Directed Motion of Proteins along Tethered Polyelectrolytes

We present the first time-resolved investigation of motions of proteins in densely grafted layers of spherical polyelectrolyte brushes. Using small-angle x-ray scattering combined with rapid stopped-flow mixing, we followed the uptake of bovine serum albumin by poly(acrylic acid) layer with high spatial and temporal resolution. We find that the total amount of adsorbed protein scales with time as $t^{1/4}$. This subdiffusive behavior is explained on the basis of directed motion of the protein along the polyelectrolyte chains.

Figure 1

Schematic representation of the uptake of proteins by spherical polyelectrolyte brushes. These particles consist of a solid polystyrene core onto which long chains of the weak polyelectrolyte poly(acrylic acid) are grafted. In salt-free solutions, the polyelectrolyte chains are strongly stretched due to the high osmotic pressure of the confined counterions. This can be seen directly in the cryogenic transmission electron micrograph shown on the left-hand side [14]. Solutions of the spherical polyelectrolyte brushes in salt-free solution are rapidly mixed with aqueous solutions of the protein BSA. The uptake of the protein by the brush layer is monitored by time-resolved small-angle x-ray scattering.

Figure 2

TR-SAXS intensities as a function of time $t$. For clarity, the upper curves have been multiplied by the factor indicated in the legend. The solid lines represent SAXS modeling by Eq. (1) with $S(q)=1$. The dotted lines (same color) depict the calculated intensities with $S(q)$ of a dispersion of interacting SPB obtained from the PRISM-approach [17]. The differences between the dotted and solid lines manifest the repulsive electrostatic interaction between the particles. The lower most curve also illustrates the decomposition of $I_0(q)$ into $I_{\mathrm{CS}}(q)$ and $I_{\mathrm{fluct}}(q)$ terms and the bulk BSA scattering.

Figure 3

The excess electron density derived from the analysis presented in Fig. 2 and the corresponding uptake of BSA per SPB is given in the parenthesis. For clarity successive plots have been shifted up by $10{\mathrm{nm}}^{-3}$ as indicated in the legend.

Figure 4

Overall amount of adsorbed protein, ${\tau }_{\mathrm{ads}}$ as a function of time. ${\tau }_{\mathrm{ads}}$ has been obtained from the electron density profiles shown in Fig. 3. The inset schematically depicts the motion of proteins through the polyelectrolyte layer in terms of Eqs. (3, 4, 5). The proteins interact with the polyelectrolyte chains through the positive patches on their surface.