This report describes a computer-aided CW- and pulsed-electron paramagnetic resonance (EPR) investigation on the structural modification of dimyristoylphosphatidylcoline (DMPC) vesicles, which occur upon interaction with starburst dendrimers (SBDs). Probes used for this study included doxyl-functionalized stearic acids, with the doxyl group attached at different positions of the stearic chain (5DXSA, 12DXSA, and 16DXSA). Mainly mobility and polarity parameters were evaluated from the analysis of the CW-EPR spectra, whereas the analysis of the decay and modulation of the electron spin−echo (ESE) signal provided information on the structural environment of the paramagnetic center. Due to the interaction with the SBD surface, the vesicle structure became more rigid and ordered. The enhanced rigidity of the structure also caused the tilting of the chains of about 50° with respect to the surface line. The permeability of water at the chain beginning level increased, thus increasing the rotational mobility of the probe. The perturbing effects lessened toward the end of the chains. A fraction of 16DXSA (15%) was in the bent conformation, with the chain inserted into the lipid layer and the two polar heads at the external surface. The interaction with protonated dendrimers caused the swelling of the vesicle structure. This study indicated that the bilayer structure is modified but only partially perturbed by the addition of the dendrimers, and the integrity of the vesicle, as a model cell membrane, is preserved after the interaction with the dendrimers. This is encouraging for the use of the SBDs as drug and gene carriers.

A computer aided analysis of both cw-and pulsed-Electron Paramagnetic Resonance (EPR) spectra of 5doxylstearic acid (5DXSA) as a probe was carried out to compare the aggregation process of Sodium Dodecyl Sulfate (SDS) surfactants in the absence and in the presence of Starburst Dendrimers (SBDs), and to provide information on the interactions between SDS and SBDs. Mobility and polarity parameters were extracted from the cw-EPR analysis, whereas the analysis of the Eleetron Spin Echo Envelope Modulation (ESEEM) signal provided details about the doxyl environment in the SDS mieeUes. In the absence of SBDs, the formation of SDS mieelles was revealed by the decrease in mobility of the probes inserted in the micelles. The high packing of SDS chains in the rniceUes prevented the water permeability at the doxyl site. In the presence of the dend¡ the analysis of the EPR spectra suggested the formation of SDS aggregates at the dendfimer surface (cooperative interaction). The larger the size of the dendrimers and the protonation of their surface, the stronger the interactions resulted between the SDS surfactants and the SBD surface. The analysis of the ESEEM pattern indicated that the cooperative interaction of the surfaetant with the SBD surface led to a less packed structure of the aggregates. A schematic view was proposed to describe the local structure of the doxyl group and its environment in the absenee and in the presence of the dendrimers.