Conformational changes of DNA in the presence of 12-s-12 gemini surfactants (s = 2 and 10). Role of the spacer's length in the interaction surfactant-polynucleotide

• The presence of the dimeric (gemini) cationic surfactants in calf thymus DNA solutions induces a partial condensation of the polynucleotide.
• Micellar aggregates act as multivalent ions which lead to the formation of a nucleation center in the DNA strands.
• Spacer length of dimeric surfactants plays a main role in the condensation process of DNA.
• The breaking of the partial condensation and the formation of a new polynucleotide state happen for the higher [surfactant]/[DNA] values.
• Kinetic measurements can also be used to determine the structural change of a polymer.

A multifaceted study on the interaction of calf-thymus DNA with two different cationic gemini surfactants alkanediyl-α-ω-bis(dodecyldimethyl-amonium)bromide, 12-s-12,2Br− (with s = 2, G2, and 10, G10) was carried out. The measurements were done at different molar ratios X = [surfactant]/[DNA]. Results show two different conformational changes in DNA: a first compaction of the polynucleotide corresponding to a partial conformational (not total) change of DNA from an extended coil state to a globular state that happens at the lower molar ratio X. A second change corresponds to a breaking of the partial condensation, that is, the transition from the compacted state to a new more extended conformation (for the higher X values) different to the initial extension. According to circular dichroism spectra and dynamic light scattering measurements, this new state of DNA seems to be similar to a ψ-phase. Measurements confirm that interactions involved in the compaction are different to those previously obtained for the analog surfactant CTAB. X values at which the conformational changes happen depend on the length of the spacer in the surfactant along with the charge of the polar heads.

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