Interactions between planar grafted neurofilament side-arms.

The side-arms of neurofilaments (NFs) have been proposed to be highly disordered, leading to an entropically and electrostatically based repulsion that modulates interfilament spacing. To characterize the behavior of two interacting polymer brushes in a system of this type, we performed molecular dynamics simulations of neurofilament brushes using a four bead reduced amino acid set coarse-grained model. In these simulations, we examined components of the neurofilament brush, NF-L, NF-M, and phosphorylated NF-H (NF-HP), individually. Each protein type was grafted to planar surfaces and simulations were performed for a range of separations of two apposed grafted surfaces. The calculated force-separation curves show the force increases as the reciprocal separation as predicted for polyelectrolyte brushes at high salt. All three systems can be overlapped on a single force-separation curve, which is not expected given the variation in amino acid sequence and charges on the polymers. Examination of structural properties shows scaling behavior in the average brush height, end-to-end distance, and the density interpenetration. Some of this scaling can be understood in terms of treating the NF proteins as effective polyelectrolytes, but some cannot suggesting a distinct polyampholyte behavior. Correlations are found between oppositely charged residues in opposite brushes. However, these correlations are weak in comparison to the strong correlations within each brush. In comparison with recent experimental data that observes condensed and expanded gel states, our results suggest that the condensed state structure involves significant interdigitation of the side-arms.