Physicochemical characterization of branched chain polymeric polypeptide carriers based on a poly-lysine backbone

A systematic study is reported on the physicochemical characteristics of two branched chain polymers (based on a poly-L-lysine backbone) with a general formula poly[Lys-(DL-Alam-Xi)], where X = Orn (OAK) or N-acetyl-Glu (Ac-EAK) and m approximately equal to 3, using surface pressure and fluorescence polarization methods. These data are compared with those of the linear poly(L-Lys) from which OAK and Ac-EAK are derived. These two polymers show a moderate surface activity, able to form stable monomolecular layers at the air-water interface. Poly(L-Lys), the most hydrophilic, has the lowest surface activity. The interaction of these polymers with phospholipid bilayers either neutral or negatively charged was studied with vesicles labeled with two fluorescent probes: ANS and DPH. Results indicate that these polymers are able to accommodate in their internal structure, mainly through electrostatic interactions, a certain amount of ANS marker molecules, but fluorescence increases of the ANS-polypeptide complexes were so low that its influence in further polarization measurements could be discarded. After interaction with liposomes, these polymers induce an increase in the polarization of the probes, thus indicating a rigidification of the bilayers. Electrostatic forces seem to be very important in this interaction; cationic polymers are clearly more active, with PG-containing liposomes, than Ac-EAK. Moreover, in these assays poly(L-Lys) behaves as the more active compound. This fact is probably due to its major ability to form alpha-helical structures that could insert easily in the bilayers. These results indicate that the polymeric structures studied can be used as carriers for biologically active molecules, because their interactions with bilayers remain soft and have a positive effect on the stability of the membranes.