A betaine 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) that bears no hydrogen atom and with a negatively charged functional group such as a carboxylate group that may not be adjacent to the cationic site. It is a modified amino acid consisting of glycine with three methyl groups serving as methyl donor for various metabolic pathways. Historically, the term was reserved for trimethylglycine (TMG) which is involved in methylation reactions and detoxification of homocysteine.