Betaine

A betaine () is any neutral chemical compound with a positively charged cationicfunctional group that bears no hydrogen atom, such as a quaternary ammonium or phosphonium cation (generally: onium ions), and with a negatively charged functional group, such as a carboxylate group that may not be adjacent to the cationic site. They are thus zwitterions, but cannot isomerize because there is no labile hydrogen atom attached to the nitrogen atom.

Historically, the term was reserved for trimethylglycine (TMG), which is involved in methylation reactions and detoxification of homocysteine.