In contrast, the absence of a frequency shift in this band for BPEI suggests weaker binding to the phosphate backbone that does not alter the backbone conformation of DNA. Instead, the significant decrease in intensity of this band suggests that the primary amines of BPEI likely form hydrogen bonds with the phosphate oxygen atoms of DNA in addition to electrostatic interactions. Additionally, the substantial intensity changes observed in the nucleobase bands at 1680 cm -1 and between 1300 -1500 cm -1 indicate that BPEI also intercalates into the DNA duplex, most likely through hydrogen bonding interactions with nucleobases. The concomitant increase in the intensity of the bands between 1300-1500 cm -1 indicates that BPEI interactions with the nucleobases disrupts their native π-stacking interactions in the DNA.