Diffusion in Quasi Two-Dimensional Macromolecular Solutions

Membranes composed of a linearly polymerized amphiphile (with molecules interconnected via functional groups attached to the head groups via spacers) and dimyristoylphosphatidylcholine (DMPC) were studied in order to test their usefulness as models of two-dimensional macromolecular solutions. For that purpose a comparative experimental and Monte Carlo simulation study of the lateral diffusion of monomeric tracers and the self-diffusion of macrolipids was performed. Measurements of the diffusion coefficients were performed by the photobleaching technique in giant vesicles and in asymmetric bilayers supported on argon-sputtered glass substrates, with the inner (proximal) monolayer being composed of DMPC and the outer (distal) of the macromolecular solution. In the former case polymerization was performed by UV (285-nm) excitation and carried out chemically in the latter by using a hydrophilic initiator. The hydrodynamic radii of the photochemically and chemically polymerized macrolipids were obtained from the self-diffusion coefficients (of the macrolipids) by the application of a recent theory of diffusion in membranes in frictional contact with solid surfaces. The friction is transmitted by a thin lubricating water film (of some 10-Å thickness) between the proximal monolayer and the solid surface and can lead to a strong dependence of the diffusion coefficient on the size of the diffusant. We found a ratio of the radii of the macrolipid (a_p) to the monomer (a_m) of a_p/a_m ≈ 7 for the photochemically polymerized species and of a_p/a_m ≈ 7 for the chemically polymerized species. By assuming that the 2D mean-square radius of gyration scales as N3/4, we obtained a degree of polymerization of N = 20 for the former and of N = 300 for the latter case showing that chemical polymerization is essential for the preparation of large macrolipids. Monte Carlo simulations in order to calculate the diffusion coefficients of monomeric tracers and macrolipids were carried out for fractions of area covered by the macromolecules varying from 10% to 80%. Two polymer conformations, representing extreme cases, were considered: a collapsed two-dimensional coil and an extended chain. The best agreement was found to be between experimental and simulated diffusion coefficients obtained for a linear combination of the collapsed and extended chain models, with the probability of the polymers to be in their collapsed states being about 0.4. The comparison suggests a value of N ≈ 24 for the degree of polymerization of the photochemically polymerized species in good agreement with the experimental value.