A betaine (BEET-ah-een, ) in chemistry is any neutral chemical compound with a positively charged cationic functional group such as a quaternary ammonium or phosphonium cation (generally: onium ions) which bears no hydrogen atom and with a negatively charged functional group such as a carboxylate group which may not be adjacent to the cationic site. A betaine thus may be a specific type of zwitterion. Historically the term was reserved for TMG (trimethylglycine) only. Biologically, betaine is involved in methylation reactions and detoxification of homocysteine.