Structural organization and phase behavior of DNA-calcium-dipalmitoylphosphatidylcholine complex

Freeze-fracture study of ultrastructure of DNA-calcium-dipalmitoylphosphati dylcholine (DPPC) complex was carried out at different temperatures. For hi gh-speed cryofixation from controllable initial temperatures, a special the rmostatic chamber was designed. The fracture surface of the complex was Fou nd to be considerably different from the initial DPPC liposomes: 1) the per iod of ripple phase was 25 nm in contrast to 15 nm for control samples; 2) the ripple phase was observed at temperatures ranging from 6 degrees C to l ipid melting temperature; 3) at temperature above the lipid melting unorder ed worm-like folds were formed on the fracture surface. Their length was co rrelated with the length of DNA fragments used in the experiment. We suppos e that DNA molecules adsorbed on the membrane surface were segregated to cl usters, resulting in formation of a new phase with specific structure and p roperties.