Conformation of Peroxynitrite: Determination by Crystallographic Analysis

Peroxynitrite is an inorganic toxin of biological importance. It is formed in vivo from the diffusion-limited reaction of nitrogen monoxide with superoxide. Due to the partial double bond between the nitrogen and the first peroxide oxygen, peroxynitrite can occur in two conformations, cis and trans. The synthesis of tetramethylammonium peroxynitrite in ammonia [Bohle, D. S., et al. (1994) J. Am. Chem. Soc. 116, 7423-7424] yields small crystals if the ammonia is left to evaporate slowly. X-ray structure analysis shows that peroxynitrite crystallizes in the cis form, relative to the N-O bond. Crystal twinning or disorder prevents the determination of accurate bond lengths and bond angles. However, a nearly flat (torsion angle of 22°) molecule with O=N, N-O, and O-O bond lengths of 1.16, 1.35, and 1.41 A, respectively, would fit the observed electron density best. The space group of tetramethylammonium peroxynitrite is Pmmn (59), and it has the following unit cell dimensions: a = 7.1778(11) A, b = 8.6893(13) A, and c = 5.7266(9) A.