The structural diversity of DNA–neutral phospholipids–divalent metal cations aggregates: a small-angle synchrotron X-ray diffraction study

We investigate the structure of aggregates formed due to DNA interaction with saturated neutral phosphatidylcholines [dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine] in presence of Ca2+ and Mg2+ cations using simultaneous synchrotron small- and wide-angle X-ray diffractions. For DPPC:DNA = 3:1 mol/base and in the range of 1–50 mM Ca2+, the diffractograms show structural heterogeneity of aggregates. We observe the coexistence of two lamellar phases in aggregates prepared at 1 mM Ca2+: Lx phase with the DNA strands (of unknown organization) intercalated in water layers between adjacent lipid bilayers and LDPPC phase of DPPC bilayers without any divalent cations and DNA strands. Aggregates prepared in the range 2–50 mM Ca2+ show a condensed gel lamellar phase Lgc with the lipid bilayer periodicity d ≈ 8.0 nm, and the DNA–DNA interhelical distance dDNA ≈ 5.1 nm. The increase of temperature induces the decrease in the intensity and the increase in the width of the DNA related peak. In the fluid state, the condensed lamellar phase Lαc gradually converts into Lx phase. The aggregates do not exhibit rippled Pβ phase. The thermal behaviour of aggregates was investigated in the range 20–80°C. Applying heating–cooling cycles, the aggregates converted into energetically more favourable structure: a condensed lamellar phase Lc (or Lx) is preserved or we observe lateral segregation of the DNA strands and metal cations (Lx phase) in coexistence with LPC phase of pure phospholipids.