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 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.