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The Gramicidins
The gramicidins are a family of
naturally-occurring bacterial antibiotics whose action results from their
ability to form cation-specific channels. The 15 residue peptides
have an unusual structure, containing high proportions of D-amino acids,
which induce a strong propensity towards helical (β6.3)
conformations in lipid bilayers. Additionally, the C-terminus is
modified by the formation of an amide with ethanolamine, whilst the N-terminus
is found as the formyl amide. Channels are formed by the head to
head arrangement of two gramicidin monomers to produce a membrane traversing
species (Woolley, G. A. and Wallace, B. A., J. Membr. Biol., 129,
109-136 (1992) and Hu, W., et al, Biochemistry, 34,
14138 (1995)). These channels are selective for monovalent cations
and show conductance levels of 3E+07 ions/sec/channel at a membrane potential
of 100 mV. Derivatisation of the C-terminal ethanolamine with carbamates
bearing pendant positive charges has been shown to induce modulation of
channel conductance (Woolley, G. A., et al, J. Am. Chem. Soc.,
117,
4448-4454 (1995)).
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