The corresponding binding curves (Figure ) were fit to a cooperative binding model using the Hill equation: The data was fit to a cooperative binding model using the Hill equation. The goodness-offit was assessed using the R 2 value, which is 0.99 for LPEI and 0.97 for BPEI. Two replicates were measured and the standard deviation was calculated to be less than 2% of the mean for each data point shown.