Fig. 30.30 Gramicidin A, (Val–Gly–Ala–Leu–Ala–Val3–(Trp–Leu)3–Trp–ethanolamine) is made up of 15 alternating L- and D-configured amino acids and forms a narrow channel of about 4 Å through the membrane along which monovalent cations such as Na+ and K+ can migrate.