29. DEOXYRIBONUCLEIC ACIDS AS MACROMOLECULES 



31 



1-1 



0.5- 



(o) 



r i.o 



.? 0.5 



Fig. 15. (a) The G(S) function for a monodispersed solution. (6) The experimental 

 G(S) function. [From J. A. V. Butler, D. J. R. Laurence, A. B. Robins, and K. V. 

 Shooter, Proc. Rotj. Soc. A250, 1 (1959).] 



a. Sedimentation Constant: The Polydispersity in DNA Solutions 



At such large dilutions, it is almost necessary to measure the distribution 

 of the concentrations in the ultracentrifuge cell by using an ultraviolet 

 absorption method. Consequently, one does not obtain directly the dis- 

 tribution function g(S) of the sedimentation constant, but the integral 

 function : 



G(S) = [ g(S) dS 



Ja 



G(S) is then the weight fraction of the particles with a sedimentation con- 

 stant smaller than S. If S were the same for all the DNA particles, G(S) 

 would be represented by Fig. 15a. The result of the experiment (Fig. 156) 

 clearly shows that the S values cover a wide range beginning at 10 and 

 up to more than 50 in Svedberg units. 22a The DNA solutions then are 

 clearly polydispersed. 



If we refer to the definition of S (Section 11,1, a) and if we assume that 

 the partial specific volume V ap is the same for all the particles, we have to 

 conclude that the ratio M/f of the molecular weight to the friction coeffi- 

 cient differs from one particle to another. But we cannot say, from the 

 single sedimentation determination, if the heterogeneity of M/f is only 

 due to a heterogeneity of the molecular weight. 



In order to answer this very important question it would be necessary 

 either to measure the distribution of the Brownian diffusion constant A (see 

 Section 11,1,6) — but we have seen that no data are available as yet — or to 

 determine, by a proper comparison with the viscosity and light-scattering 

 22a The Svedberg unit is equal to 10 -3 c.g.s. units. 



