1.L-Cysteine 
Synonyms: (+)-2-Amino-3-mercaptopropionic acid; L-beta-Mercaptoalanine; CySH
CAS No.: 52-90-4
Purity: 99%Min
Molecular Formula: C3H7NO2S
Molecular Weight: 121.15

Cysteine (abbreviated as Cys or C) is a semi-essentialproteinogenic amino acid with the formula HO2CCH(NH2)CH2SH>. It is encoded by the codons UGU and UGC. The thiol side chain in cysteine often participates in enzymatic reactions, as a nucleophile. The thiol is susceptible to oxidization to give the disulfide derivative cystine, which serves an important structural role in many proteins. When used as a food additive, it has the E number E920.