Differential effects of spermine and its analogues on the structures of polynucleotides complexed with ethidium bromide

The interactions of spermine and polyamine analogues with synthetic polynucleotides of various base sequences complexed with ethidium bromide (EB) were investigated using measurements of fluorescence intensity and steady-state fluorescence polarization. Spermine and polyamine analogues displaced some but not all of the EB bound to poly(dA-dT) poly(dA-dT) or poly(dG-dC) poly(dG-dC), suggesting that polyamines may stabilize these polynucleotides in a conformation with reduced affinity for EB. Modifications of the aliphatic backbone of spermine have pronounced effects on its ability to displace EB from poly(dA-dT) poly(dA-dT) but not from poly (dG-dC) poly(dG-dC). Spermine and some but not all of the polyamine analogues caused fluorescence depolarization when they interacted with the complex of EB and poly(dA-dT) poly(dA-dT). Neither spermine nor any of the analogues, however, induced fluorescence depolarization in the complex of EB with poly(dG-dC) poly(dG-dC) or poly(dA) poly(dT). This suggests that spermine and some spermine analogues induce structural changes specific to alternating A-T sequences.