The theory of metal electronucleation applied to the study of fundamental properties of liposomes

This short review describes how the theory of electrochemical metal nucleation considering non-stationary effects due to the activation of latent nucleation sites has been successfully translated and applied to describe phenomena observed on lipid membranes. This rather unexpected connection is merely formal, but has resulted in a completely new approach in liposome research. It has been proposed that hydrophobic active sites spontaneously and constantly appear and disappear on lipid membranes. These sites control the affinity of liposomes for hydrophobic surfaces and determine the permeability of the lipid membrane to small hydrophilic molecules. Thus, the kinetic models for liposome adhesion on hydrophobic substrates and for the spontaneous leakage of liposomal content are identical to that of non-stationary nucleation mentioned above. Therefore, the broad scope of the available work on metal nucleation has facilitated the interpretation of the data obtained in liposome research. Future applications of the nucleation model in the realm of liposomes are also discussed

An overview of surface forces and the DLVO theory

This lecture text focuses on surface forces and interactions in a liquid medium, with particular emphasis on the surface-surface interactions described by the DLVO theory, i.e., van der Waals attraction and electric double-layer repulsion. The text begins by describing the fundamental forces of nature, their connection to intermolecular interactions, and how the latter result in measurable forces between surfaces and macroscopical objects. A step-by-step reasoning on how DLVO forces arise is then presented, accompanied by a simplified description of the mathematical derivations of the main equations within the framework of the theory. The connection between the DLVO theory and the prediction of the stability of colloidal systems is presented. Examples on how the colloidal stability can be controlled or tuned are presented. The shortcomings of the original DLVO theory are discussed, and recent extended models dealing with these issues are briefly described. The text closes with a general overview of some of the most relevant non-DLVO interaction

Avoiding artifacts in liposome leakage measurements via cuvette- and liposome-surface modifications

The barrier properties of lipid membranes are often determined by investigating their solute permeability with the help of spectroscopic methods and the use of liposome-encapsulated self-quenching fluorescent dyes, for example, Carboxyfluorescein (CF). It was shown previously that liposome-surface interactions, and thus the choice of cuvette material, influence the result of such spectroscopic permeability/leakage experiments. In this work, we explore different methods to minimize the artifacts observed in spontaneous leakage measurements performed with cholesterol-containing liposomes. The spontaneous leakage of CF from liposomes with different composition and surface properties is monitored in cuvettes composed of quartz, polystyrene (PS), and Poly(methyl methacrylate) (PMMA). Our results show that significantly different leakage profiles are recorded for the exact same liposome batch depending on the cuvette material used. Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D) experiments indicate that these discrepancies likely arise from side processes occurring at the solution-cuvette interface, mainly, the attaching and spreading of liposomes. Further, we show that in some cases it is possible to minimize liposome-cuvette interactions, and reduce the experimental artifacts, by supplementing the liposomes with polyethylene glycol (PEG)-grafted lipids or gangliosides, and/or by pre-adsorbing free PEG to the cuvette walls. The collected data suggest that quartz cuvettes modified by adsorption of PEG8000 are suitable for spontaneous leakage experiments with POPC:cholesterol-based liposomes, while other cuvette materials perform poorly in the same experiments

Adhesion and Structural Changes of PEGylated LipidNanocarriers on Silica Surfaces

PEGylated lipid nanoparticles have a continuously expanding range of applications,particularly within pharmaceutical areas. Hereby, it is shown with the help of the Quartz CrystalMicrobalance with Dissipation monitoring (QCM-D) and other surface sensitive techniques that, atroom temperature, PEGylated liposomes and lipodisks adhere strongly to silica surfaces resultingin the displacement of the hydration layer of silica and the formation of immobilized nanoparticlefilms. Furthermore, it is shown that drastic changes in the structure of the immobilized films occurif the temperature is increased to >35 ◦C. Thus, intact immobilized PEGylated liposomes ruptureand spread, even in the gel phase state; immobilized lipodisks undergo complete separation of theircomponents (bilayer forming lipids and PEGylated lipids) resulting in a monolayer of adsorbedPEGylated lipids; and PEGylated supported lipid bilayers release part of the water trapped betweenthe lipid membrane and the surface. It is hypothesized that these changes occur mainly due to thechanges in the configuration of PEG chains and a drastic decrease of the affinity of the polymer forwater. The observed phenomena can be applied, e.g., for the production of defect-free supportedlipid bilayers in the gel or liquid ordered phase states

Improved accuracy and reproducibility of spontaneous liposome leakage measurements by the use of supported lipid bilayer-modified quartz cuvettes

