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 preedge 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 structure of the primary photoproduct could have included five-or six-coordinate species with water ligation, or a four-coordinate square-planar species. A four-coordinate Co(II) species is expected to be unstable but its biological relevance is highlighted by our recent discovery of a four-coordinate Co(II) species, (existing as the inactive, as isolated, form) in the corrinoid protein of Clostridium thermoaceticum. The X-ray preedge spectra of five-and sixcoordinate species have a strong 1s-3d transition at about 10 eV below the edge. In four-coordinate, squareplanar species the 1s-3d intensity is significantly reduced, but they show a 1s-4p z 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 z peak. This indicates the formation of a four-coordinate species. The four-coordinate species in the free cobalamin is very unstable and can only be detected by time-resolved methods. This indicates a special role for the protein in maintaining an unusual four-coordinate Co(II) corrinoid.

Glutamate Ligation in the Ni(II)- and Co(II)-Responsive Escherichia coli Transcriptional Regulator, RcnR

“…Thus, there is no obvious change in geometry associated with the variation of Glu34 or Glu63 as compared to WT- Ec RcnR. 75, 76 However, the XANES spectra for WT, E34A-, E34C-, E63A- and E63C - Ec RcnR are non-overlapping, and thus indicative of some change in the Co(II) coordination sphere. These differences were addressed by EXAFS analysis.…”

Preparation of non-precious metal catalysts for PEMFC cathode from pyrolyzed vitamin B12

“…In previous studies, the Co oxidation state of pristine B12 was 3þ [25,26]. Scheuring et al used timeresolved X-ray absorption spectroscopy to study pristine B12-Co(III) and base-off B12-Co(II), and found that the E 0 of Co (II) was lower than that of Co(III) by approximately 4.0e6.0 eV [27]. The Co K-edge E 0 of py-B12-300 is lower than that of pristine B12 by about 2.9 eV, indicating that the Co oxidation state may be a mixture of 3þ and 2þ.…”

“…Scheuring et al examined various six-, five-and four-coordinated B12 species. They found that the six-coordinated B12 species has an octahedral structure with a central Co(III) and the four-coordinated B12 species has a square-planar Corrin -structure with a central Co(II) [27]. Okada et al identified four-, five-and six-coordinated Co(II)-or Co(III)-based nitrogen-containing compounds, and concluded that the best catalysts of the ORR are four-coordinated Co(II)-based nitrogen-containing compounds with a coplanar chelate structure [33].…”

Momentaufnahmen photoangeregter Moleküle in fehlgeordneten Medien mit gepulster Synchrotron‐Röntgenstrahlung

“…Mit dem Auftauchen von Synchrotrons der dritten Generation mit extrem hohen Photonenflüssen pro Röntgen-puls wurden die Untersuchungen von Molekülen in angeregten Zuständen in fehlgeordneten Medien dann entscheidend verbessert. Frühe Studien mit älteren Synchrotrons gingen hauptsächlich von drei Ansätzen aus: 1) Zeitauflösung durch Sperren des Detektors oder Unterbrechen des Rönt-genpulses, [22][23][24][25][26][27][28][29][30] 2) Abfangen von Molekülen im angeregten Zustand in kryogenen Matrices und Untersuchung mit stationären Methoden [31][32][33][34][35] und 3) energiedispersive Rönt-genabsorptionsspektroskopie, bei der die Struktur des angeregten Zustands innerhalb der Auslesezeit des Detektors ermittelt wird. [36][37][38][39][40][41] Die Zeitauflösung bei diesen frühen Arbeiten lag nur im Mikrosekundenbereich, weshalb viele angeregte Zustände, besonders Singulettzustände mit kurzer Lebensdauer, nicht untersucht werden konnten.…”