Orientational Alignment of Amyloidogenic Proteins in Pre-Aggregated Solutions

In the present study we combine dielectric relaxation spectroscopy with generalized Born simulations to explore the role of orientational order for protein aggregation in solutions of bovine pancreatic insulin at various $p\mathrm{H}$ conditions. Under aggregation-prone conditions at low $p\mathrm{H}$, insulin monomers prefer antiparallel dipole alignments, which are consistent with the orientation of the monomeric subunits in the dimer structure. This alignment is also true for two dimers, suggesting that already at moderate protein concentrations the species assemble in equilibrium clusters, in which the molecules adopt preferred orientations also found for the protomers of the corresponding oligomers.

Figure 1

The electric dipole moments (orange vectors) indicate a tilted mutual orientation of the protomers in the insulin dimer. The dipole vector of the dimer [with respect to its center of mass (blue sphere)] stands perpendicular to the paper plane (for simplicity not shown).

Figure 2

Dielectric dispersion curve ${\epsilon }^{\prime }(\nu )$ of insulin (2 mM HCl, $25°\mathrm{C}$, $p\mathrm{H}$ 2.7, $W_P=40\mathrm{mg}/\mathrm{mL}$).

Figure 3

Fitted relaxation times ${\tau }_{\beta }$ and relaxation amplitudes $S_{\beta }$ of the tumbling modes of insulin at different solvent conditions as a function of the mass concentration $W_P$ of insulin.

Figure 4

Effective dipole moments and Kirkwood factors $g_K$ of insulin at different solvent conditions.

Figure 5

Transition of the protomer dipole moment from the structure in the dimer to a monomerlike equilibrium structure. The upper part of the figure shows that during this change the Stokes radius remained constant.

Figure 6

Contour plot of the intermolecular energy $\mathrm{\Delta }U(\mathrm{\Delta }{r}_{cm},\varphi )$ of two approaching insulin monomers. The equilibrium structure in Fig. 1 corresponds to $r=19.2\AA$ and $\varphi =0°$.

Figure 7

Potential of mean force $\mathrm{\Delta }A(\theta )$ of the angle between the dipoles of $M_1$ and $M_2$ at various distances.