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.