2 PRINCIPLES OF STRUCTURE 39 



C=:0 group have a symmetry plane. As a result of the ring forma- 

 tion, however, the i C-atom of the carbonyl group also becomes 

 asymmetric. For that reason two different configurations of the hetero- 

 cyclic ring are possible; they are called a- and /3-glucose (Fig. 35c and 

 d) and are distinguished by their optical rotation (/? shows the smaller 

 rotation). It is seen that the /S-glucose shows a regularly alternating 

 distribution of the H- and OH-groups on both sides of the ring, while 

 in a-glucose the hydroxyl groups at the i and 2 C-atoms are neighbours. 



With /3-glucose it is possible to lay a second bridge between the i 

 and the 6 C-atoms by dehydration (laevo-glucosan) ; in a-glucose this 

 is impossible. This proves that in /5-glucose the OH-group of the i 

 C-atom lies on the same side of the ring as the one of the 6 C-atom. 



The a and ^ positions of the OH-groups at the i C-atom are 

 fundamental to an understanding of the structure of disaccharides and 

 high-polymer carbohydrates. In disaccharide formation a 1-4-bridge 

 between two glucose rings is formed by loss of one molecule of water. 

 Now it is easy to see that in the case of the a-position the two rings 

 can simply be joined directly, whereas in the case of /3-position one 

 of the rings must first rotate through an angle of 180° around its 

 1-4-axis in order to bring the two OH-groups which are to react into 

 a neighbouring position. 



OH OH OH CHpOH 



CH^OH CH2OH CH^OH OH 



Fig. 37a. Maltose Fig. 37b. Cellobiose 



Disaccharides from glucose 



Both cases are realized in nature; in the first case maltose is formed 

 and in the second cellobiose, the disaccharide unit of the cellulose 

 chain (Fig. 37). In maltose the two glucose rings can be made to 

 coincide by a simple translation, whereas in cellobiose this requires 

 a digonal axis. The cellobiose molecule therefore possesses a higher 

 degree of symmetry, seeing that the coincidence must be achieved by 

 a combination of a translation and a rotation. 



The bond represented in Fig. 37a is described as a-glucosidic and 

 the one in Fig. 37b as /5-glucosidic. Instead of sugar molecules, all 



