**1.5.1 General properties**

This class of peptide is characterized by its rigid structure, given by the presence of 3 to 4 disulfid bridges. They are sub-classified through their cysteines connectivity and their secondary structure.

By virtue of the cysteine knot motif that stabilize them, all these peptides show a rigid secondary structure. Defensin are amphipatic molecules with a common defined beta sheet motif secondary structure. Indeed there is a gamma-core motif (GXCX(3-9)C), considered the structural signature of the disulfide-stabilized antimicrobial peptides that present two beta strands with an interposed loop (Sagaram et al., 2011). This motif has one hydrophobic and one hydrophilic side. The hydrophilic side of these peptides is usually constituted by several lysine or arginine aminoacids. This gives them a general positive charge at physiological pH. They are resistant to degradation and peptidase digestion because of their compact structure. As for the other antimicrobial peptide classes, there are few phylogenetic relationships even within each defensin subclass. The first three defensins class described were found exclusively in mammals.
