62 UNITY AND DIVERSITY IN BIOCHEMISTRY 



{c) Sterides 



The sterides are esters of fatty acids and sterols. Lanoline, the fat ob- 

 tained from wool, is a mixture of cholesterol oleate, palmitate, and stearate. 



B. Natural Phosphoric Esters 



{a) Phosphoric Acids 



Orthophosphoric acid, H3PO4, possesses three acid groups which ionize, 

 one after the other, as the pH increases. 



OH 



/ 



0=P— OH 



\ 

 OH 



The first ionization corresponds to a pK of 1-97, so that this group is 

 relatively strongly acidic, and the ionization is complete at a very acid 

 pH, well outside the pH range of biochemical interest. The second acid 

 group (pK = 6-82) is comparable to organic acids in strength. It is this 

 group, when combined with a strong base, which acts as a buffer in the 

 acid-base equilibrium of biological environments. The third acid group is 

 only slightly dissociated, only forming salts in very alkaline solution, out- 

 side the biochemical range. Orthophosphoric acid can form phosphoric 

 esters with alcohols. Three types exist : monoesters, diesters and triesters — 



OR OR OR 



/ / / 



0=P— OH 0=P— OR' 0=P— OR' 



\ \ \ 



OH OH OR" 



In the biosphere, triesters of orthophosphoric acid are unknown, 

 however, this does not exclude the possibility that this binding may be 

 present in certain macromolecules. The diesters of orthophosphoric acid 

 which exist in the biosphere are often mixed esters. Acid or alkaline 

 hydrolysis slowly transforms them into monoesters. Most of the complex 

 lipides are diesters and vitamin B^g also falls into this category. The mono- 

 phosphoric esters of alcohols form a very important biochemical group. 

 The two free acid groups are more strongly acidic than when they were 



REFERENCES 



Deuel, H. J. Jr. (1951) The Lipids. Their Chemistry and Biochemistry, Vol. I : 



Chemistry. Inter-science, New York. 

 HiLDiTCH, T. P. (1947). The Chemical Constitution of Natural Fats, 2nd ed., 



Chapman and Hall, London. 

 LovERN, J. A. (1955). The Chemistry of Lipids of Biochemical Significance. Methuen, 



London. 



