the protein. A subsequent decay occurs, with a lifetime of about 0.8 gsec at 23 °C, resulting in a volume increase and enthalpic change of the protein. This microsecond decay is temperature dependent and appears to be influenced by solvent viscosity. We are interested in this decay as a relaxation process in myoglobin not generally observable through the spectroscopy of the heme. We will present data on the effect of added glycerol or methanol, the source of the myoglobin, the method of deoxygenation, and the intensity of the photolysis laser pulse on the recovered photoacoustic decay parameters