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Water-Mediated Electron Transfer between Protein Redox Centers
journal contribution
posted on 2007-04-12, 00:00 authored by Agostino Migliore, Stefano Corni, Rosa Di Felice, Elisa MolinariRecent experimental and theoretical investigations show that water molecules between or near redox partners
can significantly affect their electron-transfer (ET) properties. Here we study the effects of intervening water
molecules on the electron self-exchange reaction of azurin (Az), by performing a conformational sampling
on the water medium and by using a newly developed ab initio method to calculate transfer integrals between
molecular redox sites. We show that the insertion of water molecules at the interface between the copper
active sites of Az dimers slightly increases the overall ET rate, while some favorable water conformations
can considerably enhance the ET kinetics. These features are traced back to the interplay of two competing
factors: the electrostatic interaction between the water and protein subsystems (mainly opposing the ET
process for the water arrangements drawn from MD simulations) and the effectiveness of water in mediating
ET coupling pathways. Such an interplay provides a physical basis for the found absence of correlation between
the electronic couplings derived through ab initio electronic structure calculations and the related quantities
obtained through the Empirical Pathways (EP) method. In fact, the latter does not account for electrostatic
effects on the transfer integrals. Thus, we conclude that the water-mediated electron tunneling is not controlled
by the geometry of a single physical pathway. We discuss the results in terms of the interplay between different
ET pathways controlled by the conformational changes of one of the water molecules via its electrostatic
influence. Finally, we examine the dynamical effects of the interfacial water and check the validity of the
Condon approximation.