THE MOLECULAR-CONFORMATION OF CHORISMIC ACID IN THE CRYSTALLINE STATE

Chorismic acid is the branch point intermediate in the biosynthesis of all aromatic amino acids, and is essential in the metabolism of plants, fungi, and bacteria, but not mammals, making it an attractive target for herbicide, fungicide, or bacteriocide design. Chorismate has attracted substantial attention from bioorganic chemists because it spontaneously rearranges to prephenate in a reaction that is catalyzed 106-fold by specific enzymes (chorismate mutases) and up to 104-fold by certain catalytic antibodies. The detailed molecular structure of chorismic acid, previously unknown, has been determined by X-ray crystallography. The free acid, formula C10H10O6, crystallizes with orthorhombic space group symmetry C2221 with unit cell dimensions a = 4.569 (1), b = 20.437 (7), c = 29.229 (9) A ̊, and V = 2729 (1) A ̊3. The solvent of crystallization approximates 1.5 H2O and 0.5 CCl4 per molecule of chorismic acid. Crystals were unstable at room temperature and therefore X-ray diffraction data were measured at low temperature (-125°C). The phase problem was solved by direct methods and the model was refined by a full-matrix least-squares technique to give a final R = 0.092 and Rw = 0.10. The structure shows the anticipated positions of single and double bonds with typical bond lengths. The conformation of chorismic acid in this crystal structure is trans diequatorial with respect to extracyclic groups C(3) and C(4); this is also reported to be the predominant conformer in solution. In this conformation the carbon atoms which become bonded in the enzymatic rearrangement to prephenate are quite distant from one another. This may explain why it was possible to obtain crystals which, although unstable at room temperature, could be studied at lower temperatures. © 1992.