Abstract
The octapeptide (PHGGGWGQ) is a fragment of the prion protein (PrP) and this sequence is repeated in the human PrP 60-91 region. The octarepeat is known to have great affinity to copper (II) ions. Proline can form either a Cγ-exo ring pucker or a Cγ-endo ring pucker. Due to stereoelectronic effects, an exo ring pucker is preferred if an electron-withdrawing group is on the 4R position of proline while an electron-withdrawing substituent on 4S position favors an endo ring pucker. The proline of the octarepeat peptide in the native prion protein adopts an endo pucker. In this work we have studied how the ring pucker of proline affects the binding affinity of the peptide to Cu2+ ions by using 4R-hydroxyproline (Hyp)、4R-fluoroproline (Flp)、4R-methoxyproline (Mop), 4S-hydroxyproline (hyp)、 4S-fluoroproline (flp) and 4S-methoxyproline (mop). The fluorescence quenching behavior and binding stoichiometry of proline derivatives containing PrP peptides are similar to the wild type. The dissociation constant (Kd) measured by the fluorescence and CD glycine competition experiments is similar to that of the wild type with the exception of Hyp and hyp substituted peptides. The significent change in the dissociation constants of these two peptides might be due to the hydration of the OH group, which pertubes the environments of the coordination site and affects the coordination between Cu2+ and the peptides. Especially, Hyp substituted peptide has a significently different CD spectrum from those of the other PrP peptides upon binding to Cu2+. The difference between Hyp and hyp sunstituted peptides could be due to the conformation of their pyrrolidine rings.