CARBOHYDRATES 



51 



l^liese sU uclures appear to be mirror images of each other. Rotation 

 oi one cannot yield the other, and they re])resent different com- 

 pounds. Actually, of course, the four bonds drawn above do not lie 

 in a single plane but extend at angles giving the relatively regular 

 jiyramidal structure: 



Shown in perspective, the chemical bonds are heavy lines connecting 

 the groups with the carbon atoms located at the points of intersection 

 of these lines. The dashed lines are included to show the overall 

 pyramidal shape. In the particular orientation chosen, the pyramids 

 are lying on one edge in the plane of the paper with faces partly 

 opened toward each other. In each case the bond to the group X 

 projects at an angle through the plane of the paper and away from 

 the observer. Neither structure can be superimposed on the other 

 merely by reorientation. For example, if the structure on the left is 

 flipped over to place groups W, Y, and Z on those of the right-hand 

 structure, group (or atom) X projects at an angle out of the paper to- 

 ward the observer. Thus compounds differing in this way are said to 

 have different configurations about the central carbon atom and are 

 called optical isomers when the atoms or groups differ between the 

 structures only in their spatial arrangements. 



Polarized light transmitted by a solution of one of these forms is 

 rotated one way and in the other direction by a solution of the other 

 optical isomer. The magnitudes of rotations are the same since the 

 groups themselves are identical. The difference in direction is desig- 

 nated with a plus sign, the letter d, or the prefix dextro for clockwise 

 rotation, and a minus sign, the letter /, or the prefix levo for counter- 

 clockwise rotation. Since the substituent groups of the two isomers are 

 identical, the chemical properties of the compounds are also identical 

 in many cases. Likewise, the physical properties other than optical 

 rotation are often identical. However, the biological properties are 

 usually different, often spectacularly so. Hence, this particular type 

 of structural consideration is important in the study of biochemistry. 



