Polyelectrolyte Complexes
of DNA and Linear PEI: Formation,
Composition and Properties
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Abstract
In the present study, the complexation between linear
13.4 kDa
poly(ethylene imine) (LPEI) and plasmid DNA was investigated. Analytical
ultracentrifugation (AUC) was used for size and molar mass determination.
Additionally, the morphology was studied by scanning force microscopy.
The polyplex formation was investigated in a wide range of PEI nitrogen
to DNA phosphate ratios (N/P). At N/P ratios below 1, the PEI/DNA
complex formation is characterized by an incomplete DNA condensation
and the formation of the primary DNA/PEI complexes. The merging of
the initially formed polyplexes occurs at N/P ∼2, resulting
in the formation of polyplexes with much larger size and high aggregation
rate. Stable and uniform polyplexes were formed at N/P > 10, with
average sizes of the polyplexes of about 170 ± 65 nm. The content
of uncomplexed PEI chains in the polyplex dispersion was estimated
at four different N/P ratios, 6.2, 11.6, 28.6, and 57.8, by combining
preparative centrifugation with a copper complex assay and by sedimentation
velocity analysis as an alternative method. It is demonstrated that
virtually all added PEI binds to the DNA at N/P < 2.5; further
addition of PEI results in the appearance of a large amount of free
PEI in solution. Nevertheless, PEI is able to bind in the whole range
of N/P ratios tested. According to the data collected by sedimentation
velocity analysis and scanning force microscopy, the single PEI/DNA
complexes are composed on average of 8 to 32 single condensed DNA
plasmids and 70 ± 25 PEI molecules