Structure of complexes of cationic lipids and poly(glutamic acid) polypeptides: A pinched lamellar phase

Complexes of cationic lipids with negatively charged biological polyelectro lytes such as DNA and proteins have elicited much interest recently because of their potential applications in gene delivery and in developing novel b iomolecular materials. We report on the structure of complexes made from th e anionic polypeptide poly-L-glutamic acid (PGA) and a positively charged l ipid mixture consisting of the cationic lipid didodecyl dimethylammonium br omide (DDAB) and the neutral lipid dilauroyl-sn-glycero phosphocholine (DLP C). Small-angle X-ray scattering (SAXS), small angle neutron scattering (SA NS), and optical microscopy of the complexes are consistent with a condense d multilamellar structure with PGA macromolecules sandwiched between the bi layers of the lipids. At the isoelectric point of the complex, lipid diluti on experiments at increasing ratios of the neutral lipid to the cationic li pid resulted in an unexpectedly large increase in the interlamellar "d" spa cing from 39 Angstrom for the pure DDAB membrane to 60 Angstrom at very hig h dilutions. Significantly, SAXS data shows that the lamellar complexes rem ained single phase which indicates that PGA interchain interactions are rep ulsive with their average spacing increasing with increasing lipid dilution . The data are consistent with a model of a "pinched lamellar" phase of the lipid-PGA complex where the PGA macromolecule and DDAB associate to form l ocalized pinched regions. Between PGA-membrane "pinches" large pockets of w ater stabilized by hydration repulsion are contained and the system behaves as a nearly pure DLPC membrane with a large equilibrium spacing of 60 Angs trom. These results suggest that biologically active molecules could be inc orporated in the large hydration domains between "pinched" regions for deli very applications.