Line tension of multicomponent bilayer membranes

Prognosticating the effect of temperature and pH parameters on size and stability of the nanoliposome system based on thermodynamic modeling

The main challenge of using nanoliposome systems is controlling their size and stability. In order to overcome this challenge, according to the research conducted at the Research Centre for New Technologies of Biological Engineering, University of Tehran, a model for predicting the size and stability of nanoliposome systems based on thermodynamic relations has been presented. In this model, by using the presented equations and without performing many experiments in the laboratory environment, the effect of temperature, ionic power and different pH can be considered simultaneously whereas examining the components of size, stability and any feature were considered before. Synthesis and application of liposomal nanocarriers in different operating conditions can be investigated and predicted, and due to the change in temperature and pH, the smallest size of th system can be obtained. In this study, we were able to model the synthesis and storage conditions of liposomal nanocarriers at different temperatures and acidic, neutral and alkaline pHs, based on the calculation of mathematical equations. This model also indicates that with increasing temperature, the radius increases but with increasing pH, the radius first increases and then decreases. Therefore, this model can be used to predict size and stability in different operating conditions. In fact, with this modelling method, there is no need to study through laboratory methods and analysis to determine the size, stability and surface loads, and in terms of Accuracy, time and cost savings are affordable.

Free energy of hydrophilic and hydrophobic pores in lipid bilayers by free energy perturbation of a restraint

The free energy of pore formation in lipid bilayers has been previously calculated using a variety of reaction coordinates. Here, we use free energy perturbation of a cylindrical lipid exclusion restraint to compute the free energy profile as a function of pore radius in dimyristoylphosphatidylcholine (DMPC) and dioleoylphosphatidylcholine (DOPC) bilayers. Additionally restraining the headgroups to lie on the membrane surface allows us to also calculate the free energy profile of hydrophobic pores, i.e., cylindrical pores lined by acyl chains. For certain pore radii, the free energy of wetting of hydrophobic pores is calculated using the density bias method. It is found that wetting of hydrophobic pores becomes thermodynamically favorable at 5.0 Å for DMPC and 6.5 Å for DOPC, although significant barriers prevent spontaneous wetting of the latter on a nanosecond time scale. The free energy of transformation of hydrophilic pores to hydrophobic ones is also calculated using free energy perturbation of headgroup restraints along the bilayer normal. This quantity, along with wetting and pore growth free energies, provides complete free energy profiles as a function of radius. Pore line tension values for the hydrophilic pores obtained from the slope of the free energy profiles are 37.6 pN for DMPC and 53.7 pN for DOPC. The free energy profiles for the hydrophobic pores are analyzed in terms of elementary interfacial tensions. It is found that a positive three-phase line tension is required to explain the results. The estimated value for this three-phase line tension (51.2 pN) lies within the expected range.

Regulation Mechanism of Bubbling Deformation and Fracture Toughness of the Membrane by Asymmetric Phospholipids: A Model System Study

Dynamic regulation of the deformation modulus and fracture toughness of a membrane is critical to organelles and cells for matching their conflicting needs of resilient and fractured behaviors. These properties implement the protection of the function in the normal condition and the fission function in the endocytosis condition of a membrane. Naturally, a membrane contains phospholipids that have different hydrophilic and hydrophobic group length. The diffusion and aggregation of the phospholipids with asymmetry of the hydrophilic-hydrophobic ratio on the membrane play a key role in regulating the mechanical behaviors passively to the external force. In present work, the effects of the asymmetry of phospholipids on the bubbling deformation and fracture toughness of the membrane to external stretching are investigated in a model system. A disk-shaped micelle formed from the blend of symmetric and asymmetric diblock copolymers in a selective solvent is considered as the membrane sheet. Its mechanically responsive behaviors are investigated by self-consistent field theory. By analyzing the evolution of different components during the stretching process, the mechanism of formation of the bubbling structure is revealed. Moreover, the fracture toughness depending on the asymmetry of the phospholipids is determined quantitatively.

