Condensing DNA with poly(amido amine) dendrimers of different generations: means of controlling aggregate morphology

Abstract

The morphology of the aggregates formed between DNA and poly(amido amine) (PAMAM) dendrimers depends on the dendrimer generation as previously reported in separate studies at high dendrimer/DNA charge ratios (>1). This has lead to substantial work on dendrimers as possible transfection agents. Inspired by these studies, we here present novel results from a coherent and systematic study using cryo-TEM, dynamic light scattering (DLS) and fluorescence spectroscopy to reveal how the size, composition and morphology of aggregates formed between DNA (4331 base pairs) and PAMAM dendrimers, are affected by dendrimer size and charge at low charge ratios (<1) in dilute solutions. At such conditions the process is cooperative and kinetically controlled and well-defined structured aggregates are formed for lower dendrimer generations. The smaller sized dendrimers (generation 1 and 2), which have a lower total charge per molecule, allow the formation of well-structured rods and toroids. In contrast, globular and less defined aggregates, which are less stable against precipitation, are formed with higher generation dendrimers. We were also able to directly visualise the cooperative nature of the condensation process as cryo-TEM and DLS show that dendrimer/DNA aggregates, containing condensed DNA, coexist with free extended DNA chains. In fact, the apparent hydrodynamic radii of the dendrimer/DNA aggregates, obtained using DLS, are found to be almost constant for charge ratios ≤1. The fluorescence study shows that the number of dendrimers bound per DNA chain decreases with the dendrimer generation but is independent of the charge ratio.