due to aggregates of DNA molecules. This phenomenon will be discussed at length
elsewhere. In Fig. 17 the results obtained with an initial DNA concentration of 150
jig/ml (to be diluted 10 times in the denaturing solution) are shown, for LiOH,
NaOH, KOH and CsOH. The final concentration of positive ions was in the four
cases 0.1 M. It is evident that the bigger the positive ion, the steeper the bulk denaturation process. Similar curves with a final concentrations of Na+ and K+ 0.2 M
are shown in Fig. 18 a and b. As the ionic force increases the rise-time of the irreversible hyperchromic effect becomes shorter. In Fig. 18 c and d, similar results were
obtained, measuring in a CsCl density gradient the fraction of denatured molecules
as a function of 7 for Na+ concentrations equal to 0.1 and 0.2 M. It must be said
that in many cases we have measured a rise-time of the fraction of denatured molecules a little longer than that of the irreversible hyperchromic effect of the same DNA
solution. This point however was not sufficiently investigated and we cannot say if
the effect is real.