Figure 2.

Chemical structure of cyclic antimicrobial peptides: (A) Structure of polymyxin B, including fatty acyl tail, linear peptide, and cyclic peptide (B) Chemical structure of bacitracin: bacitracin is an AMP that consists of D-aspartic acid, D-phenylalanine, D-ornithine, D-glutamic acid, and a ring of thiazoline containing amino acids. (C) Chemical structure of vancomycin: vancomycin is a branched tricyclic glycosylated non-ribosomal peptide. (D) Chemical structure of defensin: defensins comprise an N-terminal β-strand followed by an α-helix and two more β-strands. The β-strands form a triple-stranded antiparallel β-sheet that can be stabilized by disulphide bonds. Two of the disulphide bonds connect the α-helix and the central β-strand, while a third disulphide bond stabilizes the structure by linking the β-strand.