B. Andreasson et al., INTERACTIONS OF SPERMIDINE AND METHYLSPERMIDINE WITH DNA STUDIED BY NUCLEAR-MAGNETIC-RESONANCE SELF-DIFFUSION MEASUREMENTS, Biophysical journal, 70(6), 1996, pp. 2847-2856
The NMR pulsed field gradient self-diffusion method has been used to s
tudy the self-diffusion of the polyamine spermidine and the polyamine
analog methylspermidine (completely N-methylated spermidine). The self
-diffusion coefficient, D, was measured in solutions of calf thymus DN
A prepared from nucleosome core particles (with an average length of 1
20 base pairs) as a function of the concentration ratio of polyamine t
o DNA phosphate. A study of the self-diffusion quotient, D/D-0 (where
D-0 is the diffusion coefficient for free polyamine, not associated wi
th DNA), in additions of spermidine and methylspermidine to solutions
of NaDNA/NaCl, gave almost identical results with complete association
of polyamine to DNA in the initial part of the titrations, indicating
similar affinities for DNA. A large influence on the measured self-di
ffusion coefficients was detected for methylspermidine in NaDNA soluti
ons with different concentrations of NaCl, which shows a considerable
salt effect on the polyamine-DNA association. No notable differences i
n D/D-0 for methylspermidine were observed in competitive titrations o
f solutions of Li- and NaDNA, indicating that sodium and lithium ions
behave similarly in their interactions with DNA. In titration experime
nts of methylspermidine into MgDNA solution, the results showed that t
he polyamine association is less effective than in the case of NaDNA,
because of competition from magnesium binding to DNA. Comparisons with
calculations based on the electrostatic Poisson-Boltzmann cell model
were performed. It is suggested that the interaction is primarily of e
lectrostatic nature, with no binding to specific sites on the DNA mole
cule.