Abstract
In-plane lipid organization and phase separation in natural membranes play key roles in regulating many cellular processes. Highly cooperative, first-order phase transitions in model membranes consisting of few lipid components are well understood and readily detectable via calorimetry, densitometry, and fluorescence. However, far less is known about natural membranes containing numerous lipid species and high concentrations of cholesterol, for which thermotropic transitions are undetectable by the above-mentioned techniques. We demonstrate that membrane capacitance is highly sensitive to low-enthalpy thermotropic transitions taking place in complex lipid membranes. Specifically, we measured the electrical capacitance as a function of temperature for droplet interface bilayer model membranes of increasing compositional complexity, namely, (a) a single lipid species, (b) domain-forming ternary mixtures, and (c) natural brain total lipid extract (bTLE). We observed that, for single-species lipid bilayers and some ternary compositions, capacitance exhibited an abrupt, temperature-dependent change that coincided with the transition detected by other techniques. In addition, capacitance measurements revealed transitions in mixed-lipid membranes that were not detected by the other techniques. Most notably, capacitance measurements of bTLE bilayers indicated a transition at ∼38 °C not seen with any other method. Likewise, capacitance measurements detected transitions in some well-studied ternary mixtures that, while known to yield coexisting lipid phases, are not detected with calorimetry or densitometry. These results indicate that capacitance is exquisitely sensitive to low-enthalpy membrane transitions because of its sensitivity to changes in bilayer thickness that occur when lipids and excess solvent undergo subtle rearrangements near a phase transition. Our findings also suggest that heterogeneity confers stability to natural membranes that function near transition temperatures by preventing unwanted defects and macroscopic demixing associated with high-enthalpy transitions commonly found in simpler mixtures.
| Original language | English |
|---|---|
| Pages (from-to) | 10016-10026 |
| Number of pages | 11 |
| Journal | Langmuir |
| Volume | 33 |
| Issue number | 38 |
| DOIs | |
| State | Published - Sep 26 2017 |
Funding
This manuscript has been authored by UT-Battelle, LLC, under contract no. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. J.K. and F.A.H. acknowledge support from Laboratory Directed Research and Development Program award 6074 and from DOE scientific user facilities. G.J.T. and F.A.H. also acknowledge financial support from the Joint Institute for Biological Sciences. G.J.T. and S.A.S. acknowledge funding and support from the Air Force Office of Scientific Research Basic Research Initiative (grant number FA9550-12-1-0464). DIB measurements were made at the University of Tennessee, Knoxville (UTK). DSC, PPC, densitometry, and FRET experiments and GC/MS analyses were performed at Oak Ridge National Laboratory (ORNL). GUV preparation and confocal microscopy were performed at Cornell University and ORNL. ORNL is managed by UT-Battelle, LLC, under U.S. Department of Energy (DOE) contract no. DE-AC05-00OR22725.