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

Abstract

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 $⟨\varphi ⟩$ $\sim$ 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 $T_m$.

8 pages, 7 figures