224
M. J. Toda et al.
are representative of the normal type, while MeCbl and AdoCbl Abs spectra can be
classified as anomalous. The spectra of some lesser known derivatives, NO 2 Cbl,
SCNCbl, ClCbl, N 3 Cbl, CNCbl, and (i-Pr) 2 -phosphