Formation of racemic compound crystals by mixing of two enantiomeric crystals in the solid state. Liquid transport of molecules from crystal to crystal

Mixing of powdered (-)- and (+)-enantiomer crystals in the solid state gives crystals of the racemic compound. This racemic crystal formation was followed by IR spectral measurement of a 1 :1 mixture of (-)- and (+)-enantiomer crystals as a Nujol mull. As the formation of racemic crystals proceeds, the OH absorptions of the enantiomer disappear gradually and new OH absorptions due to the racemic compound appear. The formation of racemic crystals from enantiomer crystals has been studied for various kinds of chiral compounds: 2,29-dihydroxy-1,19-binaphthyl (1) and its derivatives, 10,109-dihydroxy-9,99-biphenanthryl (4), 2,29-dihydroxy-4,49,6,69-tetramethylbiphenyl (5) and its derivatives, 4,49-dihydroxy-2,29,3,39,6,69- hexamethylbiphenyl (8), 1,6-di(o-chlorophenyl)-1,6-diphenylhexa-2,4-diyne-1,6-diol (11) and its derivatives, trans-4,5-bis[hydroxy(diphenyl)methyl]-2,2-dimethyl-1,3-dioxacyclopentane (17) and itsderivatives, tartaric acid (20) dimethyl tartrate (21), malic acid (22), mandelic acid (23), and norephedrine (24). These molecular movements and blending occur rapidly in the presence of liquids such as liquid paraffin (Nujol), seed oils such as olive, coconut, rapeseed and soybean oil, artificial oil such as silicone oil and water, although the same movement also occurs in the absence of the liquid. For example, keeping a mixture of powdered (-)-1 (1a) and (+)-1 (1b) at room temperature for 48 h gives racemic crystals (1c). However, molecular aggregation sometimes occurs in solution but not in the solid state. Forexample, recrystallization of (-)-16 (16a) and (+)-16 (16b) from solvent gives racemic crystals of 16c, although mixing of these two components as powders in the presence of liquid does not give 16c. In order to determine the mechanism of the molecular movement in the solid state, X-ray crystal structures of optically active and racemic compounds and also the molecular movements from optically active crystal to racemic crystal have been studied

ADP-Ribose Pyrophosphatase Reaction in Crystalline State Conducted by Consecutive Binding of Two Manganese(II) Ions as Cofactors

Adenosine diphosphate ribose pyrophosphatase (ADPRase), a member of the Nudix family proteins, catalyzes the metal-induced and concerted general acid–base hydrolysis of ADP ribose (ADPR) into AMP and ribose-5′-phosphate (R5P). The ADPR-hydrolysis reaction of ADPRase from <i>Thermus thermophilus</i> HB8 (<i>Tt</i>ADPRase) requires divalent metal cations such as Mn²⁺, Zn²⁺, or Mg²⁺ as cofactors. Here, we report the reaction pathway observed in the catalytic center of <i>Tt</i>ADPRase, based on cryo-trapping X-ray crystallography at atomic resolutions around 1.0 Å using Mn²⁺ as the reaction trigger, which was soaked into <i>Tt</i>ADPRase-ADPR binary complex crystals. Integrating 11 structures along the reaction timeline, five reaction states of <i>Tt</i>ADPRase were assigned, which were ADPRase alone (E), the ADPRase-ADPR binary complex (ES), two ADPRase-ADPR-Mn²⁺ reaction intermediates (ESM, ESMM), and the postreaction state (E′). Two Mn²⁺ ions were inserted consecutively into the catalytic center of the ES-state and ligated by Glu86 and Glu82, which are highly conserved among the Nudix family, in the ESM- and ESMM-states. The ADPR-hydrolysis reaction was characterized by electrostatic, proximity, and orientation effects, and by preferential binding for the transition state. A new reaction mechanism is proposed, which differs from previous ones suggested from structure analyses with nonhydrolyzable substrate analogues or point-mutated ADPRases