Lipid membrane microrheology

University Of Oregon Eugene, Eugene OR

Investigators

Abstract

ID: MPS/DMR/BMAT(7623) 1006171 PI: Parthasarathy, Raghuveer ORG: University of Oregon Title: Lipid Membrane Microrheology INTELLECTUAL MERIT: Lipid membrane fluidity is not well understood and membrane viscosity, the key property that describes fluid response, remains poorly quantified. This proposal describes a coordinated experimental and theoretical program that will illuminate lipid bilayer viscosity and viscoelastic response. Experimentally, lipid membranes will be probed using recently developed methods of two point microrheology, in which dynamic correlations between the trajectories of Brownian tracer particles provide insights into the physics of complex fluids. This approach has never before been applied to lipid membranes. The experiments will involve high-speed video microscopy of nanoparticles linked to planar, unsupported lipid bilayers, followed by image analysis and particle tracking. Theoretical work will analytically extend recent models of the response of ideal two-dimensional fluids to encompass elasticity, viscoelasticity, and tension ¨C key physical properties of consequence to membranes. The resulting theoretical framework will enable quantitative maps between experimentally observed tracer dynamics and the underlying collective dynamics of membranes. The studies target three important membrane systems: (1) Experiments will address homogenous lipid bilayer membranes to correlate viscosity with molecular structure and temperature. Structural studies will examine the dependence of viscosity on the lipids¡¯ acyl chain length, which will reveal whether the hydrophobic core or the hydrophilic headgroup dominates the fluid response. More fundamentally, the data together with new theoretical models will quantify the extent to which lipid membranes behave as viscous versus viscoelastic fluids. (2) The project will also study phase-separated membranes. Cellular membranes are not homogenous in composition, a feature that is believed to help govern protein©\protein interactions. The mechanical consequences of heterogeneity remain poorly understood. Experiments will therefore probe viscosity in model bilayers that, below a critical temperature, exhibit cholesterol©\dependent phase separation into coexisting fluid phases. (3) Finally, the PI will investigate asymmetric membranes. The two lipid leaflets of cellular membranes differ with respect to composition. By constructing asymmetric lipid bilayer membranes, the proposed studies will examine the consequences of asymmetry and the existence mechanical coupling across the leaflets. BROADER IMPACTS: This project will advance the experimental methodology and the theory of viscous and viscoelastic flow in biological lipid membranes. A better understanding of membrane protein diffusion and assembly and of the mechanism of lipid domain formation should emerge. The PI has developed and will continue to deliver a general education course, The Stuff of Life, that explores biophysics for non-science members. Through readings, discussions, and hands©\on exercises, students explore the physical properties of biological materials and the constraints these properties place on living organisms. The course includes discussions of the interplay between physics and biology, of how new modes of experimental analysis (e.g., particle tracking) enable new discoveries, and of the similarities between biomaterials (e.g. membranes) and non©\biological soft matter (e.g. soap films). He will continue to lead efforts in his Department to organize weeklong Physics Day Camps for low economic status high school students. His lab will host undergraduate students participating in an REU program targeting female physical sciences students and students from another program that brings community college students to the campus for a summer research experience. As co-PI on an NSF-sponsored GK-12 project, he directs an activity that partners University of Oregon graduate students with schools in remote Oregon school districts to introduce inquiry-driven science curriculum elements, including science kits that remain with the participating schools.