CHAPTER IV 



MACROMOLECULES 



The knowledge of the various typical types of chemical structure which 

 have been identified in the biosphere, and of the principal linkages which 

 join them, still leaves us in a region where the essential identity of all 

 organisms may be'distinguished. There is no more difference between a 

 molecule of coenzyme A isolated from a bacterium and one prepared from 

 animal tissue than there is between two molecules of sodium chloride. 

 When covalency forces operate in a volume within the limits of a few cubic 

 angstroms to a few thousand cubic angstroms, we are still in the world of 

 simple molecules, or molecules joined together in the compounds described 

 in Chapter III : this is the region in which organisms are identical. This 

 truth has long intrigued biochemists, whose desire to understand the 

 chemistr}^ of life on a molecular scale has not prevented consideration of 

 the great diversity of living things. The advent of the chemistry of macro- 

 molecules introduced into biochemistry the idea of specificity, which up 

 till then was lacking. 



Macromolecules are defined as chemical compounds whose molecular 

 weight is above 10,000 and in which covalent forces are effective in all the 

 available space. This more or less arbitrar}^ boundary corresponds approxi- 

 mately to molecular sizes above which the solution of these particles takes on 

 the so-called "colloidal" properties. But we are still dealing with chemical 

 molecules, even though these very large molecules cannot pass through 

 ordinary membranes. Their constituent atoms, like the compounds des- 

 cribed in Chapter III, are united mainly by covalencies. 



As soon as one arrives in the world of macromolecular chemistry, one 

 must be careful to distinguish between the chemical molecular weight and 

 the physical molecular w^eight. The chemical molecular weight is the sum of 

 the weights of the atoms joined by covalencies, in the smallest particle of 

 that compound. The physical molecular weight is the weight of the particle 

 actually present in a gas or in a solution. An example, taken from Staudin- 

 ger, will illustrate this difference. The chemical molecular weight of 

 stearic acid CigHggOa is 284; the determination of the freezing point 

 depression in benzene reveals a physical molecular weight of 568. This 

 result is explained by the fact that the molecules of stearic acid, in which 

 the atoms are united by covalencies, are associated in pairs by the action of 

 residual valencies. The chemical molecular weight is certainly equal to 



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