Gramicidin metal complexes

An example of a channel- or pore-forming antibiotic is gramicidin A (9.57), a peptide consisting of 15 amino acids. It induces the transmembrane transport of protons, alkali-metal ions, and thallium ions at concentrations as low as 10 ° M, even though it is unable to complex these ions in solution. Gramicidin also forms several dimers with itself. 

Gramicidin S is a cyclic decapeptide isolated from Bacillius brevis whose antibiotic activity is derived from its ability to complex alkali metal ions and transport them across membranes. It acts as a channel for ion leakage which is not difficult to imagine when it is noted that it has a doughnut... 

---