RESUMEN
Femtosecond time-resolved X-ray absorption (XANES) at the Co K-edge, X-ray emission (XES) in the Co Kß and valence-to-core regions, and broadband UV-vis transient absorption are combined to probe the femtosecond to picosecond sequential atomic and electronic dynamics following photoexcitation of two vitamin B12 compounds, hydroxocobalamin and aquocobalamin. Polarized XANES difference spectra allow identification of sequential structural evolution involving first the equatorial and then the axial ligands, with the latter showing rapid coherent bond elongation to the outer turning point of the excited state potential followed by recoil to a relaxed excited state structure. Time-resolved XES, especially in the valence-to-core region, along with polarized optical transient absorption suggests that the recoil results in the formation of a metal-centered excited state with a lifetime of 2-5 ps. This combination of methods provides a uniquely powerful tool to probe the electronic and structural dynamics of photoactive transition-metal complexes and will be applicable to a wide variety of systems.
RESUMEN
Polarized time-resolved X-ray absorption spectroscopy at the Co K-edge is used to probe the excited-state dynamics and photolysis of base-off methylcobalamin and the excited-state structure of base-off adenosylcobalamin. For both molecules, the final excited-state minimum shows evidence for an expansion of the cavity around the Co ion by ca. 0.04 to 0.05 Å. The 5-coordinate base-off cob(II)alamin that is formed following photodissociation has a structure similar to that of the 5-coordinate base-on cob(II)alamin, with a ring expansion of 0.03 to 0.04 Å and a contraction of the lower axial bond length relative to that in the 6-coordinate ground state. These data provide insights into the role of the lower axial ligand in modulating the reactivity of B12 coenzymes.
Asunto(s)
Coenzimas , Vitamina B 12 , Espectroscopía de Absorción de Rayos X , Vitamina B 12/química , FotólisisRESUMEN
CarH is a protein photoreceptor that uses a form of B12, adenosylcobalamin (AdoCbl), to sense light via formation of a metastable excited state. Aside from AdoCbl bound to CarH, methylcobalamin (MeCbl) is the only other exampleâto dateâof photoexcited cobalamins forming metastable excited states with lifetimes of nanoseconds or longer. The UV-visible spectra of the excited states of MeCbl and AdoCbl bound to CarH are similar. We have used transient Co K-edge X-ray absorption and X-ray emission spectroscopies in conjunction with transient absorption spectroscopy in the UV-visible region to characterize the excited states of MeCbl. These data show that the metastable excited state of MeCbl has a slightly expanded corrin ring and increased electron density on the cobalt, but only small changes in the axial bond lengths.
RESUMEN
Time resolved spectroscopy provides unique insight into the structure and function of cobalamins. In these experiments, the cobalamin is initially excited by a short "pump" pulse in the UV-visible region and then characterized at some later time using a short "probe" pulse. The emphasis in this chapter is on both UV-visible and X-ray probe pulses, with a particular focus on the unique information provided by the latter. The principles of time-resolved spectroscopy are reviewed, with an emphasis on ultrafast measurements (time scales less than ~10ps) to characterize short-lived cobalamin excited states. Several practical considerations are discussed, with a focus on the technical details that are necessary to obtain high quality, interpretable data. These include sample delivery, polarization, and excitation power. Some of the theoretical approaches to interpreting data are discussed.