The entatic state principle describes the influence of the surrounding protein matrix on the active site of an enzyme. The stiff protein stabilizes the metal center in a distorted and energized geometry, which imitates the transition state of the electron transfer. Therefore, the activation barrier of the oxidation reaction is lowered and the electron transfer is promoted. This principle is adaptable for the coordination sphere of protein model complexes. The copper center in [Cu(GUAqu)2]+/2 complexes is coordinated by two different N-donor functions. Two hybrid guanidinoquinoline ligands coordinate the metal center having each a guanidine and an aromatic imine unit. During the oxidation process only a small change in the geometry of the complex cation is observed. The geometries of both oxidation states are highly distorted from square-planar (Cu(II)) and tetrahedral (Cu(I)). Hence, the complexes function as models for the geometric entatic state. For the evaluation of the electron transfer abilities, we determined the electron self-exchange rates k11 in different solvents. Thereby, an electron of the reduced complex species is transferred to its oxidized form. The velocity k11 of this reaction is determined via Marcus cross relation based on the Marcus theory. Additional examination of the reorganization energies of the cross reactions (experimentally) and the self-exchange (theoretically via DFT calculations) reveal a deeper insight in the entatic state features of the copper guanidinoquinoline complexes.

Copper guanidinoquinoline complexes as entatic state models of electron transfer proteins

255th National Meeting (2018)

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