CONSTITUENTS OF THE BIOSPHERE 



29 



There are also many iminoacids, such as pipecolic acid, baikiaine, 

 guvacine, etc. 



C 



/ \ 

 H2C CH:, 



H 

 C 



// \ 

 HC CH2 



H2C 



CH— COOH 



H2C 



CH— COOH 



\ / 



N 

 H 



Pipecolic acid 



\ / 



N 

 H 



Baikiaine 



H 

 C 



/ ^ 

 HjC C— COOH 



HoC CH2 



\ / 



N 

 H 



Guvacine 



(d) Stereoisomerism 

 Whilst the natural sugars are for the most part members of the D-series, 

 the natural amino acids, regardless of their optical activity, belong as a rule 

 to the L-series, 



Natural L-form 



D-form 



In addition there are certain amino acids which, in addition to the a 

 carbon atom, contain a second asymmetric carbon (threonine, hydroxy- 

 proline, isoleucine, hydroxy lysine). These amino acids can theoretically 

 exist in four isomeric forms. 



