Interactions between starburst dendrimers and mixed DMPC/DMPA-Na vesicles studied by the spin label and the spin probe techniques, supported by transmission electron microscopy

The interactions between polyamidoamine starburst dendrimers (SBDs) and vesicles consisting of dimyristoylphosphatidylcholine (DMPC) and its phosphatidylcholate salt (DMPA-Na) in various relative ratios (from 0% to 40% of DMPA-Na) were studied by electron paramagnetic resonance (EPR) and by transmission electron microscopy (TEM). The EPR spectra were computer-simulated to extract mobility and structural parameters of the probes and labels. The systems were analyzed as a function of the level of protonation of SBDs and generation (G = 2SBD and G = 6SBD). Both the hydrophobic 5doxylstearic spin probe (5DSA) and the positively charged nitroxide CAT16, a cationic surfactant with a C16 chain, inserted in the vesicles and monitored the formation of dendrimer-mixed vesicle (DMPC/DMPA-Na) complexes. 5DSA revealed a partial ordering of a double-layer-like structure occurring at the dendrimer surface. The spin labels at the vesicle/dendrimer interface modified the structural and mobility parameters upon complexation with the dendrimer. At high levels of protonation, dendrimers showed a larger interaction with the vesicles, especially for the mixed vesicles, compared to dendrimers at a lower level of protonation. In agreement with EPR results, TEM micrographs showed that the addition of DMPA-Na to DMPC modifies the vesicle shape from spherical to rodlike. The EPR analysis suggested that the vesicles wrap around the large dendrimers whereas the small dendrimer directly interacts with the vesicle surface. The results are very promising for a better understanding of the mechanism of interaction of dendrimers with cell membranes which may be an important feature in the role of SBDs as drug and gene carriers to target cells.