HIGH-MOLECULAR CELL CONSTITUENTS 3 



the recognition of the composition of the substance, provided its 

 molecular weight is known. The deviation of the analytical results 

 from simple, integral proportions is without importance in that 

 case. But this will not hold for high-molecular compounds in 

 which variations in the proportions of their several components 

 often will provide the sole indication of the occurrence of dif- 

 ferent compounds. 



In attempting to formulate the problem with some exag- 

 geration one could say: The validity of the identification of a 

 substance by the methods of classical organic chemistry ends 

 with the mixed melting point. When we deal with the extremely 

 complex compounds of cellular origin, such as nucleic acids, 

 proteins, or polysaccharides, a chemical comparison aiming at 

 the determination of identity or difference must be based on the 

 nature and the proportions of their constituents, on the sequence 

 in which these constituents are arranged in the molecule, and on 

 the type and the position of the hnkages that hold them together. 

 The smaller the number of components of such a high-molecular 

 compound is, the greater is the difficulty of a decision. The oc- 

 currence of a very large number of different proteins was recog- 

 nized early; no one to my knowledge ever attempted to postulate 

 a protein as a compound composed of equimolar proportions of 

 18 or 20 different amino acids. In addition, immunological in- 

 vestigations contributed very much to the recognition of the 

 multiplicity of proteins. A decision between identity and dif- 

 ference becomes much more difficult when, as is the case with 

 the nucleic acids, only few primary components are encountered. 

 And when we finally come to high polymers, consisting of one 

 component only, e.g., glycogen or starch, the characterization of 

 the chemical specificity of such a, compound becomes a very 

 complicated and laborious task. 



While, therefore, the formulation of the tetranucleotide con- 

 ception appeared explainable on historical grounds, it lacked an 

 adequate experimental basis, especially as regards "thymonucleic 

 acid". Although only two nucleic acids, the deoxyribose nucleic 

 acid of calf thymus and the ribose nucleic acid of yeast, had been 



References p. 23 



