Radical SAM (RS) enzymes use S-adenosyl-L-methionine (SAM) and a [4Fe-4S] cluster to initiate a broad spectrum of by radical transformations throughout all kingdoms of life. We report here that low-temperature photoinduced electron transfer from the [4Fe-4S](1+) cluster to bound SAM in the active site of the hydrogenase maturase RS enzyme, HydG, results in specific homolytic cleavage of the S-CH3 bond of SAM, rather than the S-C5' bond as in the enzyme-catalyzed (thermal) HydG reaction. This result is in stark contrast to a recent report in which photoinduced ET in the RS enzyme pyruvate formate-lyase activating enzyme cleaved the S-CH3 bond to generate a 5'-deoxyadenosyl radical, and provides the first direct evidence for homolytic S-CH3 bond cleavage in a RS enzyme. Photoinduced ET in HydG generates a trapped (CH3)-C-center dot radical, as well as a small population of an organometallic species with an Fe-CH3 bond, denoted Omega(M). The (CH3)-C-center dot radical is surprisingly found to exhibit rotational diffusion in the HydG active site at temperatures as low as 40 K, and is rapidly quenched: whereas 5'-dAdo* is stable indefinitely at 77 K, (CH3)-C-center dot quenches with a half-time of similar to 2 min at this temperature. The rapid quenching and rotational/translational freedom of (center dot)CH(3)( )shows that enzymes would be unable to harness this radical as a regio- and stereospecific H atom abstractor during catalysis, in contrast to the exquisite control achieved with the enzymatically generated 5'-dAdo*.