Nanotech 2010

Bilipidic membrane phase identification by optical properties

P.P. Favero, M. Baptista, A.C. Ferraz, R. Miotto
Universidade Federal do ABC (UFABC), BR

Keywords: cell membrane, DPPC, DFT


Although plasmatic membranes present a fundamental role in ions transport and cell physiology, its structure and composition is still a matter of debate. The cell membrane complexity is atributed to the large variety of molecules performing versatile biochemical functions. Even simplest lipid bilayer consisting of only one kind of lipid molecule exhibits already a very complex phase behavior. Fig. 1 shows four DPPC lipidic gel structures that we studied by first principles calculations. The most ordered structure is the Gel3 phase that is achieved at high pressure and low temperature conditions. Decreasing the pressure, the membrane presents a phase transition to Gel2. Its structure has a slope in comparison to the perfect Gel3 phase. Increasing the temperature, a new Gel1 phase is observed. It is characterized by a senoid modulation of the membrane profile. Finally, in the case of high pressure and temperature, the layers are translated and the two lipid molecules become out of phase. In order to contribute to the membrane structure characterization we propose a new method to identify the bilipidic membrane phase by means of reflectance anisotropy spectra (RAS) calculated from density functional theory (DFT). Although DFT calculations severe restrict the size and number of possible structures studied, its detailed electronic structure description is essential in order to calculate the optical properties of the system. The reflectance anisotropy spectra is a relevant structure identification method. It has been used for monitoring surfaces during film growth as well as for surface analysis like molecule adsorption configuration [Ref. 1]. Our calculations are sumarized in Fig. 2, where the RAS spectrum for each dppc membrane phase structure are compared. The Gel0 and Gel3 structures present similar spectrum patterns and this is easy to be explained as their structures are distinguished just by a lateral translation. As can be seen in Fig 1, the main atomic modification is observed inside the membrane. As RAS spectra is a tool of surface analysis, it is not able to clearly identify small configuration changes in the interior of the sample. The RAS ploted in Fig. 2, on the other hand, shows energy peak dislocations for each Gel1, Gel2 and Gel3 phases at energies between 2eV and 3eV. Therefore, we believe that it is fair to say that ras can be used as a phase characterization method for dppc membrane and we suggest an experimental work that confirms this prediction, as it has not been measured yet.
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