Chapter 10 



AMINO ACIDS AND 



±KU 1 hi IS J by Ernest Sondheimer 



Amino acids, peptides and proteins are among the most important and well-known 

 constituents of living matter. Consequently much of their chemistry and metabolic sig- 

 nificance is adequately described in general textbooks of biochemistry. In our treatment 

 we have chosen to stress some less familiar aspects of them and to emphasis a few un- 

 usual compounds which occur in plants. 



AMINO ACIDS 



Amino acids can be defined as carboxylic acids having at least one amino group. 

 Most naturally occurring ones can be depicted by the general formula R CH(NH3+)C02~. 

 These are the so-called alpha amino acids since the amino group is adjacent to the car- 

 boxylic acid function. If R is a group other than hydrogen, the compound has an asym- 

 metric carbon atom and can, therefore, occur in an optically active form. The isomers 

 having the L-configuration are the ones most widely distributed in nature. The first 

 amino acid isolated was asparagine. This substance was obtained by Vauquelin and 

 Robiquet in 1806 (1) who succeeded in separating it from asparagus juice by taking ad- 

 vantage of differences in the shape, transparency and flavor of the crystals. Since then 

 chemists have continued their attempts to isolate new amino acids; and although more 

 than seventy such compounds have been obtained from higher plants, new ones are being 

 discovered yearly. It is convenient to divide the amino acids into two major groups; one 

 group is found in all living systems either in the free state or condensed as peptides while 

 the members of the second group apparently occur only in a limited number of organisms 

 and do not serve as protein monomers. 



The physical properties of the amino acids are to a very large extent determined by 

 the dipolar ionic structure of these compounds. Thus, they are all white solids. They 

 either have high melting points or decompose on heating. As a group they display much 

 greater solubility in water than in organic solvents. Since they are amphoteric they form 

 salts with acids or bases. Under appropriate conditions they will migrate in an electric 

 field, i. e. show electrophoretic properties. Each amino acid has one pH value at which 

 there is no net charge on the molecule and at which, therefore, no migration in an elec- 

 tric field will occur. This pH value is called the isoelectric point. It is between pH5and 

 6 for the "neutral" amino acids - the mono carboxylic, mono amino compounds, near pH 

 3 for the dicarboxylic mono amino acids, and above 7. 5 for the basic amino acids. The 

 latter group includes diamino mono carboxylic acids and mono amino mono carboxylic 

 acids with other basic substituents. 



WIDELY DISTRIBUTED AMINO ACIDS 

 AND PROTEIN CONSTITUENTS 



The structure and Rf values of the most widely distributed amino acids are listed 

 in Table 1. These are also the amino acids commonly found as the monomer ic units of 



- 205 - 



