Time-Resolved X-ray Absorption Spectroscopy of Photoreduced Base-off Cob(II)alamin Compared to the Co(II) Species in Clostridium thermoaceticum

Abstract

We have made significant improvements in pump-probe time-resolved X-ray absorption spectroscopy that enable us to structurally describe chemical intermediates with short lifetimes. We demonstrate that X-ray pre-edge data for a 1 mM compound can be acquired with a high signal-to-noise ratio by time-resolved discrimination of fluorescent signals from a 13-element germanium detector. With the utilization of this novel time-multiplexed laser photolysis system coupled to a flow cell, we characterized the structure of the initial photoproduct of five-coordinate base-off Co(III) methylcobalamin. The X-ray pre-edge spectra of five- and six-coordinate species have a strong 1s-3d transition at about 10 eV below the edge. In four-coordinate, square-planar species the 1s-3d intensity is significantly reduced, but they show a 1s-4p peak at about 6 eV below the edge. We used this `fingerprint` to monitor the structural change upon photolysis. Since the quantum yield of the base-off species is 0.48, the observed spectrum upon photolysis is a mixture of photoproduct and initial states. The photoproduct of the base-off methylcobalamin shows a substantial decrease in the 1s-3d peak and significant increase in the 1s-4p peak. This indicates the formation of a four-coordinate species. The four-coordinate species in the free cobalamin is very unstable and can onlymore » be detected by time-resolved methods. This indicates a special role for the protein in maintaining an unusual four-coordinate Co(II) corrinoid. 23 refs., 5 figs.« less