Wavenumber (cm -1 ) Wavenumber (cm -1 ) FIG. . FTIR spectra of DNA in the presence of (a) LPEI and (b) BPEI in phosphate buffer, pH = 7.4. The black arrow in each set of spectra represents the spectral changes that occur from N/P ratios of 0, 0.25, 0.5, 1, 1.5, 2, 2.5, 3. All the spectra were blank subtracted, baselined, and normalized to the total integrated area. level insights into their interactions. To investigate this, we measured the FTIR spectra of free DNA, LPEI, and BPEI, as well as DNA in the presence these polymers at N/P ratios ranging from 0 -3. The spectra of LPEI and BPEI (Figure ) are relatively weak compared to DNA, showing bands around ∼1450 cm -1 (CH 2 deformation) and 1620 cm -1 (NH 2 scissoring) . contrast, DNA (Figure ) possesses bands at ∼1222 -1 (PO - 2 asymmetric stretching) and ∼1680 cm -1 (C--O stretching of thymine and guanine). Additional weak bands that derive from inplane ring stretching modes of the nucleobases appear between 1250 -1600 cm -137-41 .