Biomedical Engineering Reference
In-Depth Information
d 0
l
Fig. 4.2 A bilayer deforms at the bilayer gramicidin A channel coupling area, which incurs an
energetic cost (see Chap. 5 ) . This schematic diagram showing gramicidin A channels in lipid bilayers
follows identical morphology as that presented in Fig. 4.1 .The upper panel shows a lipid bilayer
without any integral membrane proteins. The lower panel shows a bilayer with integral gramicidin
A monomers and dimers of different lengths. When gramicidin A channels are formed inside a
lipid bilayer under a trans-membrane potential difference between two sides, the bilayer conducts a
current pulse with a specific average pulse width (proportional to the gramicidin A channel lifetime)
and height (proportional to the gramicidin A channel conductance) depending on the gramicidin A
channel type (the number of amino acids in the structures of gramicidin A monomers). Two types
of gramicidin A monomers are schematically illustrated here to produce two gramicidin A channels
of different lengths ( l ). d 0 is the unperturbed thickness of the bilayer [ 6 , 10 ]
hydrophobic coupling between the channel and the bilayer. A more detailed descrip-
tion of the various properties of the ordered channel versus non-channel phases of
the membrane will be provided in Chap. 5 . It is important to mention that in these
kinds of channels, only a dimer state is a stable structure for a channel. No other
channel states are physically stable.
4.1.2 Alamethicin Channels
Alamethicin channels form barrel-stave pores [ 15 , 26 ]. In this type of pore, the
peptides align in such a way as to make a cylinder where the peptides stay on the
surface of the cylinder. Many conductance states are possible, depending on the
number of monomers involved in forming the cylindrical channel. A model diagram
is shown in Figs. 4.3 and 4.4 , and will be revisited in more detail in Chap. 5 . Here,
the channel is assumed to be formed due to inter-monomer binding. The channel
also experiences hydrophobic coupling with lipid monolayers in both its longitudi-
nal terminals. Unlike gramicidin A channels, where we observe only one ordered
gramicidin A dimer state, there is no experimentally observed unique cylindrical
alamethicin channel state. Alamethicin channels with various cylindrical states are
possible. It appears physically plausible that a transition between different cylin-
drical channel states can take place. These states are modeled in various diagrams
[ 6 , 15 , 26 ]. A phenomenological explanation of how the conformational transitions
 
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