We next investigated the effect of modification on the resulting internal structure of our PEI-DNA complexes. Typically, in the presence of polycations possessing charge +3 or above, DNA helices self-assemble into a columnar hexagonal array. SAXS experiments allow the measurement of the inter-DNA spacings in the condensed phase. Within the DNA condensates, the surfaces of adjacent DNA helices do not make contact but are separated by a water layer representing a balance between attractive and repulsive forces in the condensed phase. The water layer thickness depends on the specific condensing agent employed. In our investigation, we used calf thymus DNA to create fibrous polyplex samples appropriate for X-ray analysis. It was previously observed that the DNA packaging in these fibrous samples is similar to that of polyplex nanoparticles. Representative scattering intensity profiles from unmodified and modified PEI-DNA complexes in 10 mM Tris buffer (pH 7.5) are shown in Figure . Sufficient PEI was used to ensure complete DNA condensation for each polyplex. Once fully condensed, further increases in the polymer:DNA weight ratios do not significantly alter the observed DNA-DNA spacings within the condensates. The polyplexes were incubated for an hour at RT prior to SAXS analysis, akin to polyplex incubation during transfections. Unmodified bPEI-DNA exhibited a characteristic Bragg peak at scattering vector Q = 2.533 nm -1 , corresponding to a Bragg spacing of 24.7 Å. bPEI-DNA is known to pack in a columnar hexagonal lattice, so this Bragg spacing corresponds to an Our previous studies showed that introducing a non-cationic amino acid into oligopeptide chains increases repulsive forces and reduces attractions, resulting in more loosely packed DNA.