The prion protein is a copper binding glycoprotein that in mammals can misfold into a pathogenic isoform leading to prion diseases, as opposed, surprisingly, to avians. The avian prion N-terminal tandem repeat is richer in prolines than the mammal one, and understanding their effect on conformation is of great biological importance. Here we succeeded in investigating the conformations of a single avian hexarepeat by means of NMR and molecular dynamics techniques. We found a high flexibility and a strong conformational dependence on pH: local turns are present at acidic and neutral pH, while unordered regions dominate at basic conditions. (C) 2007 Elsevier B.V. All rights reserved.