Synthesis of PAMAM dendrimers and investigations of their interaction with POPC/POPG lipids

PAMAM dendrimers are three dimensional organic polymers synthesised by repetitive steps to achieve a controlled size and shape with a choice of surface functional groups. One of the potential applications of dendrimers is for drug/gene delivery which requires the dendrimer to interact with the cellular membranes. This study is designed to probe the interactions between PAMAM dendrimers and lipid bilayers. To investigate these interactions PAMAM dendrimers up to the third generation were synthesised. 31P and 1H-1H NOESY NMR studies between these dendrimer and POPC/POPG-derived lipids were then carried out. The results obtained from the NMR experiments were then compared with those from fluorescence studies using a surface labelled PAMAM dendrimer and the nature of the dendrimer-lipid bilayer interactions was also explored using molecular dynamics modelling. The solid-state NMR study in a controlled buffer at pH 7.2 revealed that the larger dendrimer (third generation) interacts strongly with a bilayer containing POPG, but not with a bilayer containing only POPC, and no interaction between the smaller dendrimer (zero generation) was observed with either POPC or POPG. This was confirmed with the fluorescence experiments, as changes in the emission intensity of a labelled dendrimer were mainly detected for negatively charged species (SDS and POPG) and rather less for zwitterionic, neutral or cationic species (1,2-dodecyldiol, CTAB and POPC). The coarsegrained molecular dynamic simulations showed that the 3rd generation PAMAM dendrimer can interact with the surface of the membrane when dendrimer is positively charged, but not when the dendrimer is uncharged. These studies demonstrate how the positive charges and size of the dendrimer influence the interaction with negatively charged lipid, which can have an impact on both the dendrimer’s cellular uptake and potential toxicity.