24 



CARBOHYDRATES 



OO OO OO YO 



D L 



AUose 



D L 



Altrose 



D L 



Mannose 







D L 



Glucose 



D L 



Gulose 



D L 



Galactose 



D L 



Idose 



D L 



Talose 



Configuration of stereoisomeric aldohexoses 



Note that the D-forms are those in which the configuration of the asym- 

 metric carbon farthest from the aldehyde group, i.e., carbon 4 in pentoses 

 and 5 in hexoses, is the same as that in D-glyceraldehyde. 



Optical rotation 



Substances containing asymmetric carbons can also rotate polarized 

 light. ^ If such light is passed through a solution of D-glyceraldehyde, for 

 example, the emergent beam will be twisted a certain number of degrees. 

 The amount of twisting is measured in an instrument called a polarimeter. 

 This effect is called optical rotation, and substances which show it are 

 said to be optically active. When measured under specified conditions, 

 the angle of rotation is called the specific rotation and is a characteristic 

 property of the optically active substance. 



It is important to note that there is no necessary relationship between 

 configuration and the sign of optical rotation. The D-forms may show 

 either positive or negative rotation.- 



Cyclic formulas of monosaccharides 



The aldose formulas have been written above in the *'open-chain" or 

 "free aldehyde" form. Certain properties of the aldoses indicate that an 

 aldehyde group is present (combination with phenylhydrazine; for ex- 

 ample, to form hydrazones or osazones). However, other properties 



^ This light vibrates in only one plane, as contrasted with ordinary light which vi- 

 brates in all possible planes. The student ma.v visualize a polarized light beam as a 

 Hat, narrow ribbon of light. 



** Positive rotation is clockwise rotation when you look toward the light source. 



