Fig. 1. Structures and functions of channel-forming peptidic natural products. (a) Structures of gramicidin A (1), inactive analogue 2, active fluorescent probe 3, inactive fluorescent probe 4, and polytheonamide B (5). (b) Head-to-head β^6.3-helical dimer structure of 1 (PDB ID 1MAG). (c) β^6.3-Helical structure of 5 (PDB ID 2RQO). Numbers highlighted in filled circles are residue numbers. ^aEC₅₀ value of H⁺/Na⁺ transport activity toward pH-gradient liposomes comprising egg yolk phosphatidylcholine and egg yolk phosphatidylglycerol (19 : 1). ^bIC₅₀ value against the P388 mouse leukemia cell line.