20 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



HO 



(6) 

 CH.OH 



HO 



a form 



HO 



D-glucopyranose 



CH„OH 



OH 



j8 form 



Although they are both dextrorotatory, the specific rotations of the two 

 forms of D-glucopyranose are different. 



The unstable form of glucose is glucofuranose. This compound also 

 exists in the two forms a and /3. 



HOH,C — HOHC 



'(«) (n 



H ^(«) 



OH HOHj C — HOHC 



HO 



OH 



HO 



D-glucofuranose 



One can prepare crystals of the a form or of the j8 form of D-gluco- 

 pyranose. The first will be obtained by crystallization from aqueous 

 solution, and the second by crystallization from pyridine. The two forms 

 are distinguishable by a number of properties, as, for example, their melting 

 point and their specific rotation in a freshly made solution. In solution, 

 one form changes into the other, so that the rotation changes until an 

 equilibrium is reached (mutarotation equilibrium). In the formulae of 

 Haworth, the keto and aldehyde groupings are not written as in the linear 

 formula, they are replaced by a potential-aldehyde or a potential-ketone 

 group. 



Ov OH 



■Ov H 



Potential aldehyde group 



Oy^ CH^jOH 



" ^ OH 



OH 



CHoOH 



Potential ketone group 



The sugars are in effect reducing agents, the same as the aldehydes and 

 ketones, but they do not restore the colour to fuchsin bleached by SOg. 



Haworth's formulae are now generally accepted. They give a clearer idea 

 of the relationship of the various groups than the ordinary cyclic formulae 

 or the linear formulae, although any of the three may be met with. 



