Heme-solvent coupling: A Mossbauer study of myoglobin in sucrose

The Mossbauer effect of Fe-57-enriched samples was used to investigate the coupling of 80% sucrose/water, a protein-stabilizing solvent, to vibrationa l and diffusive modes of the heme iron of CO-myoglobin. For comparison we a lso determined the Mossbauer spectra of (K4Fe)-Fe-57 (CN)(6) (potassium fer rocyanide, PFC), where the iron is fully exposed in the same solvent. The t emperature dependence of the Mossbauer parameters derived for the two sampl es proved to be remarkably similar, indicative of a strong coupling of the main heme displacements to the viscoelastic relaxation of the solvent. We s how that CO escape out of the heme pocket couples to the same type of fluct uations, whereas intramolecular bond formation involves solvent-decoupled h eme deformation modes that are less prominent in the Mossbauer spectrum. Wi th respect to other solvents, however, sucrose shows a reduced viscosity ef fect on heme displacements and the kinetics of ligand binding due to prefer ential hydration of the protein. This result confirms thermodynamic predict ions of the stabilizing action of sucrose by a dynamic method.