Thioproline is a nonproteinogenic amino acid with the formula (CH2SCH2NHCH)CO2H, although it crystallizes as the zwitterion (CH2SCH2NH+2CH)CO−2. It consists of a 1,3-thiazolidine ring substituted with a carboxylic acid. It is synthesized by reaction of formaldehyde and cysteine. It occurs in nature, but rarely. It forms a coordination complex with cobalt.