Probing the interaction of spermine and 1-naphthyl acetyl spermine with DNA polynucleotides: a comparative biophysical and thermodynamic investigation.

The interaction of spermine and its analogue, 1-naphthyl acetyl spermine with four double stranded DNA polynucleotides has been studied to understand the structural and thermodynamic basis of the binding. The efficacy and specificity of DNA binding of this analogue has not yet been revealed. The energetics of the interaction was studied by isothermal titration calorimetry and differential scanning calorimetry. Circular dichroism spectroscopy, UV-thermal melting and ethidium bromide displacement assay have been employed to characterize the association. Circular dichroism studies showed that 1-naphthyl acetyl spermine caused a stronger structural perturbation in the polynucleotides. Among the adenine-thymine polynucleotides the alternating polynucleotide was more preferred by naphthyl acetyl spermine compared to the preference of spermine for the homo sequence. The higher melting stabilization revealed by the optical melting and differential scanning calorimetry results suggested that the binding of 1-naphthyl acetyl spermine increased the melting temperature and the total standard molar enthalpy of the transition of adenine-thymine polynucleotides. Microcalorimetry results revealed that unlike spermine the binding of 1-naphthyl acetyl spermine was endothermic. The interaction was characterized by total enthalpy-entropy compensation and high standard molar heat capacity values. There are differences in the mode of association of 1-naphthyl acetyl spermine and spermine. 1-naphthyl acetyl spermine binds with an enhanced affinity with the adenine-thymine hetero polynucleotide. Thus, the result suggests the importance of polyamine analogues and their ability to interfere with normal polyamine interactions.