Fully protonated BSS showed barriers of 40.4 and 72.2 kJ/mol for the forward and reverse reactions of H exchange with Cys493 via the re face of the glycyl radical, but of 62.2 and 95.8 kJ/mol via the si face. Therefore, this hydrogen transfer appears to be highly enantiospecific for the re-face, which proceeds over 5,600 times faster than via the si face and in the reverse process over 12,000 faster for the proR-H transfer compared to proS.