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Scandium Reduced Arene Complex: Protonation and Reaction with Azobenzene.

ABSTRACT: The reactivity of the reduced anthracene complex of scandium [Li(thf)₃ ][Sc{N(tBu)Xy}₂ (anth)] (2-anth-Li) (Xy=3,5-Me₂ C₆ H₃ ; anth=C₁₄ H₁₀ ^2- , thf=tetrahydrofuran) toward Brønsted acid [NEt₃ H][BPh₄ ] and azobenzene is reported. While a stepwise protonation of 2-anth-Li with two equivalents of [NEt₃ H][BPh₄ ] afforded the scandium cation [Sc{N(tBu)Xy}₂ (thf)₂ ][BPh₄ ] (3), reduction of azobenzene gave a thermolabile, anionic scandium reduced azobenzene complex [Li(thf)][Sc{N(tBu)Xy}₂ (η² -PhNNPh)] (4), which slowly lost one of the anilide ligands to form the neutral scandium azobenzene complex dimer [Sc{N(tBu)Xy}(μ-η² :η² -Ph₂ N₂ )]₂ (5). Exposure of 3 to CO₂ produced the scandium carbamate complex [Sc{κ² -O₂ CN(tBu)(Xy)}₂ ][BPh₄ ] (6) as a result of CO₂ insertion into the Sc-N bonds. In an attempt to prepare scandium hydrides, the reaction of 3 with the hydride sources LiAlH₄ and Na[BEt₃ H] led to the terminal aluminum hydride [AlH{N(tBu)Xy}₂ (thf)] (7) and the scandium n-butoxide [Sc{N(tBu)(Xy)}₂ (μ-OnBu)] (8) after Sc/Al transmetalation and nucleophilic ring-opening of THF, respectively. All reported compounds isolated in moderate to good yields were fully characterized.

SUBMITTER: Ghana P

PROVIDER: S-EPMC8596697 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Retaining glycoside hydrolases use acid/base catalysis with an enzymatic acid/base protonating the glycosidic bond oxygen to facilitate leaving-group departure alongside attack by a catalytic nucleophile to form a covalent intermediate. Generally, this acid/base protonates the oxygen laterally with respect to the sugar ring, which places the catalytic acid/base and nucleophile carboxylates within about 4.5-6.5 Å of each other. However, in glycoside hydrolase (GH) family 116, including disease-related human acid β-glucosidase 2 (GBA2), the distance between the catalytic acid/base and the nucleophile is around 8 Å (PDB: 5BVU) and the catalytic acid/base appears to be above the plane of the pyranose ring, rather than being lateral to that plane, which could have catalytic consequences. However, no structure of an enzyme-substrate complex is available for this GH family. Here, we report the structures of Thermoanaerobacterium xylanolyticum β-glucosidase (TxGH116) D593N acid/base mutant in complexes with cellobiose and laminaribiose and its catalytic mechanism. We confirm that the amide hydrogen bonding to the glycosidic oxygen is in a perpendicular rather than lateral orientation. Quantum mechanics/molecular mechanics (QM/MM) simulations of the glycosylation half-reaction in wild-type TxGH116 indicate that the substrate binds with the nonreducing glucose residue in an unusual relaxed 4C1 chair at the -1 subsite. Nevertheless, the reaction can still proceed through a 4H3 half-chair transition state, as in classical retaining β-glucosidases, as the catalytic acid D593 protonates the perpendicular electron pair. The glucose C6OH is locked in a gauche, trans orientation with respect to the C5-O5 and C4-C5 bonds to facilitate perpendicular protonation. These data imply a unique protonation trajectory in Clan-O glycoside hydrolases, which has strong implications for the design of inhibitors specific to either lateral protonators, such as human GBA1, or perpendicular protonators, such as human GBA2.

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Scandium Reduced Arene Complex: Protonation and Reaction with Azobenzene.

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