Question

Disaccharides are joined by glycosidic bonds formed between the anomeric carbon on one monosaccharide and a hydroxyl (-OH) group of another monosaccharide. Identify the types of linkages in each of the following three disaccharides.

Answer 1

Monosaccarides have to forms based on the orientation of the hydroxy group in the anomeric carbon (shown by asteri sk *). When the OH group is pointed upwards, the form is known as beta and when the hydroxyl group is facing downwards the form is known as alpha Он он OH OH Он hydroxyl up OH OH OH OH ОН hydroxy down beta alpha OH OH OH 1 OH The second unit is also beta OH 3 OH OH 2 4 OH the hydroxyl group that has reacted to form the glycosi dic bond was pointed upwards, hence, the first unit is a beta unit Hence, the Cl of first unit is joined to the C4 of the second unit via the glycosi dic bond formed from the hydroxyl group of first unit. Hence, the type of glycosi dic bond here is beta 1- 4.

OH the first hexose unit has downward hydroxyl group converted to glycosidic linkage OH он Он 6 4 CH-он 3 3 OH CH-он 1 The OH group is upwards in the pentose unit, hence it is a beta unit Since, the glycosidic bond joins C1 of alpha unit and C2 of beta unit, the type of glycosidi c bond is beta2 alpha 1 Note we have to specify anomeric fomm of both sugars as the glycosidic bond links both the anomeric carbons as OH is downward This is alpha OH Он Он OH OH 4 он This is beta as OH is upwards OH OH Here anomeric carbon of the alpha unit links to C4 of the beta unit. Hence, we need not specify the beta isom er name specifically in the glycosi dic bond Hence, the type of glycosidic bond here is alpha 1 - 4.

Hence, the answers are

$\beta 1 \rightarrow 4 , \beta 2 \leftrightarrow \alpha 1 \ and\ \alpha 1 \rightarrow 4$ respectively from left to right.