Highly polar environments catalyze the unfolding of PrP C helix 1

The first alpha-helix (H1) likely plays an important role in the conversion of the cellular prion protein (PrP(C)) into its pathogenic isoform (PrP(Sc)). In this conversion, H1 may either have to unfold or may represent a site of intermolecular contact. A recent molecular dynamics simulation suggested that H1 can unfold if it is detached from the protein core (Hirschberger et al. in Biophys J 90:3908, 2006). It has been hypothesized that the high dielectric constant epsilon (S) of the bulk water environment facilitates the unfolding of H1. To check this hypothesis, we performed a number of replica exchange molecular dynamics simulations of an H1 peptide in solvents of different epsilon (S). We found that the equilibrium helix fraction in water is less than 40%, in agreement with previous experimental findings, and that the helix unfolds much faster in water than in less polar solvents. The kinetically stabilizing effect of the organic solvents is largely unspecific and correlates well with their dielectric constant epsilon (S).