Monomeric Polyglutamine Structures That Evolve into Fibrils

We investigate the
solution and fibril conformations and structural
transitions of the polyglutamine (polyQ) peptide, D2Q10K2 (Q10), by synergistically using UV resonance
Raman (UVRR) spectroscopy and molecular dynamics (MD) simulations.
We show that Q10 adopts two distinct, monomeric solution conformational
states: a collapsed β-strand and a PPII-like structure that
do not readily interconvert. This clearly indicates a high activation
barrier in solution that prevents equilibration between these structures.
Using metadynamics, we explore the conformational energy landscape
of Q10 to investigate the physical origins of this high activation
barrier. We develop new insights into the conformations and hydrogen
bonding environments of the glutamine side chains in the PPII and
β-strand-like conformations in solution. We also use the secondary
structure-inducing cosolvent, acetonitrile, to investigate the conformations
present in low dielectric constant solutions with decreased solvent–peptide
hydrogen bonding. As the mole fraction of acetonitrile increases,
Q10 converts from PPII-like structures into α-helix-like structures
and β-sheet aggregates. Electron microscopy indicates that the
aggregates prepared from these acetonitrile-rich solutions show morphologies
similar to our previously observed polyQ fibrils. These aggregates
redissolve upon the addition of water! These are the first examples
of reversible fibril formation. Our monomeric Q10 peptides clearly
sample broad regions of their available conformational energy landscape.
The work here develops molecular-level insight into monomeric Q10
conformations and investigates the activation barriers between different
monomer states and their evolution into fibrils.