EVIDENCE OF INTERLIPIDIC ION-PAIRING IN ANION-INDUCED DNA RELEASE FROM CATIONIC AMPHIPHILE-DNA COMPLEXES - MECHANISTIC IMPLICATIONS IN TRANSFECTION

Complex formation of DNA with a number of cationic amphiphiles has bee n examined using fluorescence, gel electrophoresis, and chemical nucle ase digestion. Here we have addressed the status of both DNA and lipid upon complexation with each other. DNA upon binding with cationic amp hiphiles changes its structure in such a way that it loses the ability to intercalate and becomes resistant to nuclease digestion. Fluoresce nce anisotropy measurements due to 1,6-diphenylhexatriene (DPH) doped in cationic liposomes demonstrated that upon complexation with DNA, th e resulting complexes still retain lamellar organizations with modest enhancement in thermal stabilities. The lipid-DNA complexation is most effective only when the complexation was carried out at or around the phase transition temperatures of the cationic lipid employed in the c omplexation with DNA. The release of DNA from cationic lipid-DNA compl exes could be induced by several anionic additives. Determination of f luorescence anisotropies (due to DPH) as a function of temperature cle arly demonstrates that the addition of equivalent amounts of anionic a mphiphile into cationic lipid-DNA complexes leads to the ion-pairing o f the amphiphiles, the melting profiles of which are virtually the sam e as those obtained in the absence of DNA. In this process DNA gets re leased from its complexes with cationic lipids and regains its natural intercalation ability, movement, and staining ability on agarose gel and also the sensitivities toward nuclease digestion. This clearly sug gests that combination of ion-pairing and hydrophobic interactions bet ween cationic and anionic amphiphiles is stronger than the electrostat ic forces involved in the cationic lipid-DNA complexation. It is furth er revealed that the DNA release by anions is most efficient from the cationic lipid-DNA complexes at or around the T-m of the cationic lipi d used in DNA complexation. This explains why more effective DNA deliv ery is achieved with cationic lipids that bear unsaturated hydrocarbon chains than with their saturated hydrocarbon counterparts.