Polar solvation dynamics in Zn(II)-substituted cytochrome c:: Diffusive sampling of the energy landscape in the hydrophobic core and solvent-contact layer

We employed picosecond time-resolved fluorescence spectroscopy to characterize polar diffusive protein and solvent dynamics in Zn(II)-substituted cytochrome c. The solvent-response function, obtained from the dynamic Stokes shift of the Zn(II) porphyrin's fluorescence 0-0 transition energy over the 100-ps to 12-ns time scale, exhibits a double-exponential decay (250 ps and 1.45 ns time constants). The addition of glycerol to the external solvent medium slows only the slower of the two exponential components; in the presence of 50% (v/v) glycerol, the second time constant was lengthened to 2.2 ns. The magnitude of the decay accounts for 85% of the reorganization energy and leaves room for only a minor inertial component on the sub-ps time scale. The results show that two types of polypeptide motions are sensed by the intrinsic Zn(II) porphyrin. The slower of the motions arises from polypeptide motions in the interfacial, solvent-contact portion of the protein. The faster, sol vent-independent part of the response arises from motions of the polypeptide in the hydrophobic core of the protein. The two classes of motion are apparently not strongly coupled, nor is the dynamic Stokes shift coupled to the nonpolar reorganization that accompanies the excited-state photodissociation of the Zn(II) porphyrin's protein-derived axial ligands. The finding that the inertial response is limited to a small fraction of the total reorganization energy suggests that the hydrophobic core of the protein is largely incapable of the free-rotor or librational character of polypeptide and solvent motions that is observed near the solvent interface.