Characterization of Gd loaded chitosan-TPP nanohydrogels by a multi-technique approach combining dynamic light scattering (DLS), asymetrical flow-field-flow-fractionation (AF4) and atomic force microscopy (AFM) and design of positive contrast agents for molecular resonance imaging (MRI).

Chitosan CS-tripolyphosphate TPP/hyaluronic acid HA nanohydrogels loaded with gadolinium chelates (GdDOTA ⊂ CS-TPP/HA NGs) synthesized by ionic gelation were designed for lymph node (LN) MRI. In order to be efficiently drained to LNs, nanogels (NGs) needed to exhibit a diameter ϕ < 100 nm. For that, formulation parameters were tuned, using (i) CS of two different molecular weights (51 and 37 kDa) and (ii) variable CS/TPP ratio (2 < CS/TPP < 8). Characterization of NG size distribution by dynamic light scattering (DLS) and asymetrical flow-field-flow-fractionation (AF4) showed discrepancies since DLS diameters were consistently above 200 nm while AF4 showed individual nano-objects with ϕ < 100 nm. Such a difference could be correlated to the presence of aggregates inherent to ionic gelation. This point was clarified by atomic force microscopy (AFM) in liquid mode which highlighted the main presence of individual nano-objects in nanosuspensions. Thus, combination of DLS, AF4 and AFM provided a more precise characterization of GdDOTA ⊂ CS-TPP/HA nanohydrogels which, in turn, allowed to select formulations leading to NGs of suitable mean sizes showing good MRI efficiency and negligible toxicity.

Molecular organization of surfactin-phospholipid monolayers: effect of phospholipid chain length and polar head.

Mixed monolayers of the surface-active lipopeptide surfactin-C(15) and various lipids differing by their chain length (DMPC, DPPC, DSPC) and polar headgroup (DPPC, DPPE, DPPS) were investigated by atomic force microscopy (AFM) in combination with molecular modeling (Hypermatrix procedure) and surface pressure-area isotherms. In the presence of surfactin, AFM topographic images showed phase separation for each surfactin-phospholipid system except for surfactin-DMPC, which was in good agreement with compression isotherms. On the basis of domain shape and line tension theory, we conclude that the miscibility between surfactin and phospholipids is higher for shorter chain lengths (DMPC>DPPC>DSPC) and that the polar headgroup of phospholipids influences the miscibility of surfactin in the order DPPC>DPPE>DPPS. Molecular modeling data show that mixing surfactin and DPPC has a destabilizing effect on DPPC monolayer while it has a stabilizing effect towards DPPE and DPPS molecular interactions. Our results provide valuable information on the activity mechanism of surfactin and may be useful for the design of surfactin delivery systems.

Penetration of surfactin into phospholipid monolayers: nanoscale interfacial organization.

Atomic force microscopy (AFM) combined with surface pressure-area isotherms were used to probe the interfacial behavior of phospholipid monolayers following penetration of surfactin, a cyclic lipopeptide produced by Bacillus subtilis strains. Prior to penetration experiments, interfacial behavior of different surfactin molecules (cyclic surfactins with three different aliphatic chain lengths--S13, S14, and S15--and a linear surfactin obtained by chemical cleavage of the cycle of the surfactin S15) has been investigated. A more hydrophobic aliphatic chain induces greater surface-active properties of the lipopeptide. The opening of the peptide ring reduces the surface activity. The effect of phospholipid acyl chain length (dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine- (DPPC), and distearoylphosphatidylcholine) and phospholipid polar head (DPPC, dipalmitoylphosphatidylethanolamine and dipalmitoylphosphatidylserine) on monolayer penetration properties of the surfactin S15 has been explored. Results showed that while the lipid monolayer thickness and the presence of electrostatic repulsions from the interfacial film do not significantly influence surfactin insertion, these parameters strongly modulate the ability of the surfactin to alter the nanoscale organization of the lipid films. We also probed the effect of surfactin structure (influence of the aliphatic chain length and of the cyclic structure of the peptide ring) on the behavior of DPPC monolayers. AFM images and isotherms showed that surfactin penetration is promoted by longer lipopeptide chain length and a cyclic polar head. This indicates that hydrophobic interactions are of main importance for the penetration power of surfactin molecules.

Interaction of the macrolide antibiotic azithromycin with lipid bilayers: effect on membrane organization, fluidity, and permeability.

PURPOSE: To investigate the effect of a macrolide antibiotic, azithromycin, on the molecular organization of DPPC:DOPC, DPPE:DOPC, SM:DOPC, and SM:Chol:DOPC lipid vesicles as well as the effect of azithromycin on membrane fluidity and permeability. METHODS: The molecular organization of model membranes was characterized by atomic force microscopy (AFM), and the amount of azithromycin bound to lipid membranes was determined by equilibrium dialysis. The membrane fluidity and permeability were analyzed using fluorescence polarization studies and release of calcein-entrapped liposomes, respectively. RESULTS: In situ AFM images revealed that azithromycin leads to the erosion and disappearance of DPPC and DPPE gel domains, whereas no effect was noted on SM and SM:cholesterol domains. Although azithromycin did not alter the permeability of DPPC:DOPC, DPPE:DOPC, SM:DOPC, and SM:Chol:DOPC lipid vesicles, it increased the fluidity at the hydrophilic/hydrophobic interface in DPPC:DOPC and DPPE:DOPC models. This effect may be responsible for the ability of azithromycin to erode the DPPC and DPPE gel domains, as observed by AFM. CONCLUSIONS: This study shows the interest of both AFM and biophysical methods to characterize the drug-membrane interactions.

Real-time imaging of drug-membrane interactions by atomic force microscopy.

Understanding drug-biomembrane interactions at high resolution is a key issue in current biophysical and pharmaceutical research. Here we used real-time atomic force microscopy (AFM) imaging to visualize the interaction of the antibiotic azithromycin with lipid domains in model biomembranes. Various supported lipid bilayers were prepared by fusion of unilamellar vesicles on mica and imaged in buffer solution. Phase-separation was observed in the form of domains made of dipalmitoylphosphatidylcholine (DPPC), sphingomyelin (SM), or SM/cholesterol (SM/Chl) surrounded by a fluid matrix of dioleoylphosphatidylcholine (DOPC). Time-lapse images collected following addition of 1 mM azithromycin revealed progressive erosion and disappearance of DPPC gel domains within 60 min. We attribute this effect to the disruption of the tight molecular packing of the DPPC molecules by the drug, in agreement with earlier biophysical experiments. By contrast, SM and SM-Chl domains were not modified by azithromycin. We suggest that the higher membrane stability of SM-containing domains results from stronger intermolecular interactions between SM molecules. This work provides direct evidence that the perturbation of lipid domains by azithromycin strongly depends on the lipid nature and opens the door for developing new applications in membrane biophysics and pharmacology.