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| - Water-Mediated Electron Transfer between Protein Redox Centers
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| - Recent experimental and theoretical investigations show that water molecules between or near redox partnerscan significantly affect their electron-transfer (ET) properties. Here we study the effects of intervening watermolecules on the electron self-exchange reaction of azurin (Az), by performing a conformational samplingon the water medium and by using a newly developed ab initio method to calculate transfer integrals betweenmolecular redox sites. We show that the insertion of water molecules at the interface between the copperactive sites of Az dimers slightly increases the overall ET rate, while some favorable water conformationscan considerably enhance the ET kinetics. These features are traced back to the interplay of two competingfactors: the electrostatic interaction between the water and protein subsystems (mainly opposing the ETprocess for the water arrangements drawn from MD simulations) and the effectiveness of water in mediatingET coupling pathways. Such an interplay provides a physical basis for the found absence of correlation betweenthe electronic couplings derived through ab initio electronic structure calculations and the related quantitiesobtained through the Empirical Pathways (EP) method. In fact, the latter does not account for electrostaticeffects on the transfer integrals. Thus, we conclude that the water-mediated electron tunneling is not controlledby the geometry of a single physical pathway. We discuss the results in terms of the interplay between differentET pathways controlled by the conformational changes of one of the water molecules via its electrostaticinfluence. Finally, we examine the dynamical effects of the interfacial water and check the validity of theCondon approximation.
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