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| Author(s): |
Karin do Amaral Riske
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto de Física (IF/SBI) |
| Defense date: | 2001-04-27 |
| Examining board members: |
Maria Teresa Moura Lamy;
Iolanda Midea Cuccovia;
Rosangela Itri;
Marcel Tabak;
Iris Concepción Linares de Torriani
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| Advisor: | Maria Teresa Moura Lamy |
| Abstract | |
Aqueous dispersions of the anionic phospholipid DMPG (dimyristoylphosphatidyl glycerol) present a very interesting and peculiar thermal behavior, mainly at low ionic strength. In this work, we show that DMPG at low ionic strength presents two regimes with very different thermal behaviors, depending on the lipid concentration. Above a certain concentration range c\' (c\' = 0,4 ± 0,2 mM), the gel-fIuid main transition of DMPG does not occur in a narrow temperature interval, as shown by many lipid dispersions, but rather presents a gel-fIuid transition region de!imited by temperatures here called Tmon and T moff. At the standard iOIÚC strength condition used (Hepes buffer + 2 mM NaCI), Tmon = 17.5 °C and Tmoff ~35 ºC This region is called intermediate phase in the present work, since it occurs between the gel and fIuid phases, and due to its specific properties, such as low light and small angle X-ray scattering, and high electrical conductivity and viscosity. The temperature extension of the interrnediate phase decreases with the increase in iOIÚC strength and lipid concentration, till a single main phase transition, centered at T DV is observed. The different techniques used in this work (light scattering, differential scanrung calorimetry, electron spin resonance, fluorescence anisotropy, small angle X-ray scattering and optical microscopy), suggest that a considerable increase in the electrostatic surface potential occurs at Tmon, leading to an increase in the repulsion between adjacent headgroups, and possibly between vesicles. This process would trigger the beginrúng of the melting of the hydrocarbon chains, which would finish above TmolE. The intermediate phase is characterized by a non-usual membrane packing, that could be caused by the presence of highly curved regions presenting a loose packing, possibly with higher surface charge density. On the other hand, the thermal behavior of DMPG below é is very different. The intermediate phase is not present and the system displays a main phase transition centered at a unique temperature, Tm The value of Tm increases from 27.5 °C at 0.1 mM DMPG to 41°C at 5 µM DMPC. Such a high T m value was only found in concentrated DMPG dispersion at low pH, when DMPG is in its protonated state. We observed that the T m value, contrary to the expected for charged lipid dispersions, decreases significantly with the increase in NaCI concentration. We propose, then, that below c\' the surface bilayer markedly changes, resulting in a huge increase in the proton affinity to the DMPG phosphate groups, which would lead to a complete neutralization of the lipid surface around 5 µM. The results presented here are very interesting concerning the physical chernistry of charged amphiphilic aggregates. Furthermore, the properties of the anionic phospholipid DMPG might have some biological relevance in specific domains of prokaryotic membranes. (AU) | |