440 5. QUINONES 



sulfonate with cysteine shows a high degree of specificity and requires an 

 appropriately situated amino group, as well as the work of Neubeck and 

 Smythe (1944) on the same reaction in which a product with an absorption 

 peak around 500-510 mjti was obtained, whereas with glutathione reaction 

 occurred but the absorption curve was quite different. However, with 1,2- 

 naphthoquinones, one is less certain of the nature of the reaction, and 

 Kuhn and Hammer (1951) state that 1,2-naphthoquinone does not react 

 with cysteine. Under certain conditions quinones can react with thiols to 

 form quinone-mercaptols, 0:9^:(SR)2, which are easily oxidized to quinone- 

 disulfones, 0:9?:(S02R)2, two SH groups having replaced a quinone atom, 

 but it is doubtful if such products are formed under physiological conditions 

 (Recsei, 1927). 



Reactions with Amino Groups 



The reaction of quinones with amines and amino acids has been known 

 since the 1880's and it is generally agreed that the mechanism is a 1,4 

 addition, usually with subsequent oxidation: 



OH 



OH 



OH 



exactly as discussed for the thiols in the previous section. A second addition 

 followed by a second oxidation may occur, producing 2 hydroquinone 

 molecules and utilizing 3 quinone molecules (the hydroquinone may, of 

 course, be reoxidized), as in the classic experiments of Fischer and Schrader 

 (1910), who reacted glycine esters with 2>-benzoquinone to form 



C2H5OOC— CHj— NH 



CHj- C60C2H5 



which separated as red crystals from the alcoholic solution. Similar reac- 

 tions occur with alanine esters and with toluquinone instead of p-henzo- 