Elastic property of membranes self-assembled from diblock and triblock copolymers

The elastic property of membranes self-assembled from AB diblock and ABA triblock copolymers, as coarse-grained model of lipids and the bolalipids, are studied using the self-consistent field theory (SCFT). Specifically, solutions of the SCFT equations, corresponding to membranes in different geometries (planar, cylindrical, spherical, and pore) have been obtained for a model system composed of amphiphilic AB diblock copolymers and ABA triblock copolymers dissolved in A homopolymers. The free energy of the membranes with different geometries is then used to extract the bending modulus, Gaussian modulus, and line tension of the membranes. The results reveal that the bending modulus of the triblock membrane is greater than that of the diblock membrane. Furthermore, the Gaussian modulus and line tension of the triblock membrane indicate that the triblock membranes have higher pore formation energy than that of the diblock membranes. The equilibrium bridging and looping fractions of the triblock copolymers are also obtained. Implications of the theoretical results on the elastic properties of biologically equivalent lipid bilayers and the bolalipid membranes are discussed.

Pore formation in lipid membrane I: Continuous reversible trajectory from intact bilayer through hydrophobic defect to transversal pore

Lipid membranes serve as effective barriers allowing cells to maintain internal composition differing from that of extracellular medium. Membrane permeation, both natural and artificial, can take place via appearance of transversal pores. The rearrangements of lipids leading to pore formation in the intact membrane are not yet understood in details. We applied continuum elasticity theory to obtain continuous trajectory of pore formation and closure, and analyzed molecular dynamics trajectories of pre-formed pore reseal. We hypothesized that a transversal pore is preceded by a hydrophobic defect: intermediate structure spanning through the membrane, the side walls of which are partially aligned by lipid tails. This prediction was confirmed by our molecular dynamics simulations. Conversion of the hydrophobic defect into the hydrophilic pore required surmounting some energy barrier. A metastable state was found for the hydrophilic pore at the radius of a few nanometers. The dependence of the energy on radius was approximately quadratic for hydrophobic defect and small hydrophilic pore, while for large radii it depended on the radius linearly. The pore energy related to its perimeter, line tension, thus depends of the pore radius. Calculated values of the line tension for large pores were in quantitative agreement with available experimental data.

A comparison between liquid drops and solid particles in partial wetting

In this critical review we compare two geometries in partial wetting: a liquid drop on a planar substrate and a spherical particle at a planar liquid interface. We show that this comparison is far from being trivial even if the same physical interactions are at play in both geometries. Similarities and differences in terms of free energies and frictions will be discussed. Contact angle hysteresis, the impact of surface roughness and line pinning on wetting will be described and compared to selected experimental findings.

Lipoprotein insertion into membranes of various complexity: lipid sorting, interfacial adsorption and protein clustering

In a combined chemical-biological and biophysical approach we explored the membrane partitioning of the lipidated signaling proteins N-Ras and K-Ras4B into membrane systems of different complexity, ranging from one-component lipid bilayers and anionic binary and ternary heterogeneous membrane systems even up to partitioning studies on protein-free and protein-containing giant plasma membrane vesicles (GPMVs). To yield a pictorial view of the localization process, imaging using confocal laser scanning and atomic force microscopy was performed. The results reveal pronounced isoform-specific differences regarding the lateral distribution and formation of protein-rich membrane domains. Line tension is one of the key parameters controlling not only the size and dynamic properties of segregated lipid domains but also the partitioning process of N-Ras that acts as a lineactant. The formation of N-Ras protein clusters is even recorded for almost vanishing hydrophobic mismatch. Conversely, for K-Ras4B, selective localization and clustering are electrostatically mediated by its polybasic farnesylated C-terminus. The formation of K-Ras4B clusters is also observed for the multi-component GPMV membrane, i.e., it seems to be a general phenomenon, largely independent of the details of the membrane composition, including the anionic charge density of lipid headgroups. Our data indicate that unspecific and entropy-driven membrane-mediated interactions play a major role in the partitioning behavior, thus relaxing the need for a multitude of fine-tuned interactions. Such a scenario seems also to be reasonable recalling the high dynamic nature of cellular membranes. Finally, we note that even relatively simple models of heterogeneous membranes are able to reproduce many of the properties of much more complex biological membranes.