Dispersões lipídicas de dimiristoil fosfatidilglicerol: um estudo termo-estrutural / Lipid dispersions of dymiristoyl phosphatidylglycerol: a thermo-structural study

AUTOR(ES)
FONTE

IBICT - Instituto Brasileiro de Informação em Ciência e Tecnologia

DATA DE PUBLICAÇÃO

21/12/2010

RESUMO

Dispersions of the saturated anionic phospholipid dimyristoyl phosphatidyl glycerol (DMPG) have been extensively studied regarding their peculiar thermo-structural behavior. At low ionic strength, the gel-fluid transition is spread along nearly 17 ºC, displaying several thermal events in the calorimetric profile, quite different from the single sharp peak around 23 ºC found for higher ionic strength DMPG dispersions. This extended transition region is also characterized by low turbidity, high conductivity and high viscosity. The purpose of this thesis was to extend the knowledge about this peculiar thermostructural behavior of DMPG dispersions. Accordingly, the thermal dependence of the dispersions was investigated as a function of lipid and ionic strength concentrations, using several techniques: differential scanning calorimetry (DSC), and measurements of turbidity, electrical conductivity, viscosity and electrophoretic mobility. Moreover, the encapsulation of radioactive sucrose in the internal aqueous volume of DMPG aggregates clearly indicated that DMPG forms vesicles and not micelles or bicelles. However, viscosity measurements indicated that the DMPG vesicles formed along the transition region are certainly different from those formed in the gel and fluid phases. The studied lipid concentrations varied from 1 to 50 mM, and the 10 mM DMPG dispersion was studied at different ionic strengths, from 2 to 250 mM of added NaCl. The thermal properties of the dispersions were shown to be dependent on the lipid concentration, and this dependence was shown to be similar to the dependence on the ionic strength: the increase in lipid or salt concentrations decreased the temperature range of the transition region. The similarity between the two regimes, either the increase in salt or lipid concentration, is discussed here, in the light of their correlation with the concentration of bulk Na+. By measuring the sample electrical conductivity, and the electrophoretic mobility of the DMPG vesicles, it was possible to calculate the temperature dependence of the degree of ionization of DMPG vesicles, at different lipid and salt concentrations. It could be shown that the vesicles are more ionized along the transition region, and less ionized as the lipid or salt concentration increases. Besides the increase in conductivity along the transition region, a sharp increase in conductivity was observed at the pre-transition of the bilayers, indicating that this temperature is related to the beginning of the chain melting process. The relevance of the bilayer curvature and ionization degree to the peculiar thermal behavior of DMPG dispersions Will be discussed here. Highly ionized and deformed vesicles seem to be present along the extended thermal transition of DMPG bilayers, presenting large fluctuations of form and bilayer density. It is interesting to speculate if those large bilayer fluctuations could be biologically relevant.

ASSUNTO(S)

biofísica biophysics dispersões lipídicas dmpg dmpg lipid dispersions

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