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The formation of a glycosidic linkage results in the formation of a new stereogenic centre and therefore a mixture of products may be expected to result. The linkage formed may either be axial or equatorial (α or β with respect to glucose). To better understand this, the mechanism of a glycosylation reaction must be considered.
The reaction often favors formation of the α-glycosidic bond as shown due to the anomeric effect. A glycosidic bond is formed between the hemiacetal or hemiketal group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some compound such as an alcohol. A substance containing a glycosidic bond is a glycoside.
Mutation of the active site nucleophile to a non-nucleophilic amino acid prevents the formation of a covalent intermediate. An activated glycosyl donor with a good anomeric-leaving group (often a fluorine) is required. The leaving group is displaced by an alcohol of the acceptor sugar aided by the active site general base amino acid of the enzyme.
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Dextrins are mixtures of polymers of D-glucose units linked by α-(1→4) or α-(1→6) glycosidic bonds. Dextrins can be produced from starch using enzymes like amylases, as during digestion in the human body and during malting and mashing in beer brewing [3] or by applying dry heat under acidic conditions (pyrolysis or roasting).
Upon activation of the glycosyl donor group (Y) (usually SR, OAc, or Br group) in the next step, the tethered aglycon traps the developing oxocarbenium ion at C-1, and is transferred from the same face as OH-2, forming the glycosidic bond stereospecifically. The yield of this reaction drops as the bulkiness of the alcohol increases.
That equals about 12 ounces of regular beer (at 5% alcohol; some light beers have less) or 5 ounces of wine (at 12% alcohol) or 1.5 ounces of distilled spirits (at 40% alcohol or 80 proof ...
In the first step, the nucleophile attacks the anomeric centre, resulting in the formation of a glycosyl enzyme intermediate, with acidic assistance provided by the acidic carboxylate. In the second step, the now deprotonated acidic carboxylate acts as a base and assists a nucleophilic water to hydrolyze the glycosyl enzyme intermediate, giving ...