250
M. J. Toda et al.
cobalamin (GSCbl) have been investigated [18]. The Abs spectrum for GSCbl
(Fig. 15) is more in line with unique spectra of MeCbl or AdoCbl. In particular,
the γ region of MeCbl and GSCbl are quite similar. Generally speaking, for cobalamins, the shift of the γ band is due to the σ -donor strength of the upper axial
ligand [67]. For GSCbl, the γ band is shifted to longer wavelengths and the α/β
region is shifted to the red compared to MeCbl. Also, the upper axial ligand for
GSCbl is responsible for the split into two bands in the γ region.
Not surprisingly, there have been two different theoretical approaches to elucidate
the nature of the electronic transitions associated with the Abs spectrum. Brunold and
co-workers analyzed GSCbl with Abs, CD, and MCD spectroscopy as well as with
TD-DFT [13]. Interestingly, they chose to model the ground state of GSCbl using
a combined quantum mechanical/molecular mechanics (QM/MM) approach. Upon
ground-state optimization, they truncated the model to be much more cost effective
for TD-DFT. The upper axial ligand was simplified to be SCH 3 . The simulated Abs
spectrum was produced with the B3LYP functional, and comparisons were drawn
between the Abs spectra of MeCbl and GSCbl.
In a more thorough TD-DFT study, both B3LYP and BP86 were employed to
simulate Abs spectra of GSCbl; only this time, the upper axial ligand was not truncated [18]. As in the case for the other cobalamins discussed, the two functionals
predict different characters for the relevant transitions that give rise to the spectral
features. To summarize, Birke and co-workers [18] conclude that BP86 is the proper
functional to use to understand the electronically excited states of GSCbl. Overall,
BP86 simulated a more accurate Abs spectrum than B3LYP. BP86 indicated more
participation of d orbitals than B3LYP. An interesting conclusion from this study
was that the experimentally observed peaks at 612 and 529 nm were suggested to
arise from vibronic transitions.
9.7 Reduced Cobalamins
The Abs spectra of the reduced cobalamins, cob(II)alamin and cob(I)alamin, have
also been studied from an experimental [49, 64, 86] and theoretical [21, 33, 40,
44, 49, 64, 86] point of view. The colors of each compound are indicative of their
oxidation state. CNCbl is red, cob(II)alamin is orange, and cob(I)alamin is green.
One-electron reduction of cob(III)alamin leads to cob(II)alamin. This species
yields a unique Abs spectrum. In order to understand the experimental Abs spectrum
more fully, two different truncated models representative of the Co(II) compound
were used (Fig. 16) [21]. One was four-coordinate and the other five-coordinate.
The accuracy of the TD-DFT calculations was compared to XMCQDPT2 results.
Both methods yielded good agreement with each other and further validated the idea
that TD-DFT is a less expensive and a reliable choice for studying the electronically
excited states of the cobalamins. The α/β region is not as pronounced as in the other
cobalamins but is narrower and is found between 450–500 nm. This region results
from a mixture of transitions that included d orbitals. A sharp γ peak is not present. A
M. J. Toda et al.
cobalamin (GSCbl) have been investigated [18]. The Abs spectrum for GSCbl
(Fig. 15) is more in line with unique spectra of MeCbl or AdoCbl. In particular,
the γ region of MeCbl and GSCbl are quite similar. Generally speaking, for cobalamins, the shift of the γ band is due to the σ -donor strength of the upper axial
ligand [67]. For GSCbl, the γ band is shifted to longer wavelengths and the α/β
region is shifted to the red compared to MeCbl. Also, the upper axial ligand for
GSCbl is responsible for the split into two bands in the γ region.
Not surprisingly, there have been two different theoretical approaches to elucidate
the nature of the electronic transitions associated with the Abs spectrum. Brunold and
co-workers analyzed GSCbl with Abs, CD, and MCD spectroscopy as well as with
TD-DFT [13]. Interestingly, they chose to model the ground state of GSCbl using
a combined quantum mechanical/molecular mechanics (QM/MM) approach. Upon
ground-state optimization, they truncated the model to be much more cost effective
for TD-DFT. The upper axial ligand was simplified to be SCH 3 . The simulated Abs
spectrum was produced with the B3LYP functional, and comparisons were drawn
between the Abs spectra of MeCbl and GSCbl.
In a more thorough TD-DFT study, both B3LYP and BP86 were employed to
simulate Abs spectra of GSCbl; only this time, the upper axial ligand was not truncated [18]. As in the case for the other cobalamins discussed, the two functionals
predict different characters for the relevant transitions that give rise to the spectral
features. To summarize, Birke and co-workers [18] conclude that BP86 is the proper
functional to use to understand the electronically excited states of GSCbl. Overall,
BP86 simulated a more accurate Abs spectrum than B3LYP. BP86 indicated more
participation of d orbitals than B3LYP. An interesting conclusion from this study
was that the experimentally observed peaks at 612 and 529 nm were suggested to
arise from vibronic transitions.
9.7 Reduced Cobalamins
The Abs spectra of the reduced cobalamins, cob(II)alamin and cob(I)alamin, have
also been studied from an experimental [49, 64, 86] and theoretical [21, 33, 40,
44, 49, 64, 86] point of view. The colors of each compound are indicative of their
oxidation state. CNCbl is red, cob(II)alamin is orange, and cob(I)alamin is green.
One-electron reduction of cob(III)alamin leads to cob(II)alamin. This species
yields a unique Abs spectrum. In order to understand the experimental Abs spectrum
more fully, two different truncated models representative of the Co(II) compound
were used (Fig. 16) [21]. One was four-coordinate and the other five-coordinate.
The accuracy of the TD-DFT calculations was compared to XMCQDPT2 results.
Both methods yielded good agreement with each other and further validated the idea
that TD-DFT is a less expensive and a reliable choice for studying the electronically
excited states of the cobalamins. The α/β region is not as pronounced as in the other
cobalamins but is narrower and is found between 450–500 nm. This region results
from a mixture of transitions that included d orbitals. A sharp γ peak is not present. A