Redetermination of rifampicin pentahydrate revealing a zwitterionic form of the antibiotic.

Rifampicin belongs to the family of naphthalenic ansamycin antibiotics. The first crystal structure of rifampicin in the form of the pentahydrate was reported in 1975 [Gadret, Goursolle, Leger & Colleter (1975). Acta Cryst. B 31, 1454-1462] with the rifampicin molecule assumed to be neutral. Redetermination of this crystal structure now shows that one of the phenol -OH groups is deprotonated, with the proton transferred to a piperazine N atom, confirming earlier spectroscopic results that indicated a zwitterionic form for the molecule, namely (2S,12Z,14E,16S,17S,18R,19R,20R,21S,22R,23S,24E)-21-acetyloxy-6,9,17,19-tetrahydroxy-23-methoxy-2,4,12,16,18,20,22-heptamethyl-8-[(E)-N-(4-methylpiperazin-4-ium-1-yl)formimidoyl]-1,11-dioxo-1,2-dihydro-2,7-(epoxypentadeca[1,11,13]trienimino)naphtho[2,1-b]furan-5-olate pentahydrate, C(43)H(58)N(4)O(12)·5H(2)O. The molecular structure of this antibiotic is stabilized by a system of four intramolecular O-H···O and N-H···N hydrogen bonds. Four of the symmetry-independent water molecules are arranged via hydrogen bonds into helical chains extending along [100], whereas the fifth water molecule forms only one hydrogen bond, to the amide group O atom. The rifampicin molecules interact via O-H···O hydrogen bonds, generating chains along [001]. Rifampicin pentahydrate is isostructural with recently reported rifampicin trihydrate methanol disolvate.