Question

10. (15 pts) Answer the following questions about two disaccharides, (a) maltose, or malt sugar. and (b) lactose, or milk sugar. CH, OH CH OH CH, OH CH, OH + oH HHH OHHO CH OHH OHHA OHN H OH (b) Он н х H OH Maltose contains two glucose monomers joined by an -1.4 glycosidic bond Lactose contains one galactose and one glucose monomer joined by a 14 ghycoside bond (a) (2 pts) Explain what is meant by "a", "1", and "4" in the term a-1,4 glycosidic bond. Circle this bond in the appropriate structure. (b) (2 pts) Explain what is meant by "B", "1", and "4" in the term B-1,4 glycosidic bond. Circle this bond in the appropriate structure.

Answer 1

HOH % CH OH OH OH H1 a) In the a-1,4-glycosidic bond the oxygen on the anomeric carbon i.e. C1, of the first ring is down and the linkage is between C1 of the first ring and C4 of the second ring. This linkage is present in maltose but not in lactose. ao 4 2 OM H OH OH CHLOH b) In the B-1,4-glycosidic bond the oxygen on the anomeric carbon i.e. C1, of the first ring is up and the linkage is between C1 of the first ring and C4 of the second ring. This linkage is present in Lactose but not in maltose. 4 OH HA OHH OH OH c) In case of a-1,6-glycosidic bond the oxygen on anomeric carbon of first ring is down and the linkage is between the C1 of first ring and C6 of the second ring. This linkage is present in the glycogen polysaccharide. d) Both maltose as well as lactose be able to open into straight chain as lactose and maltose have only one of their two anomeric carbons involved in the glycosidic bond, meaning that they can convert to an open-chain form with an aldehyde group.

Онон H- CH, OH OHH H OH HO 3H H4-OH H5OH 61 OH D-Glucose но- HO НЕОН OH OH SOH D-Galacto se ring D-Galactose D-Glucose and D-galactose both have all the configurations same at C2, C3, and C5. But they have difference in the confoguration at C4. Hence they are distereomers of each other. Maltose is made up of two glucose units and lactos eis made of glucose and galactose. In the glucose OH at C4 is on the right side of Fischer projection hence in cyclic structure it will go down. Whereas, in galactose, OH at C4 is on the left side and hence in the ring form it should be going up. Thus in lactose the first ring is D-Galactose because the OH at C4 in the first ring is going up.