How many disaccharides of d-glucopyranose are possible?

There are 19 possible disaccharides of D-glucopyranose.

D-glucopyranose exists in both α and β forms.

The glycosidic bond can be formed between the anomeric hydroxyl group on one glucose unit and any hydroxyl group on the other glucose unit.

The possible linkages are:

α-Glucose + α-Glucose

• α-(1→1)-α [trehalose]
• α-(1→2) [α-kojibiose]
• α-(1→3) [α-nigerose]
• α-(1→4) [α-maltose]
• α-(1→6) [α-isomaltose]

α-Glucose + β-Glucose

• α-(1→1)-β [α,β-trehalose]
• α-(1→2) [β-kojibiose]
• α-(1→3) [β-nigerose]
• α-(1→4) [β-maltose]
• α-(1→6) [β-isomaltose]

β-Glucose + α-Glucose

• β-(1→1)-α [but this is the same as α-(1→1)-β]
• β -(1→2) [α-sophorose]
• β -(1→3) [α-laminaribiose]
• β -(1→4) [α-cellobiose]
• β -(1→6) [α-gentiobiose]

β-Glucose + β-Glucose

• β-(1→1)-β [β,β-trehalose]
• β -(1→2) [β-sophorose]
• β -(1→3) [β-laminaribiose]
• β -(1→4) [β-cellobiose]
• β -(1→6) [β-gentiobiose]

So there are 19 possible disaccharides of D-glucopyranose.