370 CARBOHYDRATE METABOLISM 



light caused by a solution made in the proportion of i g. of the sugar to i cc. 

 of solution, observed through a depth of lO cm. at a temperature of 20° C. 

 Measurements of the rotatory power of substances are made with an instru- 

 ment known as a polnrinieter. Compounds which rotate the plane of polarized 

 light to the right (in a clockwise direction) are called dextrorotatory ; those 

 which rotate it to the left (in a counter-clockwise direction), levorotatory. 



3. Isomerism. — An important fact regarding the sugars is that many 

 isomers may exist with the same molecular formula. There are two impor- 

 tant types of these, ordinary chemical isomers, and stereoisomers. The former 

 are due to differences in atomic groupings within the molecule. The difference 

 between two isomers of this type may be seen by comparing the structural 

 formulas for <f-glucose and that for ^-fructose (see later). Although the 

 molecular formula of both is the same (CgHjoOo) ^-glucose is an aldehyde 

 derivative while ^-fructose is a ketone derivative. 



Stereoisomers are illustrated diagrammatically for ^-glucose and ^-man- 

 nose. Here exactly the same atomic groupings are present, but they are 

 arranged in different patterns around the asymmetric carbon atoms. 



c=o c=o 



I I 



H— C-OH HO— C— H 



I 1 



HO— C— H HO— C— H 



I I 



H— C— OH H— C-OH 



I I 



H-C-OH H— C-OH 



H— C-OH H— C— OH 



(J -glucose d-mannose 



Two stereoisomeric forms exist for each of the simple sugars, the arrange- 

 ment for /-glucose and /-mannose being the exact reverse of the above patterns. 

 One important difference between these two forms of any compound is that 

 one (usually the d or dextro form), rotates the plane of polarized light to the 

 right, while the other form (usually the / or levo form), rotates it an equal 

 number of degrees to the left. Mixtures of equimolar quantities of these two 

 forms are optically inactive. In sugars, however, the prefixes d and / do not 

 necessarily refer to the direction of their specific rotatory power as they do 

 when prefixed to the names of all other organic compounds, (i-fructose, for 

 example, is strongly levorotatory. Sugars are designated d or I, not because 



