Stretch-Induced Interdigitation of a Phospholipid/Cholesterol Bilayer

Interdigitation of lipids for vesosomal formulation of ergotamine tartrate with caffeine: a futuristic trend of intranasal route

OBJECTIVE

This research work aimed to form vesosomes using combination of two drugs ergotamine (ERG) and caffeine for synergistic activity when given intranasally resulting in faster absorption, steric stability, and controlled release.

SIGNIFICANCE

The multicompartment vesicles viz., vesosomes of ERG tartrate proved to increase absorption of drugs post-intranasal administration, bypassing the blood-brain barrier via the olfactory pathway.

METHODS

The phospholipids like soya lecithin, cholesterol, and dipalmitoyl phosphatidylcholine (DPPC) were used to form a multicompartment structure called vesosomes using ethanol-induced interdigitation of lipids as the preparation method.

RESULTS

The formulation showed low particle size (PS) of 315.48 ± 14.27 nm with zeta potential (ZP) of -21.78 ± 4.72 mV, higher % EE of 91.13 ± 1.29%, and controlled release kinetics, when assessed for in-vitro and ex-vivo studies as 97.64 ± 5.13% and 82.25 ± 3.27% release, respectively. Vesosomes displayed several advantages over liposomes like improved stability against phospholipase-induced enzymatic degradation and higher brain uptake 3.41-fold increase of ERG via the olfactory pathway.

CONCLUSIONS

The stable vesosomes prepared using interdigitation of saturated phospholipids proved to be a viable option for ERG when administered intranasally for better absorption and bioavailability coupled with ease of administration gaining wider patient acceptance.

Changes in free energy barrier for water permeation by stretch-induced phase transitions in phospholipid/cholesterol bilayers

Water permeation through phospholipid/cholesterol bilayers is the key to understanding tension-induced rupture of biological cell membranes. We performed molecular dynamics simulations of stretched phospholipid/cholesterol bilayers to investigate changes in the free energy profile of water molecules across the bilayer and the lipid structure responsible for water permeation. We modeled stretching of the bilayer by applying areal strain. In stretched phospholipid/cholesterol bilayers, the hydrophobic tail of the phospholipids became disordered and the free energy barrier to water permeation decreased. Upon exceeding the critical areal strain, a phase transition to an interdigitated gel phase occurred before rupture, and the hydrophobic tail ordering as well as the free energy barrier were restored. In pure phospholipid bilayers, we did not observe such recoveries. These transient recoveries in the phospholipid/cholesterol bilayer suppressed water permeation and membrane rupture, followed by an increase in the critical areal strain at which the bilayer ruptured. This result agrees with experimental results and provides a reasonable molecular mechanism for the toughness of phospholipid/cholesterol bilayers under tension.Communicated by Ramaswamy H. Sarma.

Free Energy Cost of Interdigitation of Lamellar Bilayers of Fatty Alcohols with Cationic Surfactants from Molecular Dynamics Simulations

Cationic surfactant mixed with fatty alcohol as cosurfactant in excess water can form stable emulsions, known as "lamellar gel networks," that contain extended and interconnected networks of swollen bilayers, including ones with in-plane liquidlike disorder (L_α phase) and solidlike order (L_β phase). To study their structure and thermodynamics, molecular dynamics (MD) simulations with lateral pressure and temperature scans along reversible pathways were used to drive reversible phase changes, including formation at negative lateral pressure of the L_βI phase with interdigitated tails of opposing leaflets. Thermodynamic integration, with extrapolations to infinitely slow scans, yielded a free energy difference between the interdigitated L_βI and non-interdigitated L_β phases of 2.4 ± 0.5 kJ/mol, which is consistent with the spontaneous formation of the L_β phase under atmospheric pressure in simulation. Thermodynamic cycles involving temperature and lateral pressure for which the free energy difference is identically zero were constructed as negative controls to verify the method. Using lateral pressure, including negative lateral pressure, helps avoid kinetic bottlenecks that occur when temperature alone is used as the control variable. The method, using negative lateral pressure, should be widely applicable to other bilayers to identify molecular properties that control interdigitation and other bilayer properties.

Kelvin-Helmholtz-like instability of phospholipid bilayers under shear flow: System-size dependence

We performed a series of molecular dynamics (MD) simulations of phospholipid bilayers under shear flow to estimate the effect of the system size on Kelvin-Helmholtz (KH)-like instability of the bilayer at the molecular scale. To extend the estimation by the MD simulations to the microscale, we introduced linear stability analysis for the fluid-fluid interface consisting of a thin membrane. For both the MD simulations and theoretical model, the critical velocity difference across the bilayer, where instability occurs, decreased with increasing wavelength of the bilayer undulation λ, which corresponds to the system size. When λ was more than about ten times larger than the bilayer thickness, the critical velocity difference in the MD simulations was in quantitative agreement with that obtained by the theoretical model. This means that the theoretical model is applicable for the shear-induced KH-like instability of the bilayer for large λ. The theoretical model showed that the critical velocity difference for the KH-like instability was proportional to λ^{-3/2}. Based on these results, we discuss the implications of the shear-induced bilayer instability in the shear-induced cell damage observed in experiments.

Artificial Phospholipids and Their Vesicles

Phospholipids are at the heart and origin of life on this planet. The possibilities in terms of phospholipid self-assembly and biological functions seem limitless. Nonetheless, nature exploits only a small fraction of the available chemical space of phospholipids. Using chemical synthesis, artificial phospholipid structures become accessible, and the study of their biophysics may reveal unprecedented properties. In this article, the recent advances by our work group in the field of chemical lipidology are summarized. The family of diamidophospholipids is discussed in detail from monolayer characterization to the formation of faceted vesicles, culminating in the template-free self-assembly of phospholipid cubes and the possible applications of vesicle origami in modern personalized medicine.