84 UNITY AND DIVERSITY IN BIOCHEMISTRY 



284, for derivatives of stearic acid, such as the esters, contain the radical 

 CiaHggO-. (Radical = residue, group, grouping = aggregate of atoms, 

 which survive from one compound to another = residue of a molecule 

 when one or several atoms are removed. If an H atom is removed from 

 water H — O — H, the hydroxyl radical — OH remains. If an H atom is 

 removed from ammonia H — N — H, the amidogen radical — N — H 

 remains.) The idea of chemical molecular weight is derived from the idea 

 of a radical. When the chemical molecular weight is below 10,000 but is not 

 equal to the physical molecular weight, then one is dealing with molecules 

 associated in "micelles", as is the case with colloidal solutions of soaps. 

 When the chemical molecular weight, being above 10,000, is the same as 

 the physical molecular weight, then we are dealing with a solution of 

 macromolecules. When, however, the chemical molecular weight is above 

 10,000 but is less than the physical molecular weight then these molecules 

 are associated by residual valencies. In every case, the physical molecular 

 weight is either equal to, or greater than, the chemical molecular weight. 



These polymers or macromolecules are made up of monomeric residues 

 by covalencies at two or more points. The natural macromolecules are 

 generally made up of long chains of such radicals joined by covalencies; 

 these chains may also be joined by a small number of side-chains, also 

 covalent in nature, such that the resulting structure takes the form of a 

 three-dimensional network. 



The idea that proteins, cellulose, starch, etc., are polymers, that is that 

 they are made up of smaller units linked by covalencies, is not modern. 

 It dates at least from 1871, when the idea was clearly set out in a paper by 

 Hlaziwetz and Habermann. Unfortunately, these compounds were 

 classed by Graham among his "colloids", and there was for a long time 

 confusion between macromolecules and true colloids, in which the mole- 

 cules are linked by residual valencies. It was Staudinger who was respon- 

 sible for putting biochemists on to the right track once more when he 

 showed that the "colloidal" properties of solutions of macromolecules 

 persisted whatever the solvent, contrary to what is observed with micelles 

 resulting from the association of small molecules by secondary valencies. 

 Staudinger also demonstrated that the transformation of macromolecules 

 into their derivatives does not suppress their "colloidal" properties. 



Among the macromolecules we find the same classes of organic com- 

 pounds as with simple molecules ; but, particularly important, the number 

 of isomers is very much greater with these larger molecules. 



The chemical structures described in Chapters II and III have been 

 established by organic chemists, not only by means of analysis, but also 

 with the additional control furnished by synthesis. In the case of naturally 

 occurring macromolecules, synthesis is not yet possible for the chemist 

 (although Fraenkel-Conrat, after separating the nucleic acid from the 



