Tetracycline hydrochloride: a synchrotron microcrystal study.

The title compound, [(4S,4aS,5aS,6S,12aS)-2-aminohydroxymethylene-1, 2,3,4,4a,5,5a,6,11,12a-decahydro-6,10,12,12a-tetrahydroxy-6-methyl -1, 3,11-trioxonaphthacen-4-yl]dimethylammonium chloride, C(22)H(25)N(2)O(8)(+).Cl(-), a well known antibiotic, has been structurally characterized from an individual coarse powder grain by use of high-intensity synchrotron radiation, in conjunction with an exercise in ab initio powder diffraction structure solution. Free refinement of all H atoms establishes the major tautomeric form of the protonated tetracycline molecule without prejudice. The molecule has extensive intramolecular hydrogen bonding involving most of the potential donors and acceptors, and all intermolecular hydrogen bonding uses the chloride anion as acceptor.