... Standards: FCC4,AJI92,USP26,EP4 L-ThreonineIndications and UsageThreonine (abbreviated as Thr or ... amino acid is classified as polar. Together with serine, threonine is one of two proteinogenic amino acids ... (2S,3S) and (2R,3R). However, the name L-threonine is used for one single enantiomer, (2S,3R)- ... 3S), which is rarely present in nature, is called L-allo-threonine. The two stereoisomers (2R,3S)- and ...