transferring an H atom from the sulfhydryl group of Cys493 to the radical Gly829 . This crucial initial process needs to occur in all GRE, but is poorly understood up to date, and almost no approaches have been addressed to obtain computational models. Therefore, we decided to employ QM:MM modeling of this step using BSS as an example. We started our investigation with an enzyme-substrate complex (E:S), where both fumarate and toluene are bound in the active site. The reaction starts with radical Gly829 and the sulfhydryl group of Cys493, which is turned away in the direction of the neighboring residue Gln707. The spin density of the initial glycyl radical is mostly located at the C carbon of Gly (0.9) but is also distributed by resonance along the peptide backbone to the adjacent nitrogen (0.075) and carbonyl oxygen atoms (0.1) of the preceding peptide bond, and with a resulting negative spin density to the carbonyl C atom of Gly829 (-0.16). These values are remarkably similar to previously calculated spin densities of glycyl radicals in crystals of the model compound Nacetylglycine, where the C carbon was attributed to a value of 0.77 . Both residues, Gly829 and Cys493, represent the tips of two turns, which extend as loops from the inner rim to the center of the barrel structure (hereafter called G-loop and C-loop). Both residues are essential contributors to the active site of the enzyme and directly face each other . To A similar transfer can also occur when the H atom is transferred to the si face of the glycyl radical (leading to proS-TS ES ). The position of the Cys, in this case, resembles that in the proR-TS ES , but the G-loop harboring the glycyl radical is bent to present the other face of the Gly.