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Phospholipid membranes as substrates for polymer adsorption

Abstract

A largely unsolved problem in soft materials is how surface reconstruction competes with the rate of adsorption. Here, supported phospholipid bilayers of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) were employed as substrates for the adsorption of a weak polyelectrolyte, polymethacrylic acid, whose time-dependent ratio of charged to uncharged functional groups served to probe the local dielectric environment. Chains that encountered sparsely covered surfaces spread to maximize the number of segment–surface contacts at rates independent of the molar mass (which was varied by a factor of 30), but dependent on the phase of the lipid bilayer, gel or liquid crystal. Surface reconstruction rather than molar mass of the adsorbing molecules seemed to determine the rate of spreading. The significance of these findings is the stark contrast with well-known views of polymer adsorption onto surfaces having structures that are 'frozen' and unresponsive, and is relevant not just from biological and biophysical standpoints, but also in the formulation of many cosmetics and pharmaceutical products.

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Figure 1: Schematic diagram of the adsorption and subsequent conformational equilibration of flexible polymers at a supported phospholipid membrane.
Figure 2: Mass of polymethacrylic acid (PMA) adsorbed on to the supported lipid bilayer surfaces versus time.
Figure 3: Ionization of the carbonyl group plotted against elapsed time for a surface starved of PMA.
Figure 4: Ionization of the carbonyl group plotted against elapsed time for varying molecular weights of PMA spread on to starved bilayer surfaces.
Figure 5: Dichroic ratio of the −N(CH3)3+ asymmetric stretch vibration at 970 cm−1 of DMPC coated with adsorbed PMA chains in the LC phase (circles) and gel phase (squares), plotted against the fractional ionization of adsorbed PMA during spreading onto starved surfaces after the same adsorption procedure described in Fig. 3.

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Acknowledgements

We thank S. Safran for a discussion. This work was supported by the US Department of Energy, Division of Materials Science, under Award No. DEFG02-91ER45439 through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign.

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Xie, A., Granick, S. Phospholipid membranes as substrates for polymer adsorption. Nature Mater 1, 129–133 (2002). https://doi.org/10.1038/nmat738

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