PHYSICAL PROPERTIES OF NUCLEIC ACIDS 491 



tion. The earlier suggestion of Vilbrandt and Tennent"' that the fall in 

 viscosity produced by the action of acid or alkali was due to a depolymeriza- 

 tion of the nucleate appears to be untenable in \dew of the titration evi- 

 denced^ that phosphoester bonds are not broken by these reagents. 



The solutions studied by Creeth et al}^^ were too concentrated to permit 

 any real analysis in terms of molecular dimensions to be made, and more 

 recently Schwander^^^ has measured the viscosities of more dilute solutions 

 in 1 % sodium chloride solution at pH 3.70 and 6.60 and finds a much 

 smaller decrease of viscosity than that observed by Creeth et al. Sedimenta- 

 tion studies by Vilbrandt and Tennent"' and Cecil and Ogston*° showed 

 that the action of both acid and alkali causes a decrease in the sedimenta- 

 tion coefficient. Cecil and Ogston found that two separate components 

 appeared after the addition of acid {\ M HCl); one was homogeneous and 

 resembled the original nucleate in neutral solution and the other was poly- 

 disperse and had apparently been formed by the disaggregation of the 

 original material. The addition of alkali produced a larger lowering of the 

 sedimentation coefficient than did the action of acid and the disaggregation 

 was more complete. 



Creeth ct al}^'^ observed that on neutralizing to pH 7.0 a solution of 

 deoxypentose nucleate which had been treated with alkali at pH 12.0, a 

 slow increase of viscosity occurred and after 90 hours the viscosity resem- 

 bled that of the original solution. This behavioui' has been shown by 

 Zamenhof and Chargaff^^^^ to be due to an artifact having highly thixotropic 

 behaviour. The reversibility of the disaggregation was not observed on 

 treatment with acid. 



A very much clearer picture of the changes that occur in sodium deoxy- 

 pentose nucleate on treatment with dilute acid has been given in a very 

 careful study by Reichmann et al}^^ A great criticism of all the previous 

 work is that the pH of solutions was changed by adding small amounts of 

 relatively strong acid. This procedure must inevitably produce transient 

 regions of much lower pH (or higher pH if alkali is added) than that ulti- 

 mately attained at equilibrium. This treatment will thus produce, in some 

 particles, greater degradation than in the remainder. In order to prevent 

 the deoxypentose nucleate particles from coming into contact with con- 

 centrated acid, Reichmann ct al}^^ adjusted the pH by dialysis. Using the 

 light-scattering method, these authors found that at pH 6.5, 3.0, and 2.6 

 in 0.2 M sodium chloride solution, the deoxypentose nucleate ion has the 

 same molecular weight (7.7 X 10®). There is, however, a marked contrac- 

 tion of the ion which is quite pronounced at pH 3.0, but very much greater 



»" H. Schwander, Helv. Chim. Acta 32, 2510 (1949). 



15^ S. Zamenhof and E. Chargaff, J. Biol. Chem. 186, 207 (1950). 



1" M. E. Reichmann, B. H. Bunce, and P. Doty, J. Polymer Set. 10, 109 (1953). 



