A study of the possible associations between cocarboxylase and magnesium ion as seen in the crystal structure of thiamine.Cl.HCl.5[Mg(H2O)6]Cl.2H2O

Two hydrate pseudopolymorphs of thiamine pyrophosphate: a dihydrate and a trihydrate

Two hydrate pseudopolymorphs of 3-[(4-amino-2-methylpyrimidin-1-ium-5-yl)methyl]-4-methyl-1,3-thiazol-3-ium-5-yl hydrogen pyrophosphate (TPP),viz.a dihydrate, C12H18N4O7P2S·2H2O, (I), and a trihydrate, C12H18N4O7P2S·3H2O, (II), were obtained during a structural study of vitamin B1 coenzyme. In both compounds, TPP is a neutral zwitterion, with its pyrophosphate group doubly deprotonated and its pyrimidine ring protonated, and it assumes the usual `F' conformation in terms of the two torsion angles about the bonds by which the methylene group links the thiazolium and pyrimidinium rings [1.1 (3) and 79.7 (3)° for (I), and 2.0 (3) and 75.5 (3)° for (II)]. In (I), two TPP molecules are linked by a pair of O—H...O hydrogen bonds into a phosphate-pairing dimer. N—H...O hydrogen bonds connect the dimers into a sheet parallel to (101). In (II), the TPP molecules are self-assembled solely by N—H...O hydrogen bonds, generating a tape structure along [001]. A comparison of the four known hydrate pseudopolymorphs of TPP shows that the phosphate-pairing dimers are basic building units for the formation of two-dimensional networks.

The Direct Methods of X-ray Crystallography

The electron density function rho(r) in a crystal determines its difaction pattern, that is, both the magnitudes and phases of its x-ray diffraction maxima, and conversely. If, however, as is always the case, only magnitudes are available from the diffraction experiment, then the density function rho(r) cannot be recovered. If one invokes prior structural knowledge, usually that the crystal is composed of discrete atoms of known atomic numbers, then the observed magnitudes are, in general, sufficient to determine the positions of the atoms, that is, the crystal structure.