This occurs as well in PFL , but not in ARNR, where the hyperfine splitting does not change even after 12 hours of incubation in D2O-based buffers . The observed spectral change for BSS indicates a complete exchange of the remaining protium hydrogen at C of the glycyl radical within the time required for changing the buffer (ca. 1 h) . Since the hydrogen substituents of Gly829 are not acidic enough to spontaneously exchange with water as evident from experiments with model compounds , the process needs to be coupled to hydrogen transfer reactions with residues containing exchangeable hydrogens. The most likely of these reactions are either the GRE activation/deactivation processes or the hydrogen transfer cascades between Gly829 and Cys493 associated with the reaction mechanism.