To validate our spectral interpretation, we examined LPEI and BPEI polyplex formation in the presence of NaCl. We hypothesized that Na + and Cl -could screen electrostatic interactions and thereby inhibit polymer binding to the DNA phosphate backbone. If LPEI primarily interacts with DNA via electrostatic forces, its binding should be more sensitive to salt screening effects than BPEI, which our data suggests preferentially interacts with nucleobases.