Two-step pattern in the kinetics of protein adsorption onto poly(ethylene glycol)-grafted phospholipid monolayers

Adsorption of insulin and human serum albumin (HSA) onto dipalmithoylphosphatidylethanolamine-succinyl (DPPE-succinyl) monolayers grafted with poly(ethylene glycol) chains of molecular weight 2000 (PEG2000) was studied by monitoring changes in surface pressure, Δπeq, for up to 10 h after injecting proteins into the subphase underneath the monolayers. The increase in the PEG2000 grafting density was simulated by varying the content of the PEG-grafted phospholipid, DPPE-PEG2000, in DPPE-succinyl monolayers from 1 to 9 mol%. At the surface pressure of ∼15 mN/m chosen for our protein adsorption study, increasing PEG grafting density had a modest effect on the insulin adsorption onto mixed DPPE-succinyl/DPPE-PEG2000 monolayers. For all monolayers, the adsorption equilibrium was reached in one fast step with Δπeq values indicating a noticeable penetration of monolayers by insulin. By contrast, the HSA adsorption exhibited a two-step kinetic pattern including (1) “fast” initial adsorption reaching an equilibrium after ∼1 h and (2) “delayed burst” in HSA adsorption that occurred after ∼3–5 h. Increasing PEG2000 grafting density substantially impeded the HSA adsorption so that it took longer for the protein to squeeze in between grafted polymeric chains and embed itself into the monolayer.