Noncontact dipole effects on channel permeation. I. Experiments with (5F-indole)Trp13 gramicidin A channels

Gramicidin A (gA), with four Trp residues per monomer, has an increased conductance compared to its Phe replacement analogs. When the dipole moment of the Trp13 side chain is increased by fluorination at indole position 5 (FgA), the conductance is expected to increase further. gA and FgA conductance...

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Published in:Biophysical journal 1998-12, Vol.75 (6), p.2830-2844
Main Authors: Busath, D D, Thulin, C D, Hendershot, R W, Phillips, L R, Maughan, P, Cole, C D, Bingham, N C, Morrison, S, Baird, L C, Hendershot, R J, Cotten, M, Cross, T A
Format: Article
Language:English
Subjects:
Biophysical Phenomena
Biophysics
Electric Conductivity
Fluorine - chemistry
Glycerides - chemistry
Gramicidin - chemistry
In Vitro Techniques
Indoles - chemistry
Ion Channels - chemistry
Kinetics
Lipid Bilayers - chemistry
Membrane Potentials
Models, Chemical
Molecular Conformation
Onium Compounds - chemistry
Permeability
Thermodynamics
Tryptophan - chemistry
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Summary:Gramicidin A (gA), with four Trp residues per monomer, has an increased conductance compared to its Phe replacement analogs. When the dipole moment of the Trp13 side chain is increased by fluorination at indole position 5 (FgA), the conductance is expected to increase further. gA and FgA conductances to Na+, K+, and H+ were measured in planar diphytanoylphosphatidylcholine (DPhPC) or glycerylmonoolein (GMO) bilayers. In DPhPC bilayers, Na+ and K+ conductances increased upon fluorination, whereas in GMO they decreased. The low ratio in the monoglyceride bilayer was not reversed in GMO-ether bilayers, solvent-inflated or -deflated bilayers, or variable fatty acid chain monoglyceride bilayers. In both GMO and DPhPC bilayers, fluorination decreased conductance to H+ but increased conductance in the mixed solution, 1 M KCl at pH 2.0, where K+ dominates conduction. Eadie-Hofstee plot slopes suggest similar destabilization of K+ binding in both lipids. Channel lifetimes were not affected by fluorination in either lipid. These observations indicate that fluorination does not change the rotameric conformation of the side chain. The expected difference in the rate-limiting step for transport through channels in the two bilayers qualitatively explains all of the above trends.
ISSN:0006-3495
1542-0086
DOI:10.1016/s0006-3495(98)77726-4