The Effects of Ionic Strength on the Morphology, Scattering, and Mechanical Response of Neurofilament-derived Protein Brushes

Protein brushes not only play a key role in the functionality of neurofilaments but also have wide applications in biomedical materials. Here, we investigate the effect of ionic strength, phosphorylation, and sequence modification on the morphology of protein brushes using a continuous-space self-consistent field theory. A coarse-grained multi-block charged macromolecular model is developed to capture the chemical identity of amino acid sequences. For neurofilament heavy (NFH) brushes at pH 2.4, we predict three morphological regimes: swollen brushes, condensed brushes, and coexisting brushes which consist of both a dense inner layer and a diffuse outer layer. The dramatic height decrease is a result of the electrostatic screening-induced transition from the overlapping state to the isolated state of the coexisting brushes. For phosphorylated NFH brushes at physiological pH, we predict a strong electrostatically driven height extension. The brush heights predicted by our theory are in quantitative agreement with experimental data for a wide range of ionic strengths. We also study the evolution of the scattering and mechanical responses accompanying the morphological change. The oscillation in the reflectivity spectra characterizes the existence and microstructure of the inner condensed layer, whereas the shoulder in the force spectra signifies the swollen morphology.