Balance of Solvent and Chain Interactions Determines the Local Stress State of Simulated Membranes.

Characterization of the internal mechanical state of model lipid membranes is essential to understand the microscopic underpinnings of biological functions such as membrane fission and organelle shaping within the context of elastic theories such as the Helfrich framework. Here, we compute lateral stress or pressure profiles from molecular dynamics simulations of lipid bilayers and water-vacuum interfaces to understand the role that solvent treatment and force-field parametrization plays on the local mechanical features of membranes. We focus on two atomistic models, GROMOS 43A1-S3 and CHARMM36, and several variants of the MARTINI coarse-grained force-field, including the single-bead nonpolar water, three-point polarizable water, big multipole water, and solvent-free variants. Our results show that the various atomistic and coarse-grained force-fields produce contrasting lateral stress profiles as a result of the balance of solvent-solvent and solvent-solute forces at the hydrocarbon-water interface and fundamentally different treatment of pairwise (e.g., van der Waals, Coulomb, etc.) and multibody interactions (angles and torsions). Numerical integration of the second moment of the bilayer stress profiles indicates that different local distributions of repulsive and attractive stresses across the membrane, due to distinct force-field parametrizations, may result in substantial variations in macroscopic elastic properties.

Application of liposomes to sonoporation.

A method to prepare liposomes is presented. Liposomes made in our laboratory were characterized acoustically and optically. The phase velocity and attenuation of liposomes in suspension (concentration = 10(9)/mL) were measured, ranging from 2 to 14 MHz, using ultrasound spectroscopy. Anti-rabbit IgG conjugated with Alexafluor 647 was delivered into Jurkat cells in suspension, using the liposomes, by 10 % duty cycle ultrasound tonebursts of 2.2 MHz (the in situ spatial peak-pressure amplitude = 80 W/cm2) with an efficiency of 13 %. It has been experimentally shown that liposomes may be an alternative stable agent to Optison for delivering macromolecules into cells.