The effect of the poloxamer’s polypropylene oxide block on the self-assembly behaviour of a novel 16-3-16/pDNA complex

• Isothermal titration calorimetry shows lower critical concentrations than similar systems without DNA. • Ethidium bromide studies show DNA de-condensation is linearly related to Pluronic PPO block length. • Surface tension shows systems with PO blocks >55 units self-assemble differently than <50 units. • Pluronic PPO block length is the primary dictator of the type of self-assembly occurring in these systems. The self-assembly behaviour of a novel 16-3-16/pDNA complex (10:1 charge ratio of N + : P − ) with several Pluronics (P105, F68. F77, L62, F108, F127) was investigated using isothermal titration calorimetry, surface tension, dynamic light scattering, and ethidium bromide fluorescence. The calorimetry results showed that for all the tested systems, the 16-3-16/pDNA complex starts forming mixed aggregates with the Pluronics at concentrations <2.5 × 10 −6 M. This concentration is much lower than was seen in previous ITC investigations of gemini surfactants and Pluronics without DNA. The mixing interactions between the 16-3-16/pDNA and Pluronics are endothermic. Tensiometry was used to confirm the critical concentrations obtained from ITC. For the systems containing Pluronics with longer PPO blocks (>50 units), uniquely shaped surface tension plots were observed. This suggests that Pluronics with longer PPO blocks preferentially interact with free 16-3-16 in solution, preventing the 16-3-16/pDNA complex from forming it’s micellized structure, DLS results further supported this conclusion. Ethidium bromide fluorescence results showed that the amount of uncondensed DNA in the solution is linearly proportional to the length of the PPO block. This relationship between PPO block size and the amount of decondensed DNA in solution may explain why systems containing longer PPO blocks exhibit significantly less transfection efficiency compared to Pluronics with shorter PPO blocks.