The entire scheme of all possible H and D exchange reactions in BSS is shown in Fig. . The calculated barriers indicate immediately that the conservative exchange reactions resulting in the retention of either H to D in the glycyl radicals are highly favored over those leading to isotope exchange. Judging from the much lower barriers for both the forward and reverse reactions in the case of GlyH . , this process (with deuterated Cys493) occurs mostly via the reface (labeled by blue arrows in Fig. ), whereas in the case of GlyD . , it occurs mostly via the siface (with protiated Cys493, labeled green in Fig. ). To obtain any exchange of H to D (or reverse) in the glycyl radical, some of the less favorable reactions in the other stereochemical orientation have to occur. For exchanging H to D, the more favorable pathway is the si-face directed attack of deuterated Cys493 combined with the transfer of the 'proR'-protium from the S-glycyl intermediate back to the thiyl radical (red arrows in Fig. ). Although the barrier of the first step is significantly higher than that for the re-attack (64.8 vs. 43.2 kJ/mol, indicating a 5,400-fold slower rate), the second step of protium vs. deuterium transfer is highly favored (barriers 72.4 vs. 101 kJ/mol), while the alternative pathway via R-Gly829 contains a prohibitively high barrier at the second step (95.6 kJ/mol).