Structural Organization and Phase Behavior of DNA--Calcium--Dipalmitoylphosphatidylcholine Complex

Y. S. Tarahovsky^1*, A. A. Deev¹, I. S. Masulis², and G. R. Ivanitsky¹

¹Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences, Pushchino, Moscow Region, 142292 Russia; fax: 7 (0967) 79-0553; E-mail: tarahovsky@venus.iteb.serpukhov.su

²Institute of Cell Biophysics, Russian Academy of Sciences, Pushchino, Moscow Region, 142292 Russia

^* To whom correspondence should be addressed.

Received November 17, 1997; Revision received December 5, 1997

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Freeze-fracture study of ultrastructure of DNA--calcium--dipalmitoylphosphatidylcholine (DPPC) complex was carried out at different temperatures. For high-speed cryofixation from controllable initial temperatures, a special thermostatic chamber was designed. The fracture surface of the complex was found to be considerably different from the initial DPPC liposomes: 1) the period 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°C to lipid melting temperature; 3) at temperature above the lipid melting unordered worm-like folds were formed on the fracture surface. Their length was correlated with the length of DNA fragments used in the experiment. We suppose that DNA molecules adsorbed on the membrane surface were segregated to clusters, resulting in formation of a new phase with specific structure and properties.

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KEY WORDS: ripple phase, lipid polymorphism, DNA--lipid interactions

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