Monocopper center embedded in a biomimetic cavity: From supramolecular control of copper coordination to redox regulation

The electrochemical behavior of diversely substituted Cu-N-3-calix[6]arene, enzyme-like, funnel complexes is analyzed. The Cu(II)/Cu(I) redox process is regulated by the supramolecular organization of the Cu coordination. The presence of a shoetree alkyl nitrile guest molecule inside the host cavity is a prerequisite for a dynamic redox behavior. Combination of supramolecular CH-pi weak interactions with the calixarene cavity and electronic/steric effects from the N-3 substituting groups (pyridine, imidazole, pyrrolidine) enforces the preferential geometrical pattern adopted by Cu. This dictates the pathway of the electron-transfer process and, thus, the thermodynamics and kinetics of the redox reaction in the framework of a square-scheme mechanism. The present observations recall strongly the redox control exerted by the protein matrix on copper proteins through biological concepts such as induced fit mechanism, protein foldings, and entatic and allosteric effects.