Interactions between Dendritic Polymer Nanoparticles and Lipid Mesophases: Swollen Lamellae, Suppressed Curvature, and Augmented Structural Disorder

Abstract

Understanding interactions between nanoparticles and model membranes is relevant to functional nano-composites and the fundamentals of nanotoxicity. In this study, the effect of polyamidoamine (PAMAM) dendrimers as model nanoparticles (NP) on the mesophase behaviour of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) has been investigated using high-pressure small-angle X-ray scattering (HP-SAXS). The pressure-temperature (p-T) diagrams for POPE mesophases in excess water were obtained in the presence and absence of G2 and G4 polyamidoamine (PAMAM) dendrimers (29 Å and 45 Å in diameter, respectively) at NP-lipid number ratios 0.0002-0.02) over the pressure range p = 1-3000 bar and temperature range T = 20-80 °C. The p-T phase diagram of POPE exhibited the L_β, L_α and H_II phases. Complete analysis of the phase diagrams, including the relative area pervaded by different phases, phase transition temperatures (T_t) and pressures (p_t), the lattice parameters (d-spacing), the pressure-dependence of d-spacing (Δd/Δp), and the structural ordering in the mesophase as gauged by the Scherrer coherence length L, permitted insights into the size- and concentration-dependent interactions between the dendrimers and the model membrane system. The addition of dendrimers changed the phase transition pressure and temperature and resulted in the emergence of highly swollen lamellar phases, dubbed L_β-den and L_α-den. G4 PAMAM dendrimers at the highest number ratio (0.02) suppressed the formation of the H_II phase within the temperature range studied, whereas the addition of G2 PAMAM dendrimers at number ratio = 0.02 promoted an extended mixed lamellar region in which L_α and L_β phases coexisted.