Adsorption of semiflexible block copolymers on homogeneous surfaces.

We present the results of extensive numerical off-lattice Monte Carlo simulations of semiflexible block-copolymer chains adsorbed onto flat homogeneous surfaces. We have compared the behavior of several chain structures, such as homopolymers, diblocks, (A(alpha)B(alpha)) block copolymers, and random heteropolymers. In all the cases studied, we have found the adsorption process to be favored with an increase of the chain rigidity. Particularly, the adsorption of diblock structures becomes a two-step process characterized by two different adsorbing temperatures that depend on the chain stiffness kappa, the chain length N, and the adsorbing energies epsilon(A) and epsilon(B). This twofold adsorbing process changes to a single one for copolymers of reduced block size alpha. Each block of the stiff copolymer chain is found to satisfy the classical scaling laws for flexible chains, however, we found the scaling exponent phi to depend on the chain stiffness. The measurement of the radius of gyration exhibits a typical behavior of a polymer chain composed of Nl(p) blobs whose persistence length follows l(p) approximately (kappa/k(B)T)(0.5) for large stiff chains.

Translational diffusion of flexible lipid chains in a Langmuir monolayer: a dynamic Monte Carlo study.

We report a computer simulation study of the lateral diffusion of conformationally disordered lipid molecules in a monolayer structure. The simulations were carried out with dynamic Monte Carlo methods, employing two different representations of the internal motions of the lipid chains. The classical Cohen-Turnbull theory is found to provide a good description of the simulated lateral diffusion coefficients at moderate densities. The substantial deviations found at low densities are attributed to the small density fluctuations needed to create the free volume required for the lateral diffusion process.

Protein attraction in membranes induced by lipid fluctuations.

The nonspecific lipid-mediated attraction between two proteins embedded in a bilayer membrane have been investigated for a model system using Monte Carlo simulations. We found two types of attraction with different regimes. A depletion-induced attraction in the range r < sigmaL, where sigmaL is the diameter of a lipid and r is the distance between the surfaces of the two proteins, and a fluctuation-induced attraction in the range 1 < r/sigmaL < 6, which originates from the gradients of density and orientational fluctuations of the lipids around each protein. The effective potential of the latter type of attraction decays exponentially with U(r) approximately exp(r/vi) where the correlation length is vi/sigmaL approximately 3.2 in the present model system.