This image is the electrostatic potential of the
neurotransmitter 5HT (5-hydroxytryptamine or serotonin).
The cationic charge on the nitrogen of the amine
sidechain (blue color) is considered to be important for the molecular
attraction between the molecule and the negative charge (red color) on the
aspartate residue in the third transmembrane domain of the receptor. (See this link to receptor binding