Molecular Insights into the Binding of Linear Polyethylenimines and Single-Stranded DNA Using Raman Spectroscopy: A Quantitative Approach

Establishing how polymeric vectors
such as polyethylenimine
(PEI)
bind and package their nucleic acid cargo is vital toward developing
more efficacious and cost-effective gene therapies. To develop a molecular-level
picture of DNA binding, we examined how the Raman spectra of PEIs
report on their local chemical environment. We find that the intense
Raman bands located in the 1400–1500 cm–1 region derive from vibrations with significant CH2 scissoring
and NH bending character. The Raman bands that derive from these vibrations
show profound intensity changes that depend on both the local dielectric
environment and hydrogen bonding interactions with the secondary amine
groups on the polymer. We use these bands as spectroscopic markers
to assess the binding between low molecular weight PEIs and single-stranded
DNA (ssDNA). Analysis of the Raman spectra suggest that PEI primarily
binds via electrostatic interactions to the phosphate backbone, which
induces the condensation of the ssDNA. We additionally confirm this
finding by conducting molecular dynamics simulations. We expect that
the spectral correlations determined here will enable future studies
to investigate important gene delivery activities, including how PEI
interacts with cellular membranes to facilitate cargo internalization
into cells.