The binding of polyamines and magnesium to DNA.

1. The binding of spermine and Mg2+ to DNA has been investigated using the dye arsenazo III to measure unbound cations. 2. The apparent dissociation constant, Kd, of DNA for spermine has been found to be 7.4 +/- 3.9 x 10(-8) M and that for Mg2+, 6.5 +/- 3.3 x 10(-7) M. 3. Binding of spermine in the presence of 1 mM Mg2+ has been shown to have a Kd of about 4 x 10(-6) M. 4. Magnesium ion (2 mM) halves the concentration of spermine needed to cause DNA aggregation. 5. Spermidine binds to DNA with a similar affinity to spermine but 3,3'-iminobispropylamine and 1,5,9,13-tetra-azatridecane bind with a lower affinity. The naturally occurring polyamines thus have a higher affinity for DNA than the related polyamines which do not occur naturally. 6. Binding of spermine or spermidine to DNA alters the spectrophotometric absorbance of DNA at 260 nm.