Structural and Binding Studies of Solvent Reorganization Energy ProbesPublic Deposited
Electron transfer theory predicts reorganization energy to be one factor that determines the electrochemical potential of a metal ion. This paper describes part of a study that aims to increase understanding of solvent reorganization energy in noncovalent systems by systematically varying the environment around a metal ion, specifically ruthenium. The local environment can be changed in various ways; here a protein is introduced to displace the solvent, creating a very different electronic environment for the metal ion by changing the dielectric constant as well as the refractive index. The avidin-biotin system was chosen for study because much is already known about this system, such as the crystal structure. New probes were developed that incorporate biotin and biotin derivatives into ruthenium complexes. The electrochemistry of these complexes was investigated in the presence and absence of avidin. A change in the dielectric constant of the surrounding medium is expected to result in a shift in the electrochemical potential of the uthenium complexes. However, a lack of current signal in the cyclic voltammogram resulted in the presence of avidin. Addition of mediators did not resolve the signal, indicating that the protein blocks the metal ion from access to the electrode. Theoretical models developed from calculations indicate that the metal should be openly accessible to mediators, if not the electrode itself. To resolve this discrepancy, structural and binding studies were undertaken. Singlecrystal x-ray crystallography was used to further understand the structure of the probes and the protein-ruthenium complex. A crystal structure was determined for one probe, 4-DMP, and its morphology was found to be monoclinic, meaning that two of this crystal’s three-unit cell axes of symmetry are perpendicular to one another. The probes were coupled to ruthenium pentaamine and then bound to the protein. These protein complexes were crystallized, yielding a diffraction pattern. Further preliminary kinetic experiments to determine the dissociation rate constant (kd) were conducted, yielding a value of 1.25 x 10-4 sec-1.