Double isotope fractionation: test for concertedness and for transition-state dominance

The racemization of D-[2-2H]- and L-[2-2H] proline by the enzyme proline racemase in mixed H2O-D2O has been used in a double isotopic fractionation experiment in order to define the timing of the bond-making and bond-breaking steps of this reaction. The method is general provided that substitution at one site (2H for 1H) can be specified while isotopic fractionation at the other site (bond) (e.g., 1H vs 2H, etc.) is measured and provides a test of concertedness as well as a definition of whether the isotopically sensitive transition states are rate determining. The proline racemization was found to be either concerted or to involve enzyme catalytic groups (thiols), having fractionation factors near 0.5