 When serotonin binds to the receptors, the most important contacts are in TM helixes 3, 5 and 6 (Figure 3), while the other four TM helixes do not interact directly with the serotonin compound.When binding of serotonin takes place, the protonated primary amine site forms a salt bridge with D134 residue in TM 3, as well as forming a hydrogen bond with residue S138 in TM 3.The aromatic indole ring forms a strong Van der Waals interaction with residues F223 in TM 5 and F328 in TM 6.