PHYSICAL PROPERTIES OF NUCLEIC ACIDS 489 



deoxypentose nucleate ion. Such conclusions must, as yet, be tentative 

 owing to the marked disagreement which exists between some of the ex- 

 perimental results and more particularly in the conclusions draw^n there- 

 from. The maximum dimension of the particle has been given as 8000 A.'^^ 

 from streaming birefringence measurements and as 6500 A.,^^ 6800 A.''''* 

 and 5100-7200 A.^*' from light-scattering measurements on solutions of 

 ionic strength 0.2. There is little information on the shorter dimension, but 

 Schwander'^^ gives the value of 10 A. as determined from streaming bire- 

 fringence. This value is somewhat less than the value of 15-24 A. given by 

 Rowen ct al.^'^'^ from high-resolution electron microscopy, which is in agree- 

 ment with the previous value of 15 A. given by Williams'^- and also ob- 

 tained from measurements with the electron microscope. If these values 

 are of the correct order then the axial ratio will be about 350, which is to 

 be compared with the approximate value of 120 obtained by Cecil and 

 Ogston^" from sedimentation measurements. 



The relatively small deformability possessed by the nucleate ion when 

 compared to a typical polyelectrolyte can be attributed to the hydrogen- 

 bonded structure suggested by Gulland ct al.'^^ and to the presence of strong 

 intramolecular Van der Waals forces as postulated by Schwander and Sig- 

 ner.'^^ There is also, as has been pointed out by Conway and Butler,'^^ 

 an important difference between the nucleate ion and a typical polyelec- 

 trolyte, since in the former, the charges (— >P0~) are carried on the phos- 

 phoester "backbone" and in the latter are generally carried on short side 

 chains, thereby permitting greater relative movement of the charged 

 groups. 



Comparison of these views on the shape and structure of the molecule 

 with the Crick and Watson structure^^ is interesting. This structure indi- 

 cates a comparatively rigid molecule in view of the strong hydrogen-bond- 

 ing and Van der Waals forces between the purine and pyrimidine ring sys- 

 tems of the two chains. The length and rigidity of the molecule will be 

 determined by the equilibrium between the repulsion between the charged 

 groups tending to extend the molecule on the one hand and the hydrogen- 

 bonding and Van der Waals forces tending to contract the molecule on the 

 other. Absorption of sodium ions, with the resulting neutralization of some 

 of the charged groups through ion-pair formation, Avill disturb this equilib- 

 rium and lead to a more compact molecule. The evidence for a rigid non- 

 deformable molecule rests on the independence of the viscosity of infinite 

 dilution on ionic strength as determined from viscosity measurements^^'* -^^^ 

 and on the variation of the streaming birefringence in solutions of different 

 viscosity."" The latter evidence is capable of different interpretation, ^^^•"^ 



"1 R. F. Steiner, Trans. Faraday Soc. 48, 1185 (1952). 

 1" R. C. Williams, Biochim. et Biophys. Acta 9, 237 (1952). 



