Aggregation of amyloid β-protein (Aβ) into amyloid oligomers and fibrils is pathologically linked to Alzheimer’s disease (AD). Hence, the inhibition of Aβ aggregation is essential for the prevention and treatment of AD, but the development of potent agents capable of inhibiting Aβ fibrillogenesis has posed significant challenges. Herein, we designed Ac-LVFFARK-NH2 (LK7) by incorporating two positively charged residues, R and K, into the central hydrophobic fragment of Aβ17–21 (LVFFA) and examined its inhibitory effect on Aβ42 aggregation and cytotoxicity by extensive physical, biophysical, and biological analyses. LK7 was observed to inhibit Aβ42 fibrillogenesis in a dose-dependent manner, but its strong self-assembly characteristic also resulted in high cytotoxicity. In order to prevent the cytotoxicity that resulted from the self-assembly of LK7, the peptide was then conjugated to the surface of poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) to fabricate a nanosized inhibitor, LK7@PLGA-NPs. It was found that LK7@PLGA-NPs had little cytotoxicity because the self-assembly of the LK7 conjugated on the NPs was completely inhibited. Moreover, the NPs-based inhibitor showed remarkable inhibitory capability against Aβ42 aggregation and significantly alleviated its cytotoxicity at a low LK7@PLGA-NPs concentration of 20 μg/mL. At the same peptide concentration, free LK7 showed little inhibitory effect. It is considered that several synergetic effects contributed to the strong inhibitory ability of LK7@PLGA-NPs, including the enhanced interactions between Aβ42 and LK7@PLGA-NPs brought on by inhibiting LK7 self-assembly, restricting conformational changes of Aβ42, and thus redirecting Aβ42 aggregation into unstructured, off-pathway aggregates. The working mechanisms of the inhibitory effects of LK7 and LK7@PLGA-NPs on Aβ42 aggregation were proposed based on experimental observations. This work provides new insights into the design and development of potent NPs-based inhibitors against Aβ aggregation and cytotoxicity.Keywords: Alzheimer’s disease; amyloid β-protein; nanoparticle; peptide inhibitor; protein aggregation; self-assembly

Protein A from the bacterium Staphylococcus aureus (SpA) has been widely used as an affinity ligand for purification of immunoglobulin G (IgG). The affinity between SpA and IgG is affected differently by salt and pH, but their molecular mechanisms still remain unclear. In this work, molecular dynamics simulations and molecular mechanics Poisson–Boltzmann surface area analysis were performed to investigate the salt (NaCl) and pH effects on the affinity between SpA and human IgG1 (hIgG1). It is found that salt and pH affect the interactions of the hot spots of SpA by different mechanisms. In the salt solution, the compensations between helices I and II of SpA as well as between the nonpolar and electrostatic energies make the binding free energy independent of salt concentration. At pH 3.0, the unfavorable electrostatic interactions increase greatly and become the driving force for dissociation of the SpA–hIgG1 complex. They mainly come from the strong electrostatic repulsions between positively charged residues (H137, R146, and K154) of SpA and the positively charged residues of hIgG1. It is considered to be the molecular basis for hIgG1 elution from SpA-based affinity adsorbents at pH 3.0. The dissociation mechanism is then used to refine the binding model of SpA to hIgG1. The model is expected to help design high-affinity peptide ligands of IgG.

Considerable experimental evidence indicates that trehalose can counteract the denaturing effects of urea on proteins. However, its molecular mechanism remains unknown due to the limitations of current experimental techniques. Herein, molecular dynamics simulations were performed to investigate the counteracting effects of trehalose against urea-induced denaturation of chymotrypsin inhibitor 2. The simulations indicate that the protein unfolds in 8 mol/L urea, but at the same condition the protein retains its native structure in the ternary solution of 8 mol/L urea and 1 mol/L trehalose. It is confirmed that the preferential exclusion of trehalose from the protein surface is the origin of its counteracting effects. It is found that trehalose binds urea via hydrogen bonds, so urea molecules are also expelled from the protein surface along with the preferential exclusion of trehalose. The exclusion of urea from the protein surface leads to the alleviation of the Lennard-Jones interactions between urea and the hydrophobic side chains of the protein in the ternary solution. In contrast, the electrostatic interactions between urea and the protein change little in the presence of trehalose because the decrease in the electrostatic interactions between urea and the protein backbone is canceled by the increase in the electrostatic interactions between urea and the charged side chains of the protein. The results have provided molecular explanations for the counteraction of urea-induced protein denaturation by trehalose.