Immucillin-H [ImmH; (1S)-1-(9-deazahypoxanthin-9-yl)-1,4-dideoxy-1,4-imino-D-ribitol] is a 23 pM inhibitor of bovine purine nucleoside phosphorylase (PNP) specifically designed as a transition state mimic [Miles, R. W., Tyler, P. C., Furneaux, R. H., Bagdassarian, C. K., and Schramm, V. L. (1998) Biochemistry 37, 8615-8621]. Cocrystals of PNP and the inhibitor are used to provide structural information for each step through the reaction coordinate of PNP. The X-ray crystal structure of free ImmH was solved at 0.9 A resolution, and a complex of PNP.ImmH.PO(4) was solved at 1.5 A resolution. These structures are compared to previously reported complexes of PNP with substrate and product analogues in the catalytic sites and with the experimentally determined transition state structure. Upon binding, ImmH is distorted to a conformation favoring ribosyl oxocarbenium ion formation. Ribosyl destabilization and transition state stabilization of the ribosyl oxocarbenium ion occur from neighboring group interactions with the phosphate anion and the 5'-hydroxyl of the ribosyl group. Leaving group activation of hypoxanthine involves hydrogen bonds to O6, N1, and N7 of the purine ring. Ordered water molecules provide a proton transfer bridge to O6 and N7 and permit reversible formation of these hydrogen bonds. Contacts between PNP and catalytic site ligands are shorter in the transition state analogue complex of PNP.ImmH.PO(4) than in the Michaelis complexes of PNP.inosine.SO(4) or PNP.hypoxanthine.ribose 1-PO(4). Reaction coordinate motion is dominated by translation of the carbon 1' of ribose between relatively fixed phosphate and purine groups. Purine and pyrimidine phosphoribosyltransferases and nucleoside N-ribosyl hydrolases appear to operate by a similar mechanism.

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Synchrotron-based high-resolution photoemission and first-principles density functional calculations (DFT-GGA) were used to study the interaction of SO2 with clean and modified (OH, Oδ-, O vacancies, or Cu adatoms present) MgO(100) surfaces. The reaction of the molecule with pure and hydroxylated powders of MgO was investigated using X-ray absorption near-edge spectroscopy (XANES). At 100 K, the main product of the adsorption of sulfur dioxide on MgO(100) is sulfite (SO2,gas + Olattice → SO3,ads). No evidence is found for bonding of SO2 to Mg sites of the surface or decomposition of the molecule. DFT calculations show that a η3-S,O,O adsorption configuration leads to a SO3-like species, and this is much more stable than configurations which involve bonding to only Mg sites or formation of SO4. On a flat MgO(100) substrate, the formation of SO4 is not energetically viable. A SO3 → SO4 transformation is observed at temperatures between 150 and 450 K with a substantial reconstruction of the oxide surface. From 450 to 650 K, the adsorbed SO3/SO4 species decompose and SO2 desorbs back into gas phase. The presence of OH groups and Oδ- (δ < 2) species on MgO favors the formation of SO4 at the expense of SO3. On the other hand, the creation of O vacancies in MgO(100) by ion sputtering leads to decomposition of SO2. The chemistry of SO2 on Cu/MgO(100) surfaces is rich. At 150 K, the SO2 molecule chemisorbs intact on the supported Cu particles and forms SO3 on the oxide substrate. Heating to room temperature induces full decomposition of SO2 and the formation of SO4. The Cu adatoms facilitate the decomposition of SO2 by providing electronic states that are very efficient for interactions with the lowest unoccupied molecular orbital (S-O antibonding) of the molecule.

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