Phase behavior of DPPC in a DNA-calcium-zwitterionic lipid complex studied by small-angle X-ray scattering

In the presence of DNA and calcium, multilamellar liposomes of DPPC when in excess form complexes in which two distinct lamellar lipid phases coexist. The structure and phase behavior of both of these have been investigated using high-resolution small-angle X-ray scattering. The coexisting lipid phases exist within the same precipitated condensate with the phase behavior of the unbound lipid being affected by the complexed lipid phase. While the thermotropic phase transitions of the uncomplexed lipid from L-beta'-P-beta'-L-alpha occur as expected, the transitions of the bound lipid are substantially altered. This is manifested as an increase in the main transition temperature, T-m, of the bound lipid, which has been observed before (Tarahovsky, Y. S.; Khusainova, R. S.; Gorelov, A. V.; Nicolaeva, T. I.; Deev, A. A.; Dawson, K. A.; Ivanitsky, G. R. FEBS Lett. 1996, 390, 133. Kharakoz, D. P.; Khusainova, R. S.; Gorelov, A. V.; Dawson, K. A. FEBS Lett. 1999, 446, 27). However, we also observe an apparent disappearance of the ripple phase (P-beta') with increasing calcium concentration in the bound lipid phase. In addition we are in a position to comment on the mechanism of formation for complexes prepared in this way.