Unusual Salt and pH Induced Changes in Polyethylenimine Solutions

Linear PEI is a cationic polymer commonly used for complexing DNA into nanoparticles for cell-transfection and gene-therapy applications. The polymer has closely-spaced amines with weak-base protonation capacity, and a hydrophobic backbone that is kept unaggregated by intra-chain repulsion. As a result, in solution PEI exhibits multiple buffering mechanisms, and polyelectrolyte states that shift between aggregated and free forms. We studied the interplay between the aggregation and protonation behavior of 2.5 kDa linear PEI by pH probing, vapor pressure osmometry, dynamic light scattering, and ninhydrin assay. Our results indicate that: 1. At neutral pH, the PEI chains are associated and the addition of NaCl initially reduces and then increases the extent of association. 2. The aggregate form is uncollapsed and co-exists with the free chains. 3. PEI buffering occurs due to continuous or discontinuous charging between stalled states. 4. Ninhydrin assay tracks the number of unprotonated amines in PEI. 5. The size of PEI-DNA complexes is not significantly affected by the free vs. aggregated state of the PEI polymer. Despite its simple chemical structure, linear PEI displays intricate solution dynamics, which can be harnessed for environment-sensitive biomaterials and for overcoming current challenges with DNA delivery.