488 



D. O. JORDAN 



at the ionic strengths studied is less than the theoretical value for the fully charged 

 ion, thus indicating a considerable degree of ion-pair formation. Shack et al.^" ob- 

 serve only a small variation of the charge with ionic strength, the average value in 

 the range 0.005 and 0.1 being 1.89 negative charges for every four atoms of phosphorus. 

 The variation of charge observed by both Creeth and Jordan and Shack et al. was an 

 increase of negative charge with increase of ionic strength. The reason for this varia- 

 tion is not clear, and, in view of the limitations and inaccuracies of the membrane- 

 potential method, it is clearly desirable that the charge should be investigated by 

 other methods. 



Changes in the ultraviolet absorption spectrum on the addition of electro- 

 lyte which have been studied systematically by Thomas/^s Shack et al.,^^^ 

 and Lawleyi^o are also consistent with the view that the nucleate ion is 

 capable of some deformation. These authors find that the value of the ex- 

 tinction coefficient at 260 m^ is lowered by the addition of electrolytes, the 

 cations having a specific effect which is dependent upon their charge. The 

 adsorption of Na+, K+, Mg^, Ba++, and Zn^ probably occurs at the phos- 

 phate groups and is nonspecific since the overall shape of the adsorption 

 band is not affected. H3O+ and Ag+ ions, however, appear to have a more 

 specific action since the shape of the absorption curve changes and Xmax. 

 shifts to longer wavelengths. The lowering of the absorption is unlikely to 

 be due to a shadowing effect and is more probably a true difference in the 

 absorption of the ring systems due to changes in the configuration of the 

 rings in the macro-ion. In the Watson and Crick structure^^ the extent of 

 stretching of the polynucleotide chain is determined by an equilibrium be- 

 tween the hydrogen-bonding and the Van der Waals forces on the one hand 

 and the repulsion between the charged groups on the other. A decrease of 

 the latter due to ion-pair formation will permit a small decrease in the 

 length of the ion and, owing to the new relative configurations of the purine 

 and pyrimidine rings, a change in the absorption spectrum. [Cf. Beaven, 

 Holiday, and Johnson, Chapter 14.] 



6. Conclusions as to the Size and Shape of the Deoxypentose Nucleate Ion in 

 Neutral Solution 



The problem of determining the dimensions of a polyelectrolyte of un- 

 known macro-structure is a most exacting one, as the small number of 

 successful studies with synthetic polyelectrolytes of known structure 

 testifies. However, from the experimental evidence discussed above it is 

 possible to draw some conclusions concerning the size and shape of the 



1" J. Shack, R. J. Jenkins, and J. M. Thompsett, J. Biol. Chem. 198, 85 (1952). 

 "* R. Thomas, Bull. soc. chim. biol., in press. 



1" J. Shack, R. J. Jenkins, and J. M. Thompsett, J. Biol. Chem., 203, 373 (1953). 

 '^» P. D. Lawley, Studies in the Behaviour of Polyelectrolytes Doctoral thesis, Not- 

 tingham Univ., Nottingham, England, 1953. 



