Amphiphilic molecules of lipids self-assemble in water to minimise the exposure of their hydrophobic moieties to water. It was found that both the energetics at the lipid–water interface and the lipid molecular shape play a very important role in the aggregation and formation of resultant structures. While the hydrophobic interaction has a tendency to decrease the total surface area, repulsive interactions tend to increase the surface area. These opposing forces give rise to an optimal equilibrium area per lipid molecule. Packing restrictions determine the curvature of lipid monolayer and can also give rise to an optimum aggregate size (Israelachvili et al., 1976). Thus, the hydrophobic effect and structural diversity of lipidic molecules are responsible for the formation of a high variety of their supra-molecular assemblies. Fig. 1 illustrates a few structures of the lyotropic liquid crystalline mesophases formed by lipids: (A) one-dimensional lamellar phase known as multilamellar liposomes (onion-like structure) in excess of water; (B) two-dimensional columnar hexagonal phase and (C) three-dimensional cubic phases of symmetries characterised by space groups. Multilamellar and particularly unilamellar liposomes formed by single lipid bilayer frequently serve as a model system of lipid bilayer of biological membrane. Lipidic mesophases attract attention due to their capability to accommodate a drug into both the water phase and the hydrophobic matrix. Since 1975, when the first demonstration of the improved
Antimicrobial peptides (AMPs) emerged as an interesting alternative to antibiotics that fight against infectious diseases. AMPs are considered membrane-active agents leading to cell death by acting on the phospholipid membrane. All proposed bactericidal mechanisms have the same main initialisation – adsorption of AMPs onto the membrane due to electrostatic interactions between the cationic peptides and the accessible anionic groups of hydrophilic phospholipid headgroups at the membrane surface. Thereafter, accumulation and positional change eventually lead to the formation of pores, membrane permeabilisation or its micellisation (Teixeira et al., 2012). The interaction must be selective regarding the distinction between mammalian cells and pathogen cells (bacteria, fungi, protozoa). Thus, knowledge of the details of AMPs interaction with lipid bilayers of composition mimicking the pathogen membrane is of great importance to evaluate their antimicrobial activity. Small-angle X-ray and neutron scattering (SAXS and SANS, respectively) studies can give the necessary information on the AMPs’ ability to affect the structure and integrity of the membrane.
We studied the interaction of cecropin A–melittin (CAM, nominal charge +6), a hybrid peptide composed of the cationic region of cecropin A and the hydrophobic and non-haemolytic region of melittin with a bacterial model lipid membrane composed of zwitterionic palmitoyl-oleoyl-glycerophosphoethanolamine (POPE) and negatively charged palmitoyl-oleoyl-phosphoglycerol (POPG). SANS and SAXS were used to unravel the mechanism of the peptide antimicrobial activity as described in Silva et al. (2018). Fig. 2A depicts the normalised SANS intensity as a function of the scattering vector
In another study (Silva et al., 2013), the bilayer composed of dimyristoylphosphatidylcholine/dimyristoylphosphoglycerol (DMPC/DMPG = 3:1 mol/mol) mimicking the membrane of
Lipoplexes are formed due to the electrostatic interaction between positively charged liposomes and DNA polyanion. Positively charged complexes show enhanced interaction with the negatively charged cytoplasmic membrane and, therefore, higher cellular uptake via endocytosis. However, for successful DNA delivery, lipoplexes must escape from the endosome inside the cell and DNA must be released from the complex in cytoplasm. High positive surface charge density of the lipoplex is crucial for a successful endosomal fusion. On the other hand, it can be an obstacle for DNA release from the complex. A way to overcome this problem is to use pH-sensitive surfactants with
We prepared pH-sensitive liposomes composed of homologues of series of
In acidic conditions, DNA–C8NO/DOPE at C8NO/DOPE = 0.4 mol/mol forms a condensed inverted hexagonal phase (
Transfection efficiency for plasmid DNA (pDNA, EGFP-N1) was tested on human bone osteosarcoma epithelial cells (U2OS line) and evaluated after 24 and 48 hours by flow cytometry. Commercially available Lipofectamine 2000 (LF) was used as a control. Transfection efficiency follows a quasi-parabolic dependence on the length of C
Pulmonary surfactant (PS) is a surface active film lining the alveoli of the lung (Fig. 6A). Its principal function is to lower the surface tension at the air/liquid interface, facilitate the exchange of gasses and stabilise alveoli during breathing. PS is composed of ~90% lipids and 8%–10% of a few specific surfactant-associated proteins. Phospholipids predominate; saturated dipalmitoylphosphatidylcholine (DPPC) is the most abundant in PS of mammals, up to ~50% by mass. Unsaturated phosphatidylcholines (PC) create ~20% and a smaller fraction (~10%) of anionic species such as POPG. A high content of DPPC plays a key function to reach the necessary, extremely low surface tension (<1 mN/m). Hydrophilic SP-A, SP-D and hydrophobic SP-B, SP-C are surfactant-associated specific proteins. SP-B is crucial for proper spreading of the lipid interfacial monolayer. The surface active function of PS is critically dependent on the existence of a multilayered film (~100 nm in thickness) at the air–water interface. An inactivation, deficiency or an absence of PS results in respiratory distress syndrome (RDS) that can be lethal for premature babies. RDS is therapeutically treated by application of PS preparations obtained from animals (exogenous natural), like porcine Curosurf® or bovine Survanta®.
The exogenous natural PS contains at least 50 different phospholipids and a small fraction of hydrophobic proteins (~1–2 wt%) (Calkovska et al., 2016). Morphologically, it is a mixture of various vesicles, from unilamellar through oligolamellar up to multilamellar.
We studied the effect of bacterial toxin, lipopolysaccharide (LPS), on the structure of clinically modified porcine pulmonary surfactant, mimicking pathological conditions. Consecutively, Polymyxin B (PxB), a cyclic amphiphatic antibiotic, was applied. Fig. 6B shows the SAXS patterns of modified porcine surfactant at 37 °C when the mixture is in liquid crystalline state (fluid phase). Two peaks (
The study was focused on the mechanism underlying the structural changes in surface-reducing features (for details, see Kolomaznik et al., 2018). The function of ‘infected’ and consecutively ‘treated’ PS was tested with pulsating bubble surfactometer. The infected PS (surfactant/LPS 10%) was not able to reach the necessary physiologically relevant low surface tension. Intriguingly, the minimal surface tension decreased alongside with structural recovery when applying PxB (Kolomaznik et al., 2018). The obtained results accurately reflect the situation with a native lung surfactant as confirmed by a recent
Amphiphilic molecules of lipids and surfactants self-assemble into structures of various morphologies ranging from micelles, lamellar and non-lamellar phases to microemulsions. Each of these morphologies possesses its own unique physical properties and offers the possibility for pharmaceutical applications. In the text above, we demonstrated just a few examples of ‘usability’ of phospholipid bilayers: as a model membrane mimicking the lipid bilayer of bacterial membrane in an effort to unravel the mechanisms of AMPs’ antimicrobial activity, as a carrier designed for genetic material delivery, and finally, a lipid mixture extracted from animal's lungs that is clinically used in neonatal medicine. Findings presented here result from the collaboration between several institutions and have been published to the full extent in Kolomaznik et al. (2018), Liskayová et al. (2019), Silva et al. (2013) and Silva et al. (2018).