340 ERWIN CHARGAFF 



with ultrasound,^"^'^''^ or with radiomimetic agents, such as the sulfur and 

 nitrogen mustards." '^"^'^^^ Many of these studies have been considered in 

 detail by Errera.^'" The effect of phenol and urea has also been studied.^^^ 



V. Some Partial Degradation Products 

 1. General 



Even if the assumption is made — and there is little justification for it, 

 as will be shown later — that a deoxypentose nucleic acid preparation from 

 a given cell is composed of only one molecular species, it will be readily 

 understood that the problem of desciibing its fine structure still is in- 

 solvable. It will affect this difficulty very little whether we are dealing 

 with a single polynucleotide chain or with two complementary chains 

 holding each other in a complicated embrace and exposing identical back- 

 sides to the outer world. A nucleic acid chain must, according to the molecu- 

 lar weight assigned to it, be composed of 2500 to 15,000 mononucleotides 

 of 4 or 5 or more varieties; and only through the most stultifying over- 

 simplification could a particular sequence be predicted. It will suffice to 

 point out that a chain consisting of 2500 nucleotides in the proportions 

 found for the total deoxypentose nucleic acid of ox tissues (see Section VII) 

 could exist in something like lO'*"" sequential isomers.'' Since the human 

 mind does not enjoy contemplating the impossible for a long time, it either 

 forgets, neglects, or reduces it. The latter operation results in the more 

 modest desire, not to write the entire sequence of nucleotides, but to 

 discern certain more general structural features, if any can be found. 



A stepwise degradation of the nucleic acid chain appears to offer pos- 

 sibilities of distinction between different entities. That it may be carried 

 out in two essentially different directions has been pointed out in the 

 preceding section (IV.2). Owhig to the great difference in stability of the 

 glycosidic linkage in the purhie and in the pyrimidine nucleosides (compare 

 Chapters 5 and 9), it is possible to remove the purines preferentially from 

 a deoxypentose nucleic acid chain by a carefully controlled acid hydrolysis. 

 Products of this type for which the name apurinic acid has been proposed"" 

 will be discussed in the next section. Less well-defined substances resulting 

 from a more vigorous breakdown liave long been known as thymic acid. 



206 S. G. Laland, W. G. Overend, and M. Stacey, J. Chern. Soc. 1952, 303. 

 20' I. E. El'piner and A. V. Gerasimova, Doklady Akad. Nauk S.S.S.R. 86, 797 (1952) ; 

 Chem. Abstr. 47, 2225 (1953). 



208 D. T. Elmore, J. M. Gulland, D. O. Jordan, and H. F. W. Taylor, Biochem. J. 42, 

 308 (1948) . 



209 J. A. V. Butler, L. Gilbert, and D. W. F. James, J. Chem. Soc. 1952, 3268. Compare 

 this paper for references to other work. 



2'o M. Errera, M^canismes de I'Action des Radiations sur le Noyau Cellulaire, Les 



Editions "Acta Medica Belgica." Bruxelles, 1952. 

 211 B. E. Conway and J. A. V. Butler, /. Chem. Soc. 1952, 3075. 



