30 UNITY AND DIVERSITY IN BIOCHEMISTRY 



For example in the case of threonine, the four stereoisomers would have 

 the following formulae : 



COOH COOH COOH COOH 



The designation L- is reserved for the form found present in proteins, its 

 isomer having opposite configurations on carbon atoms a and j8, is distin- 

 guished by the prefix D-. The other isomers, which also form a pair of 

 optical isomers, but which are diastereoisomers, are denoted by the prefix 

 alio, as shown in the diagram for threonine. (In the case of cystine, there 

 are also two asymmetric carbon atoms, but since cystine is formed by the 

 union of two molecules of cysteine, then, as in tartaric acid, there are only 

 three isomers : ll, dd and meso, the meso form being optically inactive.) 



Although it is true, in general, that the natural amino acids are a-amino 

 acids of the l- series, it sometimes happens that in organisms one encoun- 

 ters stereoisomers of the d- series, for example, D-^S-thiolvaline, D-leucine, 

 D-alanine, etc. 



II. OTHER CHEMICAL STRUCTURES OF GENERAL INTEREST 



A. Terpenes 



The terpenes and their derivatives are a group of natural substances 

 related to isoprene CgHg. 



CH3 



I 

 CH2=C— CH=CH2 



Isoprene 



Isoprene does not exist as such in nature, but it is found in the biosphere 

 in the form of numerous polyisoprenes. These substances may be divided 

 into three groups : the lower terpenes, the carotenoids and the polyterpenes. 



REFERENCES 



DESNtTELLE, P. : The general chemistry of amino acids and peptides, in H. Neurath 



& K. Bailey, The Proteins, Vol. I, part A, 87-180. 

 Dunn, M. S. & Rockland, L. B. (1947) The preparation and criteria of purity of 

 the amino acids. Advanc. Protein Chem. 3, 295-382. 



