Characterization of Starburst Dendrimers and Vesicle Solutions and Their Interactions by CW- and Pulsed-EPR, TEM, and Dynamic Light Scattering

Starburst dendrimers (SBDs) of different generations (size) and level of protonations of their surface amino groups and solutions of dimyristoylphosphatidylcholine (DMPC) vesicles were analyzed, both separately and in mixtures of the two components, by negative-staining transmission electron microscopy (TEM), dynamic light scattering (DLS), and, mainly, by computer-aided continuous wave (CW)and pulsed-electron paramagnetic resonance (EPR). For the EPR study, the SBDs were labeled with a nitroxide radical (SBDT). TE micrographs showed the vesicles as multilamellar structures of spherical shape with diameters ranging from 0.2 to 1.2 m. DLS measurements provided the mean vesicle diameter (d) at ca. 400 nm, whereas the diameter of generation 6 was 7 nm. No large-sized permanent supramolecular structures (d > 400 nm) were formed. EPR measurements at room temperature were poorly informative, since (1) a fraction of the dendrimers was not interacting with the vesicles, and (b) the labels that were interacting with the vesicles were rotating quickly around the main axis. Interactions between the dendrimers and the vesicles (tested by a decrease in rotational mobility of the label) became EPR-observable and quantifiable below the freezing transition of a portion of the solution, which could not be detected by EPR analysis. The fraction of the dendrimers interacting with the vesicles underwent a glass transition. Dendrimer-vesicle interactions modified the direction of the fast-rotation axis of the radical, and the interaction was more effective for protonated dendrimers of a larger size, i.e., later generation. A “complex” was formed between one solvent molecule and the nitroxide radical. Interactions between the SBD-T and the vesicle partially compressed the hydration layer of the N-O group, and/or the hydration layer of the vesicle headgroups was compressed onto the unpaired-electron site. This study provides information on the possible utilization of starburst dendrimers as gene carriers.