29. DEOXYRIBONUCLEIC ACIDS AS MACROMOLECULES 13 



by 



(a) 



(c) 



Fig. 4. Configurations giving a negative ordinate of the asymptote (c/K)hl-k*, 

 when b > X/20. 



As a conclusion, it is seen that the use of visible light, with X as large as 

 possible, is to be chosen when one wants to measure the values of M and 

 R 2 , whereas central diffusion of X-rays is to be used for the determination 

 of the shape and specific density of the particles. 



c. Influence of Polydispersity 



In this case the light-scattering method leads to mean values of the 

 molecular weights or of the morphological parameters. 

 It can be easily shown that: 



1. Whatever the heterogeneity of shapes of the molecules in the mixture, 

 the intercept of the extrapolated curve (K/c) c =o with the axis of ordinates 

 is equal to the molecular weight average 



M w = f M 2 j(M) dM I f Mf(M) dM 



The fact that this result is quite general, even if there is no correlation 

 between the weight and the dimensions of the macromolecules, gives a 

 great advantage to the use of light scattering. 



2. The slope of the initial curve gives a mean value of R . 



If there is a biunivocal relationship between R and M this mean value 

 is of the z type for all models of particles. 



R* = 4" f Rm-M-j(M) dM (28) 



For instance, in the case of a mixture of Gaussian coils with different 



