Fig. 7.

Nitroxide labeling strategy and resulting PrP^C conformation resulting from Zn²⁺ coordination in the OR domain. (A) N-terminal labeling was achieved by introduction of a selectively placed Cys residue, followed by reaction with MTSL, resulting in the R1 side chain. (B) Labeling in the C-terminal domain involved incorporation of the unnatural amino acid para-acetyl phenylalanine, which was then reacted with hydroxylamine nitroxide reagent. (C) Energy minimization using distances derived from the nitroxide-nitroxide DEER EPR results identified a compact structure with the Zn²⁺-OR forming an interdomain cis interaction with the regulatory C-terminal domain.