Abstract
Using density functional theory we look at the quantum mechanics of charge transport along water wires both with free ends and donor/acceptor terminated. With the intermediate geometries in the DFT iterations we can follow the charge transfer mechanism and also construct the energy landscape explicitly. It shows activation barriers when a proton is transferred from one water molecule to the next. This, together with snapshots of intermediate geometries, leads to a justification and further elucidation of the Grotthuss mechanism and the Bjerrum effect. The charge transfer times and the conductivity of the proton wire are obtained in agreement with experimental results.
Highlights
In 1978 Nagle and Morowitz [1] suggested that linear water structures in membrane channels may act as carriers for proton transfer through the membrane wall
Charge transfer along a water wire is an atomic exchange mechanism in which a proton moves the short distance of less than the O − O distance leaving behind a quasineutral water molecule and forming a temporary diffuse hydronium ion H3O+
What is the overall picture of charge or proton transfer through a membrane channel? We start from the equilibrium configuration [ Zn+(NH3)3] [ H2O]4 [ NH3]+
Summary
In 1978 Nagle and Morowitz [1] suggested that linear water structures in membrane channels may act as carriers for proton transfer through the membrane wall. They coined the term proton wires for these water whiskers. Proton wires are found or suspected in bacteriorhodopsin [4,5,6,7], photosynthetic centers [8], in enzymes [9] and even in viruses [10] In most of these systems amino acid residues (His, Glu, Asp, Ser) act as proton donors and acceptors at the respective ends of the water whisker. A comprehensive review, both experimental and theoretical, was given by Cukierman [11]
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