Interestingly, the barriers for both hydrogen transfer reactions from Cys493 to the deuterated glycyl radical turned out to be slightly lower than those for the corresponding H/H exchange reactions due to a secondary kinetic isotope effect: the barriers for protium transfer to the re and si faces are 39.8 and 61 kJ/mol (Fig. ), which is 0.6 and 1.2 kJ/mol lower than the respective values for the H/H transfer and associated with an inverse KIE (0.78 and 0.63, respectively). Similarly, the secondary kinetic isotope effect facilitates also deuteron transfer when the glycyl radical is already deuterated (GlyD . ). Such a process is associated with a barrier of 42.6 kJ/mol for the re-face attack and 61.0 kJ/mol for the si face attack. (Fig. ), resulting in a lower intrinsic KIE with respect to deuteron transfer to a non-deuterated glycyl radical (decrease from 3.0 for GlyH . to 2.24-2.35 for GlyD . ). Thus, it is easier to transfer a second deuteron if the first one is already present at the glycyl radical.