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Charge Delocalization in Proton Channels, I: The Aquaporin Channels and Proton Blockage

Hanning Chen ^*, Boaz Ilan ^*, Yujie Wu ^*, Fangqiang Zhu , Klaus Schulten  and Gregory A. Voth ^*

^* Center for Biophysical Modeling and Simulation, Department of Chemistry, University of Utah, Salt Lake City, Utah; and Theoretical and Computational Biophysics Group, Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois

Correspondence: Address reprint requests to Gregory A. Voth, Tel.: 801-581-7272; E-mail: voth{at}chem.utah.edu.

The explicit contribution to the free energy barrier and proton conductance from the delocalized nature of the excess proton is examined in aquaporin channels using an accurate all-atom molecular dynamics computer simulation model. In particular, the channel permeation free energy profiles are calculated and compared for both a delocalized (fully Grotthuss shuttling) proton and a classical (nonshuttling) hydronium ion along two aquaporin channels, Aqp1 and GlpF. To elucidate the effects of the bipolar field thought to arise from two -helical macrodipoles on proton blockage, free energy profiles were also calculated for computational mutants of the two channels where the bipolar field was eliminated by artificially discharging the backbone atoms. Comparison of the free energy profiles between the proton and hydronium cases indicates that the magnitude of the free energy barrier and position of the barrier peak for the fully delocalized and shuttling proton are somewhat different from the case of the (localized) classical hydronium. The proton conductance through the two aquaporin channels is also estimated using Poisson-Nernst-Planck theory for both the Grotthuss shuttling excess proton and the classical hydronium cation.

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