Colloids versus solution state adsorption of proteins: Interaction of Myoglobin with supported lipid bilayers.

This study examines adsorption of Myoglobin (Mb) in solution and as colloid on supported lipid bilayers of neutral phospholipids and a mixture of neutral+cationic lipids formed on gold coated quartz in a Quartz crystal microbalance (QCM). Results indicate that thin adsorbed films of Mb in solution and as colloids, show atleast 3 steps in the interaction with the bilayers: i) An initial strain of a viscoelastic film ii) Entrained water that moves in and out of the adsorbed film and iii) The coupled load from the bulk liquid which increases the strain of the film. These three components constitute an effective viscoelastic film which is rigidly coupled to the QCM. Grazing incidence XRD (GIXD) shows that the bilayer head group remains nearly undisturbed for Mb solution with pure (neutral) and (neutral+cationic) mixtures, whereas for the colloids there is an increase in head group thickness with neutral and decrease in the case of mixture. Unsaturation in the alkyl tails in the neutral lipid resulting in flexible disordered bilayers and more entrained water in the cationic system results in these changes. The sensitivity of QCM-D, makes it useful to study real-time monitoring of bilayer structural robustness cytotoxicity, drug delivery and lipid self-assembly.