Penetration of Polymers in Monolayers

Phospholipid monolayers are excellent model membranes into which the penetration of macromolecules can be sensitively monitored through changes in surface pressure. In order to establish correlation between chemical structure and penetration properties we prepared monolayers composed of DPPC, DPPC/PG (95/5 mol/mol) and of DPPC/PG (80/20 mol/mol). Insertion experiments were carried out in the same Teflon cuvette as described above. Lipid solutions were spread on the aqueous subphase to form a monolayer with a given initial surface pressure. Once monolayers were established, polymer solutions were injected in the subphase and pressure increases were recorded as a function oftime. 

In the presence of monolayer composed of DPPC and 5 or 20 % PG (Fig.5b, c), the surface activity of polycationic SAK and AK (6.0 mN/m for AK, 5.3 mN/m for SAK) was less pronounced. However, no significant changes were observed with EAK and Ac-EAK (4.8 mN/m for EAK and 4.0 mN/m for Ac-EAK at 5 mN/m in DPPC/PG, 95/5 mol/mol) as compared to values of the DPPC monolayer. Similar observation was interpreted as barrier effect of the monolayer that prevent the incorporation of the relatively more hydrophobic molecules into the surface of the phospholipid monolayer. Presence of negatively charged PG in the monolayer resulted in increased penetration ability of free e-amino group containing OAK and polylysine (e. g. surface pressure increase for polylysine at 5 mN/m initial pressure was 3.6 mN/m in the presence of DPPC, while it was 4.6 mN/m in the presence of DPPC/PG (80/20 mol/mol). 

For polymers containing e-amino group (polylysine, OAK) the negative phospholipid content resulted in increased electrostatic interaction which facilitated their penetration at low surface activity. 

The amphoteric EAK and its polyanionic derivative, Ac-EAK showed low penetration, which was almost independent from the composition of the monolayer suggesting the predominant role of hydrophobic attraction. 

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