Pterin is a heterocyclic compound composed of a pteridine ring system, with a keto group and an amino group on positions 4 and 2 respectively. Pterins, as a group, are compounds that are derivatives of the basic pterin structure, described in the beginning, with additional functional groups attached to the pyrazine subring. Pterins were first discovered in the pigments of butterfly wings and perform many roles in coloration in the biological world.