Characterization of DODAB/DPPC vesicles
Introduction
Cationic lipid-like compounds can be used to deliver DNA to cells aiming at gene therapy and drug delivery (Felgner and Ringold, 1989, Zhu et al., 1993, Felgner, 1997). A large variety of synthetic cationic amphiphiles or detergents that are physically similar but chemically different from natural polar lipids has been synthesized such as interesting phosphatidylcholine tri-esters that can be metabolized by cells (Solodin et al., 1996, MacDonald et al., 1999a, MacDonald et al., 1999b, Rosenzweig et al., 2000) or highly efficient cetylpiridinium amphiphiles (Zuhorn et al., 2002, Zuhorn and Hoekstra, 2002). However, finding optimal lipid phases for transfection is still a matter of considerable interest (Koltover et al., 1998, Koynova et al., 2006).
Characterization of mixtures of cationic lipids with other neutral lipids is important not only to determine the effect of helper lipids on lipoplex structure and activity, but also to evaluate the lipid phases arising when cellular lipids interact with cationic lipids during DNA transfection. Our group has been finding many applications in drug and vaccine delivery for the simple, inexpensive and synthetic cationic lipid dioctadecyldimethylammonium bromide (DODAB) over the years (Carmona-Ribeiro, 1992a, Carmona-Ribeiro, 1992b, Carmona-Ribeiro, 2001, Carmona-Ribeiro, 2006). However, its cytotoxicity (Carmona-Ribeiro, 2003) needs to be circumvented possibly by mixing DODAB with a natural zwitterionic lipid. In this work, a systematic study of binary DODAB/dipalmitoylphosphatidylcholine (DPPC) mixed vesicles is performed in order to characterize these dispersions in aqueous solution. A search in the literature reveals that the self-assembly of DODAB/DPPC composite bilayer vesicles in aqueous dispersion was seldom systematically studied. The pioneer work by Linseisen et al. (1996) investigated the phase behaviour of these vesicles using differential scanning calorimetry (DSC) and reported a higher melting temperature than each of the pure components together with a narrow coexistence region for the gel and liquid–crystalline phases at 50% DODAB. Recently, an electrostatic model for mixed cationic-zwitterionic lipid bilayers was developed by Mbamala et al. (2006) based on an extended Poisson-Boltzmann theory. This model recovered the reorientation of the zwitterionic lipid headgroups, the nonmonotonic behaviour of the average cross-sectional area per lipid, and the absence of these properties for a mixed anionic-zwitterionic membrane. In contrast to the monotonic increase in mean area per molecule in the mixed anionic-zwitterionic bilayer, the cationic-zwitterionic bilayer theoretically displayed contraction followed by expansion as a function of the % cationic lipid in the binary mixture (Mbamala et al., 2006). In this work, a systematic evaluation of phase behaviour, size distribution, zeta-potential and colloid stability is undertaken for DODAB/DPPC vesicles prepared by hydration and vortexing of a lipid film in aqueous solution. The results confirm Linseisen et al. (1996) data on the highest mean phase transition temperature around 50% DODAB, agree with theoretical predictions by Mbamala et al. (2006) and, in addition, suggest that the highest colloid stability for DODAB/DPPC vesicles occurs precisely at the molar ratio where stability of the bilayer structure (molecular packing) is at highest.
Section snippets
Materials
Dioctadecyldimethylammonium bromide (DODAB), and dipalmitoylphosphatidylcholine (DPPC) were obtained from Sigma Chemical Co. and used as such without further purification. Extrinsic fluorescent probes 1,6-diphenyl-1,3,5-hexatriene (DPH) and 6-dodecanoyl-2-dimethyl aminenaphthalene (Laurdan) were purchased from Molecular Probes (Oregon, USA). One should recall that DPH senses the inner parts of the lipid bilayer (Repakova et al., 2005) whereas Laurdan reports the hydrophobic/hydrophilic region (
The effect of % DODAB on the phase transition of the composite bilayers
Turbidimetry has often been used to assess the phase transition of aqueous bilayer dispersions (Lee, 1977, Carmona-Ribeiro and Chaimovich, 1983, Nascimento et al., 1998, Vieira et al., 2006). In Fig. 1, turbidity at 400 nm displays steep changes over narrow ranges of temperature as typically expected from cooperative phase transitions in large and closed bilayer vesicles. For pure DPPC dispersions in 1 mM NaCl, two transitions were obtained upon heating: the first around 33.1 °C and the second
Acknowledgments
FAPESP and CNPq are gratefully acknowledged for financial support. CNCS is the recipient of an undergraduate CNPq fellowship. MAS thanks VRAID Project 2006/31 of Pontificia Universidad Católica de Chile for financial support.
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