Interactions of spermidine and methylspermidine with DNA studied by nuclear magnetic resonance self-diffusion measurements

The NMR pulsed field gradient self-diffusion method has been used to study 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 DNA prepared from nucleosome core particles (with an average length of 120 base pairs) as a function of the concentration ratio of polyamine to DNA phosphate. A study of the self-diffusion quotient, D/D-0 (where D-0 is the diffusion coefficient for free polyamine, not associated with 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-diffusion coefficients was detected for methylspermidine in NaDNA solutions with different concentrations of NaCl, which shows a considerable salt effect on the polyamine-DNA association. No notable differences in D/D-0 for methylspermidine were observed in competitive titrations of solutions of Li- and NaDNA, indicating that sodium and lithium ions behave similarly in their interactions with DNA. In titration experiments of methylspermidine into MgDNA solution, the results showed that the 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 electrostatic nature, with no binding to specific sites on the DNA molecule.