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 $L_{\beta^{\prime}}$ 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayer are presented. The effective retardance ($\Delta S = \frac{2 \pi (n_e-n_o)t}{\lambda}$), where $t$ is the thickness of the bilayer and $\lambda$ is the wavelength of light used and the direction of the projection of the acyl chains ($\theta$) were measured simultaneously. From $\Delta S$ the birefringence ($n_e-n_o$) of the bilayer was determined. A change of $\Delta S$ $\sim$ (3.8 $\pm$ 0.3)~mrad at the transition temperature $T_m \sim$ 41$^{\circ}$C between the gel $L_{\beta^{\prime}}$ to liquid disorder $L_{\alpha}$ was observed in a single planar bilayer. This agrees well with previous values of ($n_e-n_o$) in the $L_{\beta^{\prime}}$ phase and translates to an assumed $\langle \phi \rangle$ $\sim$ 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 $T_m$.