SummaryWe investigate the structure of aggregates formed due to DNA interaction with saturated neutral phosphatidylcholines (DPPC and DMPC) in presence of Ca 2+ and Mg 2+ cations using simultaneous synchrotron small-and wide-angle X-ray diffraction. For

Dipalmitoylphosphatidylcholine (DPPC) was hydrated in 0.2–60 mM solution of CaCl2in heavy water and thoroughly homogenized by freezing-thawing process. Small-angle neutron scattering (SANS) shows formation of unilamellar vesicles in the range 1–60 mM of CaCl2. From the Kratky–Porod plot ln [I(Q)Q2] vs.Q2of SANS intensityI(Q)in the range of scattering vectorsQcorresponding to the interval 0.001 Å−2≤Q2≤0.006 Å−2, the vesicle bilayer radius of gyrationRgand the bilayer thickness parameterdgwere obtained. The structure of the bilayer displays different behavior for the gel phase and the liquid-crystalline phase: In the gel phase (at 20°C), the values ofdgindicate nonlinear changes in the lipid bilayer thickness, with a maximum at ~5 mM CaCl2. In the liquid-crystalline phase (at 60°C), the parameter of the lipid bilayer thicknessdg=43.2±0.3 Å is constant within the concentration range 1≤cCa≤40 mM. Vesicles prepared at 60 mM CaCl2show within experimental error, the same values ofdgas pure DPPC unilamellar vesicles prepared by extrusion using polycarbonate filter with pores of diameter 500 Å.

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The structure of aggregates formed due to DNA interaction with dipalmitoylphosphatidylcholine (DPPC) in presence of Ca 2+ and Zn 2+ is examined using smallangle synchrotron X-ray diffraction (SAXD) and small-angle neutron scattering (SANS). SAXD detected two structures: L C-condensed lamellar phase and L X-lamellar phase with DNA strands intercalated between the adjacent lipid bilayers, without regular packing at low cation concentration (~1 mM). The high concentration of Zn 2+ induces a macroscopic phase separation in mixtures. The SANS curves of DPPC+ions 2+ vesicles evaluated using the strip function model have shown different modes of cations binding to the DPPC bilayers.