For FTIR experiments, we prepared stock solutions of polymer and DNA in phosphate buffer (pH 7.4) with and without NaCl. For NaCl screening experiment, we used concentrations of 50, 100, 150, 200, and 250 mM. The final concentration for the DNA stock solutions were 2 mM, while the final concentration for the polymer stock solutions were 5 mM. We diluted the polymer stocks further to create solutions with different final concentrations. We then added these solutions dropwise to DNA to create polyplex samples with N/P ratios ranging between 0.25 -3. The final DNA concentration after adding polymer solutions was 1 mM. We maintained the pH of all samples at 7.4. We also prepared DNA samples without polymer at 1 mM concentration with and without NaCl to use as a reference. In addition, we prepared LPEI and BPEI poly-mer solutions as controls with a final concentration of 1.8 mM and 2.6 mM, respectively. Samples were allowed to equilibrate for one hour prior to measurements.