Off to the Races: Comparison of Excited State Dynamics in Vitamin B₁₂ Derivatives Hydroxocobalamin and Aquocobalamin

Abstract

Ultrafast time-resolved spectroscopy was used to study the photochemistry of hydroxocobalamin (HOCbl) and aquocobalamin (HOCbl) in solution. Spectroscopic measurements and TD-DFT simulations provide a consistent picture of the spectroscopy and photochemistry. Excitation of HOCbl results in formation of an excited state followed by rapid internal conversion to the ground state (0.35 ± 0.15 ps) through an S/S seam at a slightly elongated Co-O bond length and a significantly elongated Co-N bond length. In contrast, the initial elongation of the axial bonds in HOCbl is followed by contraction to an excited state minimum with bonds slightly shorter than those in the ground state. Internal conversion to the ground state follows on a picosecond time scale (5.3 ± 0.4 ps). For both compounds, photodissociation forming cob(II)alamin and hydroxyl radicals (∼1.5% yield) requires excitation to highly excited states. Dissociation is mediated by competition between internal conversion to the S surface and prompt bond cleavage.

The Photoactive Excited State of the B12-Based Photoreceptor CarH

“…A visible transient absorption measurement by Kutta et al used near UV excitation at 370 nm to investigate the initial excited-state dynamics, suggesting a low quantum yield for the photochemical activity of AdoCbl bound to CarH and a novel mechanism . However, the photochemistry of cobalamins is known to be wavelength-dependent in some cases. − Here, we use ultrafast UV–visible spectroscopy to characterize the excited electronic state of AdoCbl bound to Thermus thermophilus CarH following 520 to 565 nm excitation (in the visible αβ-band region of the spectrum) and combine these measurements with transient X-ray absorption near edge structure (XANES) spectra to probe the changes in excited-state structure more directly.…”

Ultrafast Excited State Dynamics and Fluorescence from Vitamin B12 and Organometallic [Co]–C≡C–R Cobalamins

“…The averaged fluorescence spectra of HOPryCbl and CNCbl are plotted in Figure and compared with the fluorescence spectrum of Znbl reported previously . Analysis of the cobalamin absorption spectrum by us and by others has typically involved fitting the spectrum to a sum of Gaussian bands with the α, β, and sh peaks assigned to distinct electronic states or to a vibrational progression in ν LA , the long axis CC stretching mode of the corrin ring, within a single state. , This mode is observed at ca. 1500 cm –1 in resonance Raman measurements of cobalt corrins, with the expectation that the frequency is somewhat lower in the excited state, ca.…”

“…Cobalamins are generally considered nonfluorescent. Motion along the reactive surface or internal conversion from the ππ* state proceeds rapidly, preventing the observation of emission under most conditions. ,, In fact, an ultrafast X-ray study of cyanocobalamin (CNCbl) suggests ultrafast motion out of the initial Franck–Condon region from a “bright” corrin-centered ππ* electronic configuration to a dark ligand field πσ*­(3d z 2 ) configuration occurs within ca. 50 fs, followed by elongation of the axial bonds. , Relaxation into the excited state minimum is complete within a few hundred femtoseconds.…”

Aerobic photolysis of methylcobalamin: unraveling the photoreaction mechanism

“…The DFT and TD-DFT level of theory with the BP86 functional has been previously successfully applied for investigating the ground and excited-state properties of Cbls. 49,50,55,56,81–84 In particular, the applicability of pure-GGA BP86 functional in describing the electronic structure of Cbls has been benchmarked against several hybrid functional and compared with wavefunction-based methods, such as CASSCF/MC-XQDPT2 and EOM-CCSD. 85–87 It was shown that the hybrid functionals were providing an inaccurate description of the electronically excited states of Cbls, which was inconsistent with experimental observations.…”

“…While the biologically active forms of Cbls with alkyl axial ligands, such as MeCbl and AdoCbl as well as their analogs are considered to be photolytically active, 20–22 the non-alkyl Cbls are photostable. 54–56 It should be further pointed out that the photoreaction of MeCbl depends on the excitation wavelength. 57 While excitation with visible light (at 400 nm) leads to the partition between prompt photolysis and the formation of a metastable excited state, absorption of light with a longer wavelength (at about 520 nm) results in the formation of a metastable cob( iii )alamin photoproduct, without any prompt photolysis.…”