MODES OF LINKAGE BY COVALENT BONDS 79 



COOH 



NH,— CH,-CH,— CO— NH— CH-CHo— C=CH 



HM Ts^ 



\// 

 CH 



Carncsine 



COOH 

 I 

 NH2-CH,-CH2-CO-NH-CH-CH.-C=CH 



H3C— N N 



\// 

 CH 



Anserine 



|8-alanine, which is present in carnosine and anserine, is not a consti- 

 tuent of proteins, but one finds it in other natural substances such as panto- 

 thenic acid, which is a dihydroxy-dimethyl-butyryl-/3-alanine. 



CH3 OH 



HO-CH2-C CH-CO-NH-CH2-CH2-COOH 



1 

 CH3 



Pantothenic acid 



Interest in natural peptides has greatly increased during the last few 

 years since a great number of antibiotics have been found to be poly- 

 peptides. 



(b) Antibiotics 



Many antibiotics are peptides which are produced by microorganisms 

 and possess antibacterial properties. 



The gramicidines produced by Bacillus brevis, for example, are cyclic 

 peptides having a molecular weight around 400, and containing chiefly 

 L-trytophane and D-leucine together with smaller amounts of D-valine, 

 L-valine, L-alanine, glycine and ethanolamine. One of the characteristics 

 of natural antibiotic peptides is that one finds in their structure amino acids 

 which are never present in proteins, or the D-stereoisomers of the natural 

 L-forms present in proteins. 



