| Autor: |
Zantl, R., Baicu, L., Artzner, F., Sprenger, I., Rapp, G., Radler, J. O. |
| Zdroj: |
The Journal of Physical Chemistry - Part B; November 18, 1999, Vol. 103 Issue: 46 p10300-10310, 11p |
| Abstrakt: |
The thermotropic phase behavior of zwitterionic/cationic binary lipid mixtures is investigated and compared to its corresponding lipidic phase diagram of mixtures complexed with DNA. We focus on isoelectric cationic lipid−DNA condensates where the number of cationic lipids equals the number of phosphate groups on the DNA. Using differential scanning calorimetry, X-ray scattering, freeze fracture electron microscopy, and film balance, we studied mixtures of di-myristoyl-phosphatidyl-choline (DMPC) and the cationic lipid, di-myristoyl-tri-methyl-ammonium-propane (DMTAP). The lipid phase diagram shows the well-known Lα, Lβ, and Pβ ripple phase with peritectic behavior at a low molar fraction of cationic lipid, χTAP < 0.12. Beyond χTAP = 0.8 crystalline phases appear. A systematic variation in the hydrocarbon chain tilt in the prevailing Lβ phase is measured by wide-angle X-ray scattering. Most importantly, the Lβ phase shows strong nonideal mixing with an azeotropic point at about 1:1 molar stoichiometry. This finding is related to the reduced headgroup area for equimolar mixtures found in monolayer pressure−area isotherms. The intercalation of DNA in cationic lipid−DNA complexes affects the lipid-phase behavior 2-fold: (i) the chain-melting transition temperature shifts to higher temperatures and (ii) a demixing gap with coexistence of lipid vesicles and lipid−DNA complexes arises at a low cationic fraction, χTAP < 0.25. In agreement with experiments we present a thermodynamic model that describes the shift of the melting transition temperatures by DNA-induced electrostatic screening of the cationic membrane. |
| Databáze: |
Supplemental Index |
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