56 GENERAL BIOCHEMISTRY 



and water. When HOR is any alcohol including another carbohy- 

 drate, the product is called a glycoside, but when ROH is limited to 

 the carbohydrates, a larger carbohydrate is obtained. Repetition of the 

 latter process yields carbohydrates of great molecular size. Obviously 



CH2 O CH2 O 



/ \ / \ 



CHOH CHOH + HOR ^ CHOH CHOR + H,0 



\ / \ / " 



CHOH— CHOH CHOH— CHOH 



the reaction to form carbohydrates is difficult to control in the labora- 

 tory because both reactants are semiacetals and both possess several 

 hydroxyl groups, so a diversity of similar reactions is possible, all lead- 

 ing to acetals. 



Muta rotation 



The formation of the ring systems of the semiacetal forms of carbo- 

 hydrates introduces a center of asymmetry at the carbon atom origi- 

 nally bearing the carbonyl group. Thus two isomers become possible 

 for each carbonyl structure. These isomers possess somewhat different 

 properties and are usually known as the a and ^ forms. The conven- 

 tional designation depends on the configuration of the semiacetal 

 group relative to the configuration of the asymmetric carbon most 

 remote from this semiacetal group. 



When writing the structures of carbohydrates in the forms used here, 

 at least two different conventions must be employed. If the asymmetric 

 carbon atom of highest number (most remote) carries a free hydroxyl 

 group, the a isomer is written with this hydroxyl group on the same 

 side of the ring as the hydroxyl group of the semiacetal structure. The 

 P isomer is shown with these two hydroxyl groups on opposite sides 

 of the ring; see below. If the hydroxyl group of the asymmetric carbon 

 atom of highest number is involved in the semiacetal linkage, the 

 attached hydroxymcthyl group and the hydroxyl group of the semiace- 

 tal structure are written on opposite sides of the ring for « isomers 

 and on the same side for (S isomers. In certain cases not discussed in 

 this book, still other conventions must be employed. Both « and ^ 

 forms are stable in the solid state and may be synthesized or isolated. 



In aqueous solution, pyranose and furanose rings open reversibly. 

 A solution of a pure a-pyranose, for example, changes steadily in opti- 

 cal rotation to an equilibrium value representating a mixture of both 

 a and y8 forms with perhaps a small fraction of the molecules in the 

 carbonyl form, as in the above reactions. The relative preponderance 



