β2-Microglobulin (β2-m) forms amyloid fibrils in patients undergoing long-term hemodialysis. K3 peptide, a Ser20-Lys41 fragment of β2-m, has been known to form fibrils over a wide range of pH and solvent conditions. Recent solid-state NMR has revealed that K3 oligomer adopts a parallel U-shaped β-strand-turn-β-strand motif. In order to investigate the stability and morphologies of K3 oligomers with different sizes (dimer, trimer, and tetramer) and organizations (single and double layers), several all-atom molecular dynamics simulations were conducted at 310 K and pH 2 in water and 2,2,2-trifluoroethanol (TFE). For single-layered organizations, our results show that TFE destabilizes the stacking of K3 peptides due to the fact that TFE weakens the intermolecular hydrophobic interactions of K3 oligomers. In addition, we also identified that the loop region is stabilized by the hydrophobic cluster involving resides Y7, F11, and I16. Our results further suggest that K3 tetramer is a potential minimal nucleus seed for the formation of K3 protofibrils. For double-layered organizations in water, our data demonstrate that K3 peptides can form various stable assemblies through different interfacial arrangements, such as NN, NC, and CC, by different driving forces. We further propose that the stacking of different interfaces between two facing β-sheets of K3 peptides could be related to different fibril morphologies, which is in good agreement with the previous experimental results, showing that K3 protofibrils associated to formed mature fibrils with a wide range of diameters from 4 to 15 nm when they were transferred from 20% (v/v) TFE to aqueous solution.

Key words: β2-microglobulin (β2-m); Amyloid fibril; K3 peptide; 2,2,2-trifluoroethanol (TFE); Molecular dynamics (MD) simulation; Hydrophobic interaction; Morphology.