The human prion protein α2 helix: A thermodynamic study of its conformational preferences

We have synthesized both free and terminally-blocked peptide corresponding to the second helical region of the globular domain of normal human prion protein, which has recently gained the attention of structural biologists because of a possible role in the nucleation process and fibrillization of prion protein. The profile of the circular dichroism spectrum of the free peptide was that typical of α-helix, but was converted to that of β-structure in about 16 h. Instead, below 2.1 x 10-5 M, the spectrum of the blocked peptide exhibited a single band centered at 200 nm, unequivocally associated to random conformations, which did not evolve even after 24 h. Conformational preferences of this last peptide have been investigated as a function of temperature, using trifluoroethanol or low-concentration sodium dodecyl sulfate as α- or β-structure inducers, respectively. Extrapolation of free energy data to zero concentration of structuring agent highlighted that the peptide prefers α-helical to β-type organization, in spite of results from prediction algorithms. However, the free energy difference between the two forms, as obtained by a thermodynamic cycle, is subtle (roughly 5-8 kJ mol -1 at any temperature from 280 K to 350 K), suggesting conformational ambivalence. This result supports the view that, in the prion protein, the structural behavior of the peptide is governed by the cellular microenvironment. © 2005 Wiley-Liss, Inc.