Abstract
A novel poly(amino acid methacrylate) brush comprising zwitterionic cysteine groups (PCysMA) was utilized as a support for lipid bilayers. The polymer brush provides a 12-nm-thick cushion between the underlying hard support and the aqueous phase. At neutral pH, the zeta potential of the PCysMA brush was ∼−10 mV. Cationic vesicles containing >25% DOTAP were found to form a homogeneous lipid bilayer, as determined by a combination of surface analytical techniques. The lipid mobility as measured by FRAP (fluorescence recovery after photobleaching) gave diffusion coefficients of ∼1.5 μm2 s–1, which are comparable to those observed for lipid bilayers on glass substrates.
Highlights
The atom-transfer radical polymerization (ATRP) approach employed in this work allows good control of the chain growth kinetics and the mean brush thickness.[37−39] As the design rules for a suitable choice of polymer brush are not yet fully understood, we investigated four candidates: poly(cysteine methacrylate) (PCysMA)[46] and poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC)[40] zwitterionic brushes plus poly(methacrylic acid) (PMAA)[41] and poly(potassium 3-sulfopropyl methacrylate) (PKSPMA) anionic brushes (Figure 1)
PMPC and Poly(cysteine methacrylate) (PCysMA) brushes are zwitterionic at neutral pH, and the PMAA and PKSPMA brushes are both highly anionic under these conditions
In the case of PCysMA brushes, these adsorbed vesicles rupture to form homogeneous bilayers but only relatively low diffusion coefficients (D = 0.30 ± 0.1 μm[2] s−1) and mobile fractions of ∼70% were observed, which might be due to the strong electrostatic interaction between DOTAP and the underlying support or might indicate the presence of intact vesicles/ defects. These results suggest that electrostatic interaction is the principal mechanism for driving bilayer formation in the case of the PCysMA brush and is analogous to the electrostatics-driven bilayer formation previously observed by Cha et al, who systematically controlled the charge on the surface to show that a critical adsorbed vesicle concentration was required to cause bilayer formation.[71]
Summary
The proteins incorporated within them, have been the focus of significant research effort in recent years, mainly because of their importance in signal transduction and the control of cell function.[1−4] From their inception, planarsupported lipid bilayers (SLBs) have provided useful model systems for studying (i) the role of membrane composition, (ii) the function of membrane proteins in 2D systems, and (iii) ionchannel-based sensors.[5−9] A range of strategies have been explored for forming planar lipid membrane systems, including electrostatic interactions for supported lipid bilayers,[10,11,18] the insertion of anchoring units in the formation of tethered lipid bilayers (TLBs),[12−17] and lipid monolayer adsorption at hydrophobic surfaces for the formation of hybrid lipid bilayers (HLBs).[18]. Routes to provide thicker aqueous/gel-like supports for lipid bilayers have been the subject of significant research. The underlying hydrated polymer brush would provide a suitable “reservoir” to allow the diffusion of ions and small molecules
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