Exploring the Interaction of Polymers with Phospholipid Monolayers

Recent studies have revealed intricate details about how different polymers interact with phospholipid monolayers, particularly focusing on the presence of negatively charged phosphatidylglycerol (PG). The presence of PG significantly enhances the penetration capabilities of certain polypeptides, such as polylysine and oligoarginine (OAK), which shows a distinct increase in surface pressure when interacting with lipid layers.

When examining the surface pressure changes, various polypeptides exhibited different penetration abilities into monolayers composed of dipalmitoylphosphatidylcholine (DPPC) and its mixtures with PG. For instance, polylysine displayed a surface pressure increase of 3.6 mN/m in the presence of pure DPPC, which escalated to 4.6 mN/m when PG was included at an 80/20 mol/mol ratio. This suggests that the negative charge of PG enhances the electrostatic interactions, facilitating the penetration of these positively charged polymers.

Interestingly, the study categorized the polymers based on their penetration effectiveness. Polymers like polylysine and OAK, which possess positively charged groups, showed increased penetration in the presence of negatively charged PG. In contrast, polypeptides with only free groups at the ends of their side chains struggled to penetrate the monolayers, indicating that both electrostatic attraction and hydrophobicity play vital roles in this process.

The results further highlighted that amphoteric polypeptides like EAK and its derivative, Ac-EAK, displayed minimal penetration regardless of the monolayer composition. Their behavior suggests that hydrophobic forces predominantly govern their interactions with phospholipid membranes, contrasting sharply with the behavior of more hydrophobic polymers, which exhibited greater penetration capabilities.

The experimental setup involved monitoring the expansion of lipid monolayers in a controlled environment, where the introduction of polymers led to significant changes in the area per molecule of phospholipid. This expansion was particularly pronounced with polypeptides such as SAK and AK at lower surface pressures, further underscoring the complexity of polymer-lipid interactions at various concentrations and environmental conditions.

The implications of these findings extend to biological membranes, as the surface pressure values tested mimic those found in cellular membranes. Therefore, the distinct behaviors of different polymers in these studies could inform the design of drug delivery systems and other biomedical applications that rely on effective interactions between polymers and cell membranes.