144 BIG MOLECULES 



alcohol has the following atomic arrangement: 



H H H 



I I I 

 H— C— C— C— OH 



I I I 

 H H H 



However, in isopropyl alcohol the OH group is attached to the central car- 

 bon atom instead: 



H OHH 



I I I 

 H— C— C— C— H 



I I I 

 H H H 



"Normal" melts at -127° and boils at 98° C, while "iso" melts at -89° 

 and boils at 82° C. Normal chlorinates slowly in PC1 3 , iso chlorinates 

 rapidly. 



Not all isomers are so obvious. Consider adrenaline, which has the struc- 

 tural formula 



HO 

 HO < > — C HOH CH 2 NH CH 3 



Two forms exist, which differ only in the arrangement of the groups of atoms 

 attached to the tetrahedral carbon atom starred. The two forms differ in 

 optical rotation. One is physiologically active; the other is not. 



As we proceed through the higher alcohols— for example, those with four 

 carbon atoms or more and two OH groups— the stereoisomeric possibilities 

 mount. In the sugars and celluloses in which rings of carbon atoms are 

 linked to one another to form long chains, each carbon having an OH group, 

 physical interference with free rotation about an interatomic bond adds 

 further to the number of possibilities. In molecules of the size of nucleic 

 acid molecules, the number of structurally different possibilities is enormous. 



Thus (the example is Schroedinger's) the two characters of the Morse 

 code, dot and dash, can be arranged in groups of four-character letters in 

 30 different ways. If, however, we have a system of even five characters, and 

 if five copies of each of the five characters are arranged into linear code- 

 scripts of 25 characters, the total number of possible 25-character code- 

 scripts is an astronomical 63 x 10 12 — that is, 63 million millions! Note that 

 even though the total number of characters chosen to define uniquely the 

 "isomer" is only 25, the number of possibilities is hard to envisage; and 

 indeed this number does not count any arrangements with either side-chains 

 or rings, and is limited even further in that it excludes anything but five 



