If deuterium is transferred from Cys493 to the si face of the glycyl radical, the barrier is at 64.8 kJ/mol, representing an increase of 2.6 kJ/mol compared to the si H/H exchange. The reaction, the backward transfer of a D atom from the S-enantiomer of monodeuterated Gly829, occurs with a barrier of 101.0 kJ/mol, 5.2 kJ/mol more than for the si H/H exchange (Fig. ).