Organic Substances 



53 



known potential aldehydes occurring in the animal body are 

 certain lipid substances and the unidentified aldehyde of 

 elastic tissue. Whenever an aldehyde reaction is obtained 

 under specific conditions in a tissue defatted with lipid sol- 

 vents, it is attributed to the presence of carbohydrate. 



Aldehyde reactions are not given directly by any of the 

 carbohydrate substances occurring in the tissues. The alde- 

 hyde groups have to be "liberated" first by certain chemical 

 agents, oxidative or hydrolytic. In the case of oxidants, "liber- 

 ation" is a deceptive term to describe their action. True, all 

 aldoses have a potential aldehyde group, masked by the 

 pyranose or furanose ring formation. However, it is not this 

 aldehyde group which is "set free" or "revealed" oxidatively 

 but rather entirely new aldehyde groups which are created 

 in the middle of the carbon chain of the sugar molecule. 



The action of one of the oxidants, periodic acid, is fairly 

 well understood since the studies of Malaprade,^ Jackson and 

 Hudson,^ Nicolet and Shinn,^ Hotchkiss,'^ and others. It con- 

 sists of the breaking of the carbon chain through a glycoHc 

 or HO-C-C-NH2 group and in the oxidizing of the broken 

 ends to aldehyde groups according to the following schemes : 

 H H H H 



R— C— C— Ri + HIO4 = R— C=0 + 0=C— Ri + HIO3 + H2O or 



O O 



H H 

 H H 



H 



H 



R— C— C— Ri + HI04= R— C=0+0=C— Ri+ (NHJIOa 



O N 



H H2 



4. Malaprade, L.: Bull. soc. chim. France, 43:683, 1928; Malaprade, L.: 

 ibid., ser. 5, 1:833, 1934. 



5. Jackson, E. L., and Hudson, C. S.: J. Am. Chem. Soc, 60:989, 1938. 



6. Nicolet, B. H., and Shinn, L. A.: J. Am. Chem. Soc, 61:615, 1939. 



7. Hotclikiss, R. D.: Arch. Biochem., 16:131, 1948. 



