The D-alanine, and why it matters

Nearly every peptide in a vertebrate is built from L amino acids, and the enzymes that destroy peptides are shaped to read L chains. Dermorphin's second residue is D-alanine, a mirror image, which makes the chain unreadable to those enzymes and lets it last long enough to work. Strip the molecule back and the first four residues are enough for full activity, but remove or flip that D-alanine and the activity collapses. One mirrored atom arrangement is the whole difference between a frog's defence chemical and a fragment that does nothing.