Monolayers and planar or curved bilayers

Polymeric lipid assemblies as novel theranostic tools

Polymerizable lipids have been used in research and medical applications such as membrane models, imaging platforms, drug delivery systems, vaccine carriers, biosensors, and coating materials. The polymerization of these lipid molecules forms a covalent bond between lipid moieties, which improves the noncovalent interactions that maintain the lipid lamellar phase architecture and increases the stability of the polymerized system. Because such lipid molecules form nanoassemblies with modifiable structures that acquire the stability of polymers following covalent bond formation, these lipids are of considerable interest in the emerging field of theranostics. In this Account, we summarize the biomedical applications of polymerizable lipids (primarily phospholipids) in the context of various nanoplatforms. We discuss stable nanoplatforms, which have been used in a variety of theranostics applications. In addition, we describe methods for assembling triggerable theranostics by combining appropriate nonpolymerizable lipids with polymerizable lipids. Polymeric lipids hold promise as nanotools in the field of medical imaging, targeting, and on-demand drug delivery. Because of their similarity to biological lipids, long-term toxicity issues from polymerizable lipid nanoplatforms are predicted to be minimal. Although the field of polymeric nanocapsules is still in development, intensive efforts are underway to produce systems which could be applied to disease diagnosis and treatment. We envision that nanoimaging platforms coupled with localized drug delivery technology will have a significant impact on cancer therapy and other related diseases. The existing wealth of clinical knowledge both in the photochemistry of imaging agents and/or drugs and modifications of these agents using light will prove valuable in the further development of polymeric theranostic lipid-based nanoparticles.

Proton sponge and fatty acid interactions at the air-water interface. Thermodynamic, spectroscopic, and microscopic study

Mixed Langmuir and Langmuir-Blodgett (LB) films of a proton sponge, namely, diphenyl bis(octadecylamino)phosphonium bromide, and a fatty acid, docosanoic acid, with different molar ratios have been fabricated. Surface pressure versus area per molecule isotherms were registered, and the excess areas and excess Gibbs energy of mixing were calculated. Strong interactions between the proton sponge and the fatty acid take place at the air-water interface. The existence of a stoichiometric 1:1 acid-base reaction between the two components forming a complex on the water surface at high surface pressures has been demonstrated. Furthermore, the reaction had an efficiency close to 100% at the air-water interface; meanwhile, it hardly takes place in organic solvents such as chloroform or even a mixture of chloroform and dimethyl sulfoxide. The floating films were transferred to solid supports and characterized by means of several techniques including IR spectroscopy, X-ray diffraction, and atomic force microscopy, revealing the presence of highly ordered alkyl chains and a constant architecture along the different layers as well as the presence of different domains in the LB films, except those having a 1:1 proton sponge-fatty acid ratio that are homogeneous. Such domains have been interpreted as the presence of two different phases, the 1:1 complex plus the excess component in the mixture.

Interactions and reactions of monolayers and Langmuir-Blodgett multilayers with compounds in the bulk phase

Studies performed on the interactions and reactions of compounds in the bidimensional state, essentially in monolayers and Langmuir-Blodgett multilayers, with substances in the aqueous subphase are reported. More precisely, the following is illustrated: (i) interactions between acid amphiphiles and prevalently bivalent ions placed in the aqueous support and between compounds capable of functioning like ion carriers in monolayers and ions in the subphase, in order to build mimetic membranes capable of selective ion transport; and the complexation of amphiphiles in monolayer with ions in the bulk liquid phase, in order to build chemical sensors to ions; (ii) the reactions of photoinduced electron transfer between a partner in mono- or multimolecular films and a partner in the subphase, which may determine the fundamental parameters and the differences with the same reactions in the bulk phase; and (iii) the reactions of enzymatic hydrolysis between the monolayer of a glyceride, which constitutes the reaction support, and the enzyme in the liquid bulk phase, which constitutes the subphase. The mechanism of the reactions and its inhibition are clarified. To conclude, possible future developments connected with the areas studied are examined.

Specific binding of avidin to biotin containing lipid lamella surfaces studied with monolayers and liposomes

The interaction of avidin (from egg white) with phospholipid (monolayer and bilayer) model membranes containing biotin-conjugated phospholipids has been studied. In the first part, using surface sensitive techniques (ellipsometry and surface plasmon resonance) we demonstrated that the nonspecific adsorption of avidin to phospholipid lamella could be abolished by adding an amount of Ca2+, Mg2+ or Ba2+ that led to an electrostatic interaction. The specific binding of avidin to lipid mixtures containing biotin-conjugated phospholipids was obviously composition dependent. The ratio 1:12 of a B-DPPE/DPPE mixture was found to be the optimum molar ratio. When we compared the results from the surface sensitive techniques with those from the electron micrographs of a two dimensional crystal of avidin (obtained in our laboratory), the optimum ratio was found to be determined by the effect of lateral steric hindrance. In the second part, we observed the pattern of the layers of fluorescently labeled phospholipid and adsorbed proteins with a home-made micro fluorescence film balance. The fluorescence images showed that avidin was preferentially bound to the receptors that were in the fluid domains. Further, with a sensitive fluorescence assay method, the effect of the phase behavior of liposomes on the specific binding of avidin was measured. This showed that avidin interacted with biotinlipid more weakly in the gel state liposome than in the liquid state liposome. The major conclusion was that the binding of avidin to a membrane bound model receptor was significantly restricted by two factors: one was the lateral steric hindrance and the other was the fluidity of the model membrane.

Lateral inhomogeneous lipid membranes: theoretical aspects

The physical concepts underlying the lateral distribution of the components forming a lamellar assembly of amphiphiles are discussed in this review. The role of amphiphiles' molecular structure and/or aqueous environment (ionic strength, water soluble substances) on formation and stability of lateral patterns is investigated. A considerable effort is devoted to the analysis of the properties of patterned structure which can be different from those of randomly mixed multi-component lamellae. Examples include adhesion and fusion among laterally inhomogeneous bilayers, enhanced interfacial adsorption of ions and polymers, enhanced transport across the bilayer, modified mechanical properties, local stabilization of non-planar geometries (pores, edges) and related phenomena (electroporation, budding transition and so on). Furthermore, an analysis of chemical reactivity within or at the water interface of a laterally inhomogeneous bilayer is briefly discussed. A link between these concepts and experimental findings taken from the biological literature is attempted throughout the review.