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Biophys J, November 1999, p. 2492-2501, Vol. 77, No. 5
*Zoology Department, Brigham Young University, Provo, Utah 84062, and #Center for Interdisciplinary Magnetic Resonance at the National High Magnetic Field Laboratory, Institute of Molecular Biophysics and Department of Chemistry, Florida State University, Tallahassee, Florida 32306 USA
Proton transport on water wires, of interest for many
problems in membrane biology, is analyzed in side-chain analogs of
gramicidin A channels. In symmetrical 0.1 N HCl solutions, fluorination
of channel Trp11, Trp-13, or Trp15
side chains is found to inhibit proton transport, and replacement of
one or more Trps with Phe enhances proton transport, the opposite of
the effects on K+ transport in lecithin bilayers. The
current-voltage relations are superlinear, indicating that some
membrane field-dependent process is rate limiting. The interfacial
dipole effects are usually assumed to affect the rate of cation
translocation across the channel. For proton conductance, however,
water reorientation after proton translocation is anticipated to be
rate limiting. We propose that the findings reported here are most
readily interpreted as the result of dipole-dipole interactions between
channel waters and polar side chains or lipid headgroups. In
particular, if reorientation of the water column begins with the water
nearest the channel exit, this hypothesis explains the negative impact
of fluorination and the positive impact of headgroup dipole on proton conductance.
Biophys J, November 1999, p. 2492-2501, Vol. 77, No. 5
© 1999 by the Biophysical Society 0006-3495/99/11/2492/10 $2.00
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