X-Ray crystal analysis of the substrates of aconitase: XI. Manganous citrate decahydrate

The crystal structure of hydrated manganese citrate, Mn(H₂O)₆] Mn C₆H₅O₇(H₂O)]₂2H₂O has been determined. The crystals are monoclinic, space group P2₁/n cell dimensions a = 20.575 ± 0.005, b = 6.755 ± 0.002, c = 9.230 ± 0.002 Å, β = 96.74 ± 0.01°. The structure is isomorphous with that of the magnesium salt for which the structure has been determined⁴. There are two Mn(H₂O)₆]²⁺ ions, four Mn C₆H₅O₇(H₂O)] ions and four water molecules per unit cell. Each citrate ion forms a tridentate chelate to one manganese ion. A correlation of Mn²⁺ ? H distances for manganese citrate, found in this crystallographic study, was made with those determined by NMR studies in solutions in the absence and presence of the enzyme aconitase, and indicated that the assumption that citrate chelates to enzyme-bound manganese ion fits all the available data.