Disaccharides: Difference between revisions

<scene name='Disaccharides/Maltose/4'>Maltose</scene> (α-D-glucopyranosyl (1→4) D-glucopyranose) and <scene name='47/471802/Cellobiase/1'>cellobiose</scene> (β-D-glucopyranosyl (1→4) D-glucopyranose) are both disaccharides made of D-glucopyranose. Comparing the two structures given below you can observe that both have a 1→4 glycosidic bond<ref>[http://en.wikipedia.org/wiki/Glycosidic_bond Glycosidic bond]</ref>. C-1 (orange) of one glucose unit is bonded to the oxygen of C-4 of the second unit. The difference between the two is that maltose is α(1→4) and cellobiose is β(1→4). As you study the two structures notice that with cellobiose the second glucopyranose unit is rotated 180°, so that the oxygen bonds of both C-1 (β configuration) and C-4 of the second glucopyranose unit are projecting up so that oxygen has its normal angular geometry. In order to see the second glucopyranose in its normal position, rotate cellobiose 180° about the x axis so that C-6 is in the back of the ring and projecting upward. Both structures show the anomeric carbon (green) of the second glucose unit as the β anomer, but in an aqueous solution that designation would not be significant because there would be an equilibrium mixture of the α and β anomers and the open-chain structure. The open-chain structure in an aqueous solution provides an aldehyde group which can be oxidized, so maltose and cellobiose are reducing sugars<ref>[http://en.wikipedia.org/wiki/Reducing_sugar Reducing sugar]</ref>. Also, notice the sharp bend in the maltose at the glycosidic bond. Most text books do not represent the structure of maltose in a way that shows this bend. View this <scene name='Disaccharides/Maltose2/2'>bend</scene> or <scene name='47/471802/Cellobiase/2'>lack of bend</scene> in spacefill. This kink produced by the α(1→4) glycosidic bond plays an important role in the structures of polysaccharides that have α(1→4) linkages.