TIME-RESOLVED X-RAY-ABSORPTION SPECTROSCOPY OF PHOTOREDUCED BASE-OFF COB(II)ALAMIN COMPARED TO THE CO(II) SPECIES IN CLOSTRIDIUM-THERMOACETICUM

We have made significant improvements in pump-probe time-resolved X-ra y absorption spectroscopy that enable us to structurally describe chem ical 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-no ise ratio by time-resolved discrimination of fluorescent signals from a 13-element germanium detector. With the utilization of this novel ti me-multiplexed laser photolysis system coupled to a flow cell, we char acterized the structure of the initial photoproduct of five-coordinate base-off Co(III) methylcobalamin. The structure of the primary photop roduct could have included five- or six-coordinate species with water ligation, or a four-coordinate square-planar species. A four-coordinat e Co(II) species is expected to be unstable but its biological relevan ce is highlighted by our recent discovery of a four-coordinate Co(II) species, (existing as the inactive, as isolated, form) in the corrinoi d protein of Clostridium thermoaceticum. The X-ray preedge spectra of five- and six-coordinate species have a strong 1s-3d transition at abo ut 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 ba se-off methylcobalamin shows a substantial decrease in the 1s-3d peak and significant increase in the 1s-4p, peak. This indicates the format ion of a four-coordinate species. The four-coordinate species in the f ree cobalamin is very unstable and can only be detected by time-resolv ed methods. This indicates a special role for the protein in maintaini ng an unusual four-coordinate Co(II) corrinoid.