Observation of the Delta g mechanism resulting from the ultrafast spin dynamics that follow the photolysis of coenzyme B-12

摘要

Throughout nature, both free radicals and transient radical reaction intermediates are vital to many biological functions. Coenzyme B-12 is a case in point. This organometallic cofactor generates a radical pair upon activation in its dependent enzymes by substrate binding and following photolysis. The resulting cob(II)alamin/5'-deoxyadenosyl radical pair has unusual magnetic properties that present a challenge to detailed investigation at ambient temperatures. Here, we use femtosecond transient absorption spectroscopy adapted for magnetic field exposure to reveal that the spin dynamics of the B-12 radical pair are sufficiently fast for magnetic field effects to be observed on the ultrafast reaction kinetics. Moreover, the large difference in g-values between the radicals of the pair means that effects of the Delta g mechanism are observed for the first time for a radical pair system exposed to magnetic fields below 1 T. Spin dynamic simulations allow a value of the cob(II)alamin radical g-value (2.105) at ambient temperature to be extracted and, because the spin dynamic time scale is faster than the diffusional rotation of the cob(II)alamin radical, the observed value corresponds to the anisotropic g(parallel to) value for this radical. Published under license by AIP Publishing.