DNA INITIATES POLYMORPHIC STRUCTURAL TRANSITIONS IN LECITHIN

The inverted micellar phase, obtained by treating lecithin and Ca2+-DN A complex with chloroform, was used as an intermediate step in the pre paration of DNA-Ca2+-lecithin complex. DSC analysis demonstrated the i nvolvement of a large fraction of lipid in the interaction with DNA. F reeze-fracture electron microscopy revealed (i) rod-like structures on the hydrophobic fracture surface of membranes and (ii) regular bundle s of fibrils with a repeat distance of about 6 nm, which were located free in solution. Similar regular bundles of fibrils were also reveale d by staining the samples with uranyl acetate. According to the sugges ted model, the observed structures are hexagonally packed inverted lip id tubes, with DNA located in their central cores. The possible biolog ical relevance of the capability of Ca2+-DNA to initiate polymorphic p hase transitions of lecithin is discussed.