Ovothiol A is a L-histidine derivative that is L-histidine substituted at positions N3 and C5 on the imidazole ring by methyl and mercapto groups respectively. It has a role as an antioxidant, a marine metabolite and a radical scavenger. It is an aryl thiol, a L-histidine derivative and a non-proteinogenic L-alpha-amino acid. It is a tautomer of an ovothiol A zwitterion.