Recent studies have revealed avid interactions between liposomes and several solid materials, such as quartz, polystyrene (PS) and poly(methyl methacrylate) (PMMA), commonly found in cuvettes used for spectroscopic measurements. These interactions risk leading to detrimental changes in liposome structure and integrity that, if overlooked, may compromise the measurements. In case of leakage experiments based on probing the spontaneous release of encapsulated hydrophilic markers, the liposome-cuvette interactions may result in the recording of erroneously high degrees of leakage. In the present study we investigate the possibilities of preventing unwanted liposome-cuvette interactions through the use of quartz cuvettes passivated with supported lipid bilayers (SLBs). The results show that this strategy leads to higher reproducibility and significantly improved accuracy of the leakage measurements. The usefulness of the method is validated in comparative experiments focused on how changes in temperature and lipid phase state, as well as inclusion of poly(ethylene glycol)-conjugated lipids (PEG-lipids), affect the release of liposome encapsulated carboxyfluorescein (CF).Title in thesis: Improved accuracy and reproducibility of liposome leakage measurements by the use of supported lipid bilayer-modified quartz cuvettes</p

A closer look at calcium-induced interactions between phosphatidylserine-(PS) doped liposomes and the structural effects caused by inclusion of gangliosides or polyethylene glycol- (PEG) modified lipids

The effects of polyethylene glycol- (PEG) modified lipids and gangliosides on the Ca2+ induced interaction between liposomes composed of palmitoyl-oleoyl phosphatidylethanolamine (POPE) and palmitoyl-oleoyl phosphatidylserine (POPS) was investigated at physiological ionic strength. Förster resonance energy transfer (FRET) studies complemented with dynamic light scattering (DLS) and cryo-transmission electron microscopy (Cryo-EM) show that naked liposomes tend to adhere, rupture, and collapse on each other's surfaces upon addition of Ca2+, eventually resulting in the formation of large multilamellar aggregates and bilayer sheets. Noteworthy, the presence of gangliosides or PEGylated lipids does not prevent the adhesion-rupture process, but leads to the formation of small, long-lived bilayer fragments/disks. PEGylated lipids seem to be more effective than gangliosides at stabilizing these structures. Attractive interactions arising from ion correlation are proposed to be a driving force for the liposome-liposome adhesion and rupture processes. The results suggest that, in contrast with the conclusions drawn from previous solely FRET-based studies, direct liposome-liposome fusion is not the dominating process triggered by Ca2+ in the systems studied

Osmoprotective effect of ubiquinone in lipid vesicles modelling the E. coli plasma membrane

Bacteria need to be able to adapt to sudden changes in their environment, including drastic changes in the surrounding osmolarity. As part of this adaptation, the cells adjust the composition of their cytoplasmic membrane. Recent studies have shown that ubiquinones, lipid soluble molecules involved in cell respiration, are overproduced by bacteria grown in hyperosmotic conditions and it is thus believed that these molecules can provide with osmoprotection. Hereby we explore the mechanisms behind these observations. Liposomes with a lipid headgroup composition mimicking that of the cytoplasmic membrane of E. coli are used as suitable models. The effect of ubiquinone-10 (Q10) on water transport across the membranes is characterized using a custom developed fluorescence-based experimental approach to simultaneously determine the membrane permeability coefficient and estimate the elastic resistance of the membrane towards deformation. It is shown that both parameters are affected by the presence of ubiquinone-10. Solanesol, a molecule similar to Q10 but lacking the quinone headgroup, also provides with osmoprotection although it only improves the resistance of the membrane against deformation. The fluorescence experiments are complemented by cryogenic transmission electron microscopy studies showing that the E. coli membrane mimics tend to flatten into spheroid oblate structures when osmotically stressed, suggesting the possibility of lipid segregation. In agreement with its proposed osmoprotective role, the flattening process is hindered by the presence of Q10

Dissolution mechanism of supported phospholipid bilayer in the presence of amphiphilic drug investigated by neutron reflectometry and quartz crystal microbalance with dissipation monitoring

The influence and interaction of the ionizable amphiphilic drug amitriptyline hydrochloride (AMT) on a 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) phospholipid bilayer supported on a silica surface have been investigated using a combination of neutron reflectometry and quartz crystal microbalance with dissipation monitoring. Adding AMT solutions with concentrations 3, 12, and 50 mM leaves the lipid bilayer mainly intact and we observe most of the AMT molecules attached to the head-group region of the outer bilayer leaflet. Virtually no AMT penetrates into the hydrophilic head-group region of the inner leaflet close to the silica surface. By adding 200 mM AMT solution, the lipid bilayer dissolved entirely, indicating a threshold concentration for the solubilization of the bilayer by AMT. The observed threshold concentration is consistent with the observation that various bilayer structures abruptly transform into mixed AMT-DOPC micelles beyond a certain AMT-DOPC composition. Based on our experimental observations, we suggest that the penetration of drug into the phospholipid bilayer, and subsequent solubilization of the membrane, follows a two-step mechanism with the outer leaflet being removed prior to the inner leaflet