A structural, spectroscopic and computational study of a series of oximes was undertaken to investigate how geometric and structural changes relevant to the reaction coordinate for the Beckmann reaction (normal Beckmann) and Beckmann fragmentation (abnormal Beckmann) manifest in the ground state. X-ray structures of a range of oximes derived from dihydrolevoglucosan (Cyrene™; which undergoes the abnormal Beckmann reaction), in which the oxygen substituent was systematically varied were determined. As the electron demand of the OR group increased, the major structural changes included lengthening of the N-OR bond distance, and a decrease in the magnitude of the C2[double bond, length as m-dash]N-O bond angle, consistent with the changes seen for cyclohexanone oximes, which undergo the normal Beckmann reaction. However, unique to the Cyrene oximes, an increase in the length of the fissile C1-C2 bond was observed, which correlated with a decrease in the C-C 1-bond coupling constants as the electron demand of the OR substituent increased. Computational studies of Cyrene and cyclohexanone oximes using Natural Bond Orbital analysis support an electronic structure involving n → σ* and σ → σ* localized orbital interactions.

1,2-<i>trans</i>-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here we report the pH-rate constant profile for the hydrolysis of<i> </i>4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of <i>k</i>(H₃O⁺)/<i>k</i>(D₃O)⁺ = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of <i>k</i>_H/<i>k</i>_D = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect <i>k</i>(H₂O)/<i>k</i>(D₂O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in ¹⁸O-labelled water and H₂O/MeOH mixtures allowed detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.5 and a strongly negative entropy of activation (D<i>S</i>^‡ = –13.6 cal mol^–1 K^–1). Finally, at high pH, an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.6 indicates that the formation of 1,2-anhydrosugar is the rate determining step.<br>

<p>1,2-<i>trans</i>-Glycosides hydrolyze through a range of mechanisms under conditions of different pH, but systematic studies are lacking. Here we report the pH-rate constant profile for the hydrolysis of<i> </i>4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect (<i>k</i>(H₃O⁺)/<i>k</i>(D₃O⁺) = 0.63) in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region extrapolated to zero buffer concentration show a small inverse solvent isotope effect <i>k</i>(H₂O)/<i>k</i>(D₂O) = 1.1 and a positive entropy of activation (D<i>S</i>^‡ = 3.07 cal mol^–1 K^–1), which is consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in ¹⁸O-labelled water and H₂O/MeOH mixtures allowed detection of bimolecular hydrolysis and neighboring group participation, and to a minor degree, nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular dissociative mechanism is implicated through a solvent isotope effect of <i>k</i>(HO^-)/<i>k</i>(DO^-) = 0.5 and a strongly negative entropy of activation (D<i>S</i>^‡ = –13.6 cal mol^–1 K^–1). Finally, at high pH, an inverse solvent isotope effect of <i>k</i>(HO^-)/<i>k</i>(DO^-) = 0.6 and a weakly negative entropy of activation (D<i>S</i>^‡ = –5.5 cal mol^–1 K^–1) indicates that the formation of 1,2-anhydrosugar is the rate determining step. <b></b></p>

Sulfolactate (SL) is a short-chain organosulfonate that is an important reservoir of sulfur in the biosphere. SL is produced by oxidation of sulfolactaldehyde (SLA), which in turns derives from sulfoglycolysis of the sulfosugar sulfoquinovose, or through oxidation of 2,3-dihydroxypropanesulfonate, which are themselves important organosulfur compounds in the biogeochemical cycle. Oxidation of SLA is catalyzed by SLA dehydrogenases belonging to the aldehyde dehydrogenase superfamily. We report that SLA dehydrogenase RlGabD from the sulfoglycolytic bacterium Rhizobium leguminsarum SRDI565 can use both NAD+ and NADP+ as cofactor to oxidize SLA, and operates through a rapid equilibrium ordered mechanism. We report the cryo-EM structure of RlGabD bound to NADH, revealing a tetrameric quaternary structure and supporting proposal of organosulfonate binding residues in the active site, and a catalytic mechanism. Sequence based homology searches identified putative SLADH homologs in a range of sulfoglycolytic gene clusters in bacteria predominantly from the phyla Actinobacteria, Firmicutes, and Proteobacteria.

Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here we report the pH-rate constant profile for the hydrolysis of 4-nitrophenyl β-D-glucoside. An