Ultrafast Excited-State Dynamics in Vitamin B12 and Related Cob(III)alamins

• Published in: Journal of the American Chemical Society 2006-01, Vol.128 (3), p.801-808
• Main Authors: Shiang, Joseph J, Cole, Allwyn G, Sension, Roseanne J, Hang, Kun, Weng, Yuxiang, Trommel, Jenna S, Marzilli, Luigi G, Lian, Tianquan
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Electronic structure; Fundamental and applied biological sciences. Psychology; Hydroxocobalamin - chemistry; Molecular biophysics; Solvents; Spectrophotometry, Infrared; Spectrophotometry, Ultraviolet; Structure in molecular biology; Vitamin B 12 - analogs & derivatives; Vitamin B 12 - chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja054374+

Femtosecond transient IR and visible absorption spectroscopies have been employed to investigate the excited-state photophysics of vitamin B12 (cyanocobalamin, CNCbl) and the related cob(III)alamins, azidocobalamin (N3Cbl), and aquocobalamin (H2OCbl). Excitation of CNCbl, H2OCbl, or N3Cbl results in rapid formation of a short-lived excited state followed by ground-state recovery on time scales ranging from a few picoseconds to a few tens of picoseconds. The lifetime of the intermediate state is influenced by the σ-donating ability of the axial ligand, decreasing in the order CNCbl > N3Cbl > H2OCbl, and by the polarity of the solvent, decreasing with increasing solvent polarity. The peak of the excited-state visible absorption spectrum is shifted to ca. 490 nm, and the shape of the spectrum is characteristic of weak axial ligands, similar to those observed for cob(II)alamin, base-off cobalamins, or cobinamides. Transient IR spectra of the upper CN and N3 ligands are red-shifted 20−30 cm-1 from the ground-state frequencies, consistent with a weakened Co−upper ligand bond. These results suggest that the transient intermediate state can be attributed to a corrin ring π to Co 3d z 2 ligand to metal charge transfer (LMCT) state. In this state bonds between the cobalt and the axial ligands are weakened and lengthened with respect to the corresponding ground states.