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Membrane Biophotonics

(Major Center-Driven Research Project)

UC Davis: AN Parikh, AM Smith, T Huser,

AAMU: A Kassu, JM Taguenang, A Sharma

Supported lipid bilayers are model structures for cell membranes allowing for the precise engineering of lipid composition, as well as lipid-lipid and lipid-protein interactions in a controlled environment and enabling their investigation. Manipulating lipid structures on the scale of lipid rafts, which are believed to be at the center of cellular signaling processes, creates challenges for their creation, manipulation, and characterization. In this project we have developed an entirely photonics-based platform to enable the dynamic creation and destruction of nanometer-sized patterned obstacles to diffusion in lipid bilayers, and their characterization by fluorescence and Raman microscopy. We have taken two major approaches to membrane patterning: uv photolithography and ultra-short pulse laser writing.

This past year we have demonstrated interferometric (maskless) photolithography has the potential to produce 100 nm features in biomembranes using an 800 nm femtosecond laser. Minimum feature size and feature control was shown to be greatly improved over the 1-2 micron feature size results from lithographic masks and an Hg lamp uv source. Interferometric photolithography was then used to produce patterns in polybutadiene thin-flims, on poly-l-lysine thin-films, and on polystyrene and gold microspheres. Details of the dynamic process of uv induced membrane pattern formation was also studied.

In the area of ultra-short laser patterning we have investigated an explanation for the persistence of the patterns in what is expected to be a 2D fluid. The tight hair-pin bend is a most plausible structural motif near defects, pores, and frontiers in lipid bilayers determined by the hydrophobic effect (see figure). Using a phase-sensitive fluorescence probe and a spatially confined fluid bilayer patch, we obtained epifluorescence microscopy evidence for the spontaneous formation of a gel-like phase in the vicinity of such membrane edges. This patterned bilayer is heated above the transition temperature of the lipid. The shape holds even though the bilayer itself is mobile and fluid. It only loses its shape almost 20°C above the transition temperature of DMPC. These observations provide strong evidence that the presence of an edge (or a topological defect) induces unique molecular organization in its vicinity.  Key features of this organization include (1) a net increase in acyl-chain conformational order; (2) lowered effective lateral mobilities near the edge; (3) a gradual and continuous structural transformation from the edge toward the bulk; and (4) elevated effective temperature for the onset of lateral fluidity during gel-fluid transition of the bilayer.  Taken together, these features indicate an edge induced structural transformation in fluid lipid bilayers.