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Near-Field studies of anisotropic variations and temperature induced structural changes in a supported single lipid bilayer

Temperature controlled Polarization Modulation Near-Field Scanning Optical Microscopy (PM-NSOM) measurements of a single supported $L_{β^{\prime}}$ 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayer are presented. The effective retardance ($ΔS = \frac{2 π(n_e-n_o)t}λ$), where $t$ is the thickness of the bilayer and $λ$ is the wavelength of light used and the direction of the projection of the acyl chains ($θ$) were measured simultaneously. From $ΔS$ the birefringence ($n_e-n_o$) of the bilayer was determined. A change of $ΔS$ $\sim$ (3.8 $\pm$ 0.3)~mrad at the transition temperature $T_m \sim$ 41$^{\circ}$C between the gel $L_{β^{\prime}}$ to liquid disorder $L_α$ was observed in a single planar bilayer. This agrees well with previous values of ($n_e-n_o$) in the $L_{β^{\prime}}$ phase and translates to an assumed $\langle ϕ\rangle $ $\sim$ 32$^{\circ}$ when T $<T_m$ and 0$^{\circ}$ when T $>T_m$. Evidence of supper heating and supper cooling are presented, along with a discussion of the behavior that occurs around $T_m$.