where f b and f u are the relative fractions of bound and unbound DNA obtained from the MCR-ALS fitting analysis. The probabilities obtained from eq. 6 were then used to construct the phase diagrams for LPEI and BPEI polyplexes. The resulting phase diagrams (Figure ) show that NaCl impacts LPEI and BPEI complexation to DNA differently. NaCl significantly reduces LPEI binding to DNA by up to ∼ 70% (Figure ), depending on the N/P ratio. In contrast, it has a significantly smaller effect on BPEI to DNA, except at high NaCl concentrations (200 and 250 mM) (Figure ), where binding is reduced by ∼ 20 -30%. The significant susceptibility of LPEI binding to NaCl validates our hypothesis that it preferentially binds DNA through interactions to the phosphate backbone rather than through hydrogen bonding interactions with the nucleobases. In contrast, the insensitivity of BPEI binding to NaCl supports the notion that it interacts with DNA cargo primarily through hydrogen bonding interactions with the nucleobases.