l ‐Cysteine‐I at 30 K

The crystal structure of the orthorhombic phase I of l ‐cysteine, C 3 H 7 NO 2 S, has been determined at 30 K. The molecule adopts a gauche + conformation and the structure consists of zwitterions connected into sinusoidal layers by N—H⋯O hydrogen bonds. Further N—H⋯O hydrogen bonds connect the structure into a three‐dimensional array. Under ambient conditions, the thiol H atom is disordered in such a way as to form intermolecular S—H⋯S and S—H⋯O hydrogen bonds. At 30 K the structure is ordered with retention of the S—H⋯S contacts [S⋯S = 3.8489 (4) Å, S—H⋯S = 2.66 (3) Å and S—H⋯S = 150.8 (16)°].