A strand-loop-strand structure is a possible intermediate in fibril elongation: long time simulations of amyloid-beta...

A total of 6.2 micros molecular dynamics simulations of amyloid-beta (10-35) (Abeta) were performed in explicit water solvent. The results reveal that the collapsed-coil (cc) structure determined by experiments is stable at pH 5.6 for hundreds of nanoseconds, but it can exchange with a strand-loop-strand (SLS) structure on the microsecond time scale. The SLS structure has D23-K28 as a reverse loop and the central hydrophobic core and the C-terminal in hydrophobic contact. This SLS structure topologically resembles the proposed monomer conformation in fibrils. Since it has been suggested that a special conformation of Abeta is needed when the monomer binds to fibril ends to elongate fibrils, we propose that the SLS structure may be an important intermediate binding structure for Abeta fibril growth. Simulations at pH 2.0, which is used to mimic the mutation of E22Q and D23N, and at high temperature (400 K) indicate that the SLS structure is considerably populated under these conditions while the cc structure is disrupted. These results imply that the SLS structures may also be a binding intermediate in other conditions such as E22Q and/or D23N mutations and high temperature, which have been proved to promote fibril formation previously.