Capsaicin is the active ingredient in chilli peppers (fruits of
Capsicum species) - cayenne, jalapeños, scotch bonnet peppers etc.
Chemically, it is
8-methyl-N-vanillyl-6-nonenamide.
Nonenamide means a 9-carbon chain including a double bond, attached to a CONH amide group, and this is attached to a
vanillin (substituted benzene ring).
It gives a pungent flavour to foods, and a sensation of heat.
It is also used medicinally to deal with pain, and in pepper sprays to incapacitate people.
Interestingly, its effects are mainly felt by mammals but not by birds, which feed on the fruits and disperse the seeds.
At the molecular level, it has been found that capsaicin selectively binds to a receptor protein found in the membranes of pain and heat sensing neurons. This is a heat activated calcium channel, with a threshold to open between 37 and 45° C, i.e slightly above normal body temperature. Thus the chemical mimics a burning sensation, the nerves are overwhelmed by the influx, and are unable to report pain for an extended period of time.