272 



X-ray Analyses of Proteins and Nucleic Acids / 1 5 : 2. 



where R is either H or any of a number of different organic radicals. 

 This form is called an a-amino acid. 



If one makes a three-dimensional model of such an a-amino acid, 

 there are two steric arrangements of the a carbon which cannot be 



CH 2 OH 



CH.OH 



6 C— H 



I 

 5 H — C— OH 



4 HO — C— H 



3 H — C — OH 



2 H— C— OH 



1 H— C— OH 



I 

 H 



1(a) 



Straight-chain 



d-glucose 



OH 



CH-..OH 

 HOCH 



5 



0\ H 



\QU H/i 



h12 



i 

 H 



OH 



t 



Oh 



I ! i 



H OH H 



1(b) 1(c) 1(d) 



(X-pyranose ring, d-glucose fi-pyranose ring, d-glucose ffranose ring, d-glucose 



CH 2 OH 



°\CH 2 OH 



HO 



H 



1(e) 

 Chair-model, d-glucose 



CH 2 OH CH 2 OH 



O 



O. H 



JH HO/ 



/CH 2 OH 



H OH OH H 



3. Sucrose 



CH 2 OH CH 2 OH 



°,H HA ° x OH 



l/H \| 



-0-\? H H /< 

 h 6h h oh 



4. Maltose 



CH 2 OH CH 2 OH CH 2 OH CH 2 OH 



° X H hJ °,H H^ O v H H/\~ 



-o-^ \ / L --Q 



OH H OH 



5. Glycogen 



H OH 



Figure 3. Some typical carbohydrates found in living cells. 

 All hexoses can exist in solution in at least five forms : a straight 

 chain, two six-membered (pyranose) rings and two five- 

 membered (furnose) rings. For glucose, most of the molecules 

 are in the pyranose forms. The chair model is closer to the 

 actual molecular arrangement but is harder to draw. The 

 di- and polysaccharides exist in only one form. 



