Juan Sabín

Interactions between DMPC liposomes and the serum blood proteins HSA and IgG.

The interaction between two serum blood proteins, namely human serum albumin (HSA) and human immunoglobulin G (IgG), with 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) liposomes has been studied in detail using dynamic light scattering, flow cytometry, enzyme-linked immunosorbent assay (ELISA), electrophoretic mobility, differential scanning calorimetry (DSC), and surface tension measurements. HSA and IgG interact with liposomes forming molecular aggregates that remain stable at protein concentrations beyond those of total liposome coverage. Both HSA and IgG penetrate into the liposome bilayer. An ELISA assay indicates that the Fc region of IgG is the one that is immersed in the DMPC membrane. The liposome-protein interaction is mainly of electrostatic nature, but an important hydrophobic contribution is also present.

Interaction of a charged polymer with zwitterionic lipid vesicles.

The interaction between polyethylenimine (PEI) and phospholipid bilayers plays an important role in several biophysical applications such as DNA transfection of target cells. Despite considerable investigation into the nature of the interaction between PEI and phospholipid bilayers, the physical process remains poorly understood. In this paper, we study the impact of PEI on 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles as a function of salt concentration using several techniques including dynamic (DLS) and static (SLS) light scattering, differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR). At low salt concentration, vesicles aggregate, leading to the formation of stable clusters whose final size depends on the PEI concentration. At high salt concentration the system does not aggregate; DSC and NMR data reveal that the PEI penetrates into the bilayer, and SLS measurements are consistent with PEI crossing the bilayer. The transfectional ability of PEI is discussed in terms of these results.

Double Charge Inversion in Polyethylenimine-Decorated Liposomes

The study of the interaction of a cationic polymer as PEI with phospholipids membranes is of special relevance for gene therapy because the PEI is a potential nonviral vector to transfer DNA in living cells. We used light scattering, zeta potential, and electron transmission microscopy to characterize the interaction between DMPG and DOPC liposomes with PEI as a function of the charge molar ratio, pH, temperature, initial size of the liposomes, and headgroup of the lipids. Unexpectedly, a double charge inversion and two different ranges of PEI-liposome concentrations where an aggregation occurs were found, when the proper pH and initial size of the liposomes were chosen. The interaction is analyzed in terms of the interaction potential proposed by Velegol and Thwar for colloidal particles with a nonuniform surface charge distribution. Results show a remarkable dependence of the stability on pH and the initial size of the liposomes, which explains the low reproducibility of the experiments if no special care is taken in preparing the samples. Comparatively small changes in the pH or in the liposomes size lead to a completely different stability behavior.

Stable clusters in liposomic systems

The formation of stable clusters in colloidal systems is usually explained by the combination of a short ranged attractive force and a repulsive force with an interaction range in the order of the colloidal particle size. Using light scattering techniques we studied five different liposomic systems undergoing aggregation through five different mechanisms: aggregation by charge neutralization, DLCA and RLCA by charge screening, aggregation in a secondary minimum, depletion attraction and “patchy” electrostatic interactions. Surprisingly, three of these systems lead to the formation of stable clusters despite the Debye length of the electrostatic repulsion being less than 5% of the diameter of the liposomes. Alternative explanations for the stabilization of the clusters are discussed using non-DLVO forces. The understanding and controlling of the formation of stable clusters of liposomes may have an important impact in their application as a drug delivery system.

Influence of temperature on the colloidal stability of the F-DPPC and DPPC liposomes induced by lanthanum ions

The influence of La(3+) on the colloidal stability of liposomes made up by two zwitterionic phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-[16-fluoropalmitoyl-phosphatidylcholine (F-DPPC), in aqueous media has been investigated by dynamic light scattering and electrophoretic mobility. The critical aggregation concentration (c.a.c.) of La(3+) for F-DPPC and DPPC liposomes were experimentally obtained, and the results were compared with theoretical predictions using the Derjaguin-Landau-Verwey-Overbeek theory. In order to evaluate the influence of the state of the bilayer on the stability of liposomes, all experiments were performed at temperatures below and above the chain-melting phase-transition temperature of lipids (transition temperature of lipids). Changes in the size of both types of liposomes and high values of polydispersity in the presence of La(3+) showed that these ions induce aggregation of liposomes at 25 °C and at 60 °C. At 25 °C, when the bilayer of F-DPPC liposomes is interdigited, DPPC liposomes are more resistant to aggregation than the liposomes formed with F-DPPC. However, this difference disappears at 60 °C, when both bilayers have the same conformation. The experimental results also indicate that the c.a.c. is higher at 60 °C than at 25 °C for both types of liposomes. In fact, it has been observed by dynamic light scattering measurements that aggregation of liposomes at 25 °C can be prevented by increasing the solution temperature for La(3+) concentrations near to the c.a.c. Moreover, the behavior of these liposomes in the presence of the ion was studied at temperatures above and below the transition temperature of the phospholipids.

Insertion of semifluorinated diblocks on DMPC and DPPC liposomes. Influence on the gel and liquid states of the bilayer

Differential Scanning Calorimetry (DSC) was used to study the effect of the incorporation of a series of semifluorinated diblocks F(n)H(m) (F(6)H(10), F(6)H(16), F(8)H(14), F(8)H(16), F(8)H(18) and F(8)H(20)) on the gel and liquid states of the bilayer of large multilamellar DMPC and DPPC liposomes. The presence of the F(n)H(m) diblocks affects slightly the T(m) of the main gel-liquid transitions of DMPC and DPPC, but is accompanied by the appearance of a second transition in the calorimetric traces whose T(m) is mainly determined by the length of the F(n) segment. The DSC results are consistent with the previously established conclusion that the F(n) segments of the diblocks form a central layer in the core of the lipid bilayer, with the H(m) segments being interdigitated with the lipid chains. The DSC traces suggest that the structure of the fluorinated liposomes is a double bilayer at 3:4 and 1:2 and a trilayer at 2:1 lipid/F(n)H(m) molar ratios. At temperatures between the two phase transitions T(m)s, the fluorinated liposomes are neither in a gel-like or a liquid-like state but rather possess both characteristics.

Studying colloidal aggregation using liposomes.

Colloidal aggregation using liposomes has been studied in this chapter. As criteria of stability, the stability factor, an extension of the DLVO theory of colloidal stability, the fractal dimension of the liposome aggregates and the different regimes of aggregation (RLCA and DLCA) and the temperature have been used.

Fractal aggregates in tennis ball systems

We present a new practical exercise to explain the mechanisms of aggregation of some colloids which are otherwise not easy to understand. We have used tennis balls to simulate, in a visual way, the aggregation of colloids under reaction-limited colloid aggregation (RLCA) and diffusion-limited colloid aggregation (DLCA) regimes. We have used the images of the cluster of balls, following Forrest and Wittens pioneering studies on the aggregation of smoke particles, to estimate their fractal dimension.