Gramicidin - Composition and Structure

Composition and Structure

Gramicidin is a polypeptide with alternating L- and D-amino acids, sharing the general formula: formyl-L-X-Gly-L-Ala-D-Leu-L-Ala-D-Val-L-Val-D-Val-L-Trp-D-Leu-L-Y-D-Leu-L-Trp-D-Leu-L-Trp-ethanolamine

X and Y depend upon the gramicidin molecule. There exist valine and isoleucine variants of all three gramicidin species, and 'X' can be either. Y determines which is which; as Y gramicidin A contains tryptophan, B phenylalanine, and C tyrosine. Also note the alternating stereochemical configurations (in the form of D and L) of the amino acids; this is vital to the formation of the β-helix.

The chain assembles inside of the hydrophobic interior of the cellular lipid bilayer to form a β-helix. The helix itself is not long enough to span the membrane but it dimerizes to form the elongated channel needed to span the whole membrane.

The structure of the gramicidin head-to-head dimer in micelles and lipid bilayers was determined by solution and solid-state NMR. The structure was first proposed by D. W. Urry in 1971. In organic solvents and crystals this peptide forms different types of non-native double helices.