th  - 1988 - 51st ENC Conference

• / I
DYNAMICS OF THE GRAMICIDIN A TRANSMEMBRANE CHANNEL BY
THU 9:20 I SOLID STATE 15N NMR: L.I~ Nicholson, M. T. Brenneman, P.V. LoGrasso and
T.A. Cross, Florida State University, Institute of Molecular Biophysics and
Department of Chemistry, Tallahassee, Florida 32306.
The dynamics of specific sites in the peptide backbone of the gramicidin A cation selective
transmembrane channel have been studied using solid state 15 N NMR. Gramicidin A is a polypeptide
consisting of fifteen amio acids which dimerizes to form a single stranded helical pore in a lipid bilayer.
Its generally accepted structure is the ~6.3 helix which, due to the uniquely alternating L/D amino acid
sequence places the bydrophobic side chains on the outside of the channel where they interact with the
hydrocarbon core of the bilayer, and the polar peptide linkages along the interior of the channel which
enhances solvation of the channel ion. Although gramicidin is the most extensively studied channel, an
atomic resolution mechanism Of ion transport is not known. Characterization of motions of various groups
within the channel backbone will help to elucidate the specific interactions that result in transport of the ion
across the membrane. Motions of specific sites along the channel backbone have been detected by observing
the averaging of the 15N chemical shift anisotropy (CSA) tensor as a function of temperature in both oriented
and unoriented samples. It has previously been shown that fast overall channel rotation occurs in and
above the lipid phase transition region, and that the axis of rotation coincides with the channel axis which is
parallel to the bilayer normal. This global rotation becomes slow on the 3kHz timeframe of the NMR
experiment when the temperature is below the onset of the phase transition. Recent studies of the temperature
dependence of the 15N spectra of both oriented and unoriented samples show evidence for local motions of the
peptide linkages existing above the onset of the gel to liquid crystalline phase transition, and that the
amplitude of these motions varies along the channel backbone. These local motions have a large amplitude
at the monomoer - monomer juction where the peptide linkage planes contribute a proton to the hydrogen
bonds linking the two monomers. The temperature dependence of oriented samples where yield a very
sharp resonance above the phase transition region has proved to be a very sensitive indicator of dynamics
when the temperature is lowered. The resonance linewidth below the phase transition reflects directly on the
range of orientations swept out by the dynamic process at higher temperatures. This new tool for assessing
dynamics should have broad application in systems that can be oriented.
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