1559Tch24423-43810/20/054:54 M Page424 EQA 424 chapter 24 CARBOHYDRATES:POLYFUNCTIONAL COMPOUNDS IN NATURE 24-2and24-3. Conforr ations and Cyclic Forms of Sugars;Mutaroh typically with either five-or sixmembered ring.open-chain structure intoa picture of the cyclic hemiacetal is tricky:Here's a step-by-step way to do it. 1.Write in wedges and dotted lines and (for the carbonyl group. CHO CHO OH OH HO- -CHO OH OH OHH OH H- OH -OH H.OH CH.OH D-Glucose is shown step-by-step in the next paragraph.) By convention,the p cedure for L-series sugars is modified such that the top of the structure is rotated down d tside.That allows the andstructures tookemage when placed 川 Ho B-D-Glucopyranos B-L-Glucopyranose24-2 and 24-3. Conformations and Cyclic Forms of Sugars; Mutarotation Because sugars contain alcohols and carbonyl groups, they can (and usually do) form cyclic hemiacetals, typically with either five- or six-membered rings. Translating an open-chain structure into a picture of the cyclic hemiacetal is tricky: Here’s a step-by-step way to do it. 1. Write in wedges and dotted lines and (for a D-series sugar) lay the structure on its side, with the top moved down to the right. Then, locate the OH group that you will use to form the cyclic hemiacetal with the carbonyl group. 2. Rotate around the COC bond to the right of the OH group you picked out until the OH is horizontal and pointing away from you. (Make a model!) Then wrap this left-hand end of the chain behind the plane of the paper to put the OH near to the carbonyl carbon. Finally, make the hemiacetal bond. (This sequence is shown step-by-step in the next paragraph.) By convention, the procedure for L-series sugars is modified such that the top of the structure is rotated down to the left when it is laid on its side. That allows the L and D structures to look like mirror images when placed side-by-side. Mirror
-D-Glucopyranose CH2OH O OH OH OH HO
-L-Glucopyranose CH2OH O HO HO OH HO Will form a five-membered ring (a furanose) Will form a six-membered ring (a pyranose) HOCH2 CHO H H OH H OH OH H OH D-Glucose CH2OH CHO OH HO H H OH H OH H CH2OH CHO OH HO H H OH H OH H 424 • Chapter 24 CARBOHYDRATES: POLYFUNCTIONAL COMPOUNDS IN NATURE 1559T_ch24_423-438 10/20/05 4:54 AM Page 424