Preparation and conformation of α-l-arabinofuranosyl-pyridinium salts, and hydrolysis of the 4-bromoisoquinolinium compound

One-Step Aqueous Synthesis of Glycosyl Pyridinium Salts, Electrochemical Study, and Assessment of Utility as Precursors of Glycosyl Radicals Using Photoredox Catalysis

A single step method for the production of unprotected glycosyl pyridinium salts has been developed involving treatment of the unprotected sugar with a pyridine, triethylamine, and either 2-chloro-1,3-dimethylimidazolinium chloride (DMC) or 2-chloro-1,3-dimethyl-1 H-benzimidazol-3-ium chloride (CDMBI) as an activator, in aqueous solution. Reaction efficiency is sensitive to steric effects, and in particular, ortho-substitution of the pyridine ring significantly decreased conversion to product; para-substitution of the pyridine ring is well tolerated. Cyclic voltammetry reveals that glycosyl pyridinium salts possess reduction potentials in the range of -0.9 to -1.4 V versus the standard calomel electrode, which are modulated by the electron effects of ring substituents. However, glycosyl pyridiniums are not found to be useful precursors for the production of glycosyl radicals under photoredox conditions.

Variation in relative substrate specificity of bifunctional β-d-xylosidase/α-l-arabinofuranosidase by single-site mutations: Roles of substrate distortion and recognition

To probe differential control of substrate specificities for 4-nitrophenyl-alpha-l-arabinofuranoside (4NPA) and 4-nitrophenyl-beta-d-xylopyranoside (4NPX), residues of the glycone binding pocket of GH43 beta-d-xylosidase/alpha-l-arabinofuranosidase from Selenomonas ruminantium were individually mutated to alanine. Although their individual substrate specificities (kcat/Km)(4NPX) and (kcat/Km)(4NPA) are lowered 330 to 280,000 fold, D14A, D127A, W73A, E186A, and H248A mutations maintain similar relative substrate specificities as wild-type enzyme. Relative substrate specificities (kcat/Km)(4NPX)/(kcat/Km)(4NPA) are lowered by R290A, F31A, and F508A mutations to 0.134, 0.407, and 4.51, respectively, from the wild type value of 12.3 with losses in (kcat/Km)(4NPX) and (kcat/Km)(4NPA) of 18 to 163000 fold. R290 and F31 reside above and below the C4 OH group of 4NPX and the C5 OH group of 4NPA, where they can serve as anchors for the two glycone moieties when their ring systems are distorted to transition-state geometries by raising the position of C1. Thus, whereas R290 and F31 provide catalytic power for hydrolysis of both substrates, the native residues are more important for 4NPX than 4NPA as the xylopyranose ring must undergo greater distortion than the arabinofuranose ring. F508 borders C4 and C5 of the two glycone moieties and can serve as a hydrophobic platform having more favorable interactions with xylose than arabinofuranose.