- Switching between X-Pyrano-, X-Furano-, and Anhydro- X-pyranoside Synthesis (X = C, N) under Lewis acid Catalyzed Conditions
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A variety of C-glycosides can be obtained from the fluoroarylborane (B(C6F5)3) or silylium (R3Si+) catalyzed functionalization of 1-MeO- and per-TMS-sugars with TMS-X reagents. A one-step functionalization with a change as simple as the addition order and/or Lewis acid and TMS-X enables one to afford chiral synthons that are common (C-pyranosides), have few viable synthetic methods (C-furanosides), or are virtually unknown (anhydro-C-pyranosides), which mechanistically arise from whether a direct substitution, isomerization/substitution, or substitution/isomerization occurs, respectively.
- Seo, Youngran,Lowe, Jared M.,Romano, Neyen,Gagné, Michel R.
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Read Online
- Identification of the acid/base catalyst of a glycoside hydrolase family 3 (GH3) β-glucosidase from Aspergillus niger ASKU28
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Background The commercially important glycoside hydrolase family 3 (GH3) β-glucosidases from Aspergillus niger are anomeric-configuration-retaining enzymes that operate through the canonical double-displacement glycosidase mechanism. Whereas the catalytic nucleophile is readily identified across all GH3 members by sequence alignments, the acid/base catalyst in this family is phylogenetically variable and less readily divined. Methods In this report, we employed three-dimensional structure homology modeling and detailed kinetic analysis of site-directed mutants to identify the catalytic acid/base of a GH3 β-glucosidase from A. niger ASKU28. Results In comparison to the wild-type enzyme and other mutants, the E490A variant exhibited greatly reduced k cat and kcat/Km values toward the natural substrate cellobiose (67,000- and 61,000-fold, respectively). Correspondingly smaller kinetic effects were observed for artificial chromogenic substrates p-nitrophenyl β-d-glucoside and 2,4-dinitrophenyl β-d-glucoside, the aglycone leaving groups of which are less dependent on acid catalysis, although changes in the rate-determining catalytic step were revealed for both. pH-rate profile analyses also implicated E490 as the general acid/base catalyst. Addition of azide as an exogenous nucleophile partially rescued the activity of the E490A variant with the aryl β-glucosides and yielded β-glucosyl azide as a product. Conclusions and general significance These results strongly support the assignment of E490 as the acid/base catalyst in a β-glucosidase from A. niger ASKU28, and provide crucial experimental support for the bioinformatic identification of the homologous residue in a range of related GH3 subfamily members.
- Thongpoo, Preeyanuch,McKee, Lauren S.,Araújo, Ana Catarina,Kongsaeree, Prachumporn T.,Brumer, Harry
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p. 2739 - 2749
(2013/08/24)
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- Direct azidation of unprotected carbohydrates under Mitsunobu conditions using hydrazoic acid
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A single step procedure for the direct and regioselective synthesis of carbohydrate azides from unprotected sugars using hydrazoic acid under Mitsunobu conditions is reported. A series of mono-, di-, or triazido polyhydroxylated systems are described.
- Besset, Céline,Chambert, Stéphane,Fenet, Bernard,Queneau, Yves
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experimental part
p. 7043 - 7047
(2010/02/28)
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Azidation of some unprotected aldoses with the Ph3P-N-chlorosuccinimide-LiN3 system leads regioselectively to glycosyl axides ; 1,2-trans compounds are obtained stereoselectively.
- Larabi, Moulay-Larbi,Frechou, Catherine,Demailly, Gilles
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p. 2175 - 2178
(2007/10/02)
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