REACTIONS OF THE QUINONES 443 



(1941 a) wrote the reaction of l,2-naphthoquinone-4-sulfonate with amino 

 acids as: 



H3N-C<^^. 



. XOO 



HN— CC' 



Reaction with cysteine and thioglycolate gives similar products, but with 

 ghitathione, glufamate, and glycine the products are spectroscopically 

 much different (Neubeck and Smythe, 1944). o-Benzoquinones apparently 

 react in a comparable manner with amino acids, substitution occurring on 

 the 4- or 5-position, with further complex oxidation and polymerization 

 reactions often occurring (Hackman and Todd, 1953). Proline combines 

 with o-benzoquinone through the ring N to give a quinoneimine derivative 

 (Mason and Peterson, 1955). 



The reactions of quinones with amino acids, amines, or other N-contain- 

 ing compounds may thus be quite complex, and there is much more to be 

 learned about the mechanisms involved. Although some of the additions 

 of amino acids are reasonably rapid, in all cases directly compared, the ad- 

 dition of SH groups occurs more readily. 



Reactions with Proteins 



p-Benzoquinone reacts with many proteins (ovalbumin, seralbumin, fibrin, 

 nuclein, legumin, and others) and peptones to give deep red products (Ra- 

 ciborski, 1906). Toluquinone and p-xyloquinone generally behave similarly. 

 Wool proteins, such as keratin, readily react with a variety of quinones 

 to give colored products (Suida, 1913). Gelatin exposed to 2>-benzoquinone 

 turns yellow and then brown, and loses its ability to dissolve in water, 

 while heat-denatured ovalbumin becomes dark brown and resistant to 

 trypsin and pepsin when so treated (EUinger, 1923). The mechanism of 

 the antibacterial action of the quinones has been sought in reactions with 

 the bacterial proteins, in a manner somewhat analogous to the tanning of 

 tissues (Cooper, 1913). Reaction of proteins with formaldehyde prevents 

 the formation of colored products with quinones and it was therefore con- 

 cluded that the amino groups are the sites of attack. The relative reactivity 

 ratio for p-benzoquinone and toluquinone derived from several proteins is 

 around 1.5, less than with most free amino acids (see tabulation on page 

 441), and much less than when determined with respect to the antibacterial 

 activities (Cooper and Haines, 1928). Hilpert (1925) believed that quinones 

 oxidatively degrade the amino acids of proteins, this causing marked 

 changes in the protein structure and properties. Early work of this type 



