Structural and Mechanical Effects of Calcium on the Lipid Bilayer

Abstract

Calcium is a ubiquitous divalent cation involved in fundamental biochemical and biophysical processes. Its integral roles in cell signaling, protein-mediated fusion, and action potential propagation have been well studied with a variety of techniques. Calcium is also commonly maintained at strong asymmetric concentration gradients, which often span several orders of magnitude, across mammalian cell membranes in proper physiological conditions. Yet, there has been a comparative lack of investigation of the effect of divalent cations on lipid bilayer structure and mechanical properties. Of the few studies done, experimental methods are limited to undefined resolutions and indirect spectroscopic or fluorescent techniques. This is in part due to the inherent difficulty in preparing well aligned samples with ions for high resolution experiments; common multilayer preparation techniques are subject to miscibility limits and sample segregation. Computationally, molecular dynamics simulations have predicted a calcium “condensing” effect that is associated with bilayer thickening and increased hydrocarbon chain order. However, these methods are limited by still-developing approaches to modeling divalent cation structure and interactions. Here, we report lamellar x-ray diffraction measurements of lipid bilayer thickness in partially charged multilayers containing calcium. Additionally, we report an apparent calcium-induced mechanical softening effect in giant unilamellar vesicles (GUVs) as measured by micropipette aspiration. This effect is marked by a measurable reduction in the elastic stretching coefficient of GUVs in calcium-rich environments. Both the thickening and mechanical softening effects may be important for proteins and molecules that couple directly to the lipid bilayer in physiological calcium concentrations. Future investigations involving PG-containing membranes and bacterial protoplasts may help lead to a better understanding of how invading bacteria thrive in calcium-rich environments, or if calcium-rich membrane domains have some non-trivial influence on the mechanisms of antibiotics and/or host defense antimicrobial peptides.