29. DEOXYRIBONUCLEIC ACIDS AS MACROMOLECULES 17 



that there are no interactions between them when moving in the surrounding 

 medium; hence, the necessity of extrapolating the experimental data to 

 zero concentration. This means that the actual concentration used in the 

 experiments should be low enough to allow extrapolation with good preci- 

 sion. 



In the case of DNA particles, the concentration of solutions must be 

 very low, and this for two reasons: 



1. The DNA filament being very long — as we shall see — we have to con- 

 sider very high dilutions so that every given molecule should be far enough 

 away from the other molecules and, in this way, out of the reach of hydro- 

 dynamical perturbation. 



2. Aside from these hydrodynamical interactions, we have to consider 

 the effect of electrostatic forces. All the Na phosphates in the molecule being 

 ionized, each particle of DNA carries a great number of negative charges, 

 and moves in an electrostatic field produced by all the neighboring mole- 

 cules. Hence, when the solvent is pure water, we observe a very large effect 

 of the DNA concentration on the measurements and it is necessary, if some 

 precision in the extrapolation is required, to use such high dilutions that 

 the technique becomes impracticable. 



The situation is considerably improved if we introduce a strong elec- 

 trolyte such as NaCl. In this case the electric charges carried by the DNA 

 particles are practically saturated by the small ions, and the coulombian 

 interactions disappear. 



It is easy to illustrate this fact in considering the value of r} ap /c in DNA 

 solutions. From Eq. (4) this quantity should be a constant when c changes. 

 One can see in Fig. 7A that this is not the case: tjsp/c increases as a function 

 of c, but this effect becomes gradually smaller when the concentration of 

 NaCl increases. 



The same situation is met when we consider the sedimentation constant 

 S, and it is established by experience that correct values of S can be ob- 

 tained only if c is smaller than 1 X 1(T 4 gm. per milliliter. In fact such a 

 dilution is an obstacle to the use of a refractive optical method (such as 

 Philpot's) to detect the changes of concentration in the solution; and this 

 makes necessary the use of more sensitive devices based, for instance, on 

 the ultraviolet absorption of DNA in the 2.600 A. band. 12 



b. Intramolecular Interactions: Rigidity of DNA 



Since, in consequence, it is necessary to use salt-containing solutions of 

 DNA, the question arises whether the intrinsic character of DNA particles 

 and especially its configurations depend upon the presence of NaCl. 



It is well known that in the case of a chain of polyelectrolytes the electro- 



11 J. Pouyet, results on CV 9, unpublished (1955). 



12 K. V. Shooter and J. A. V. Butler, Trans. Faraday Soc, 52, 734 (1956). 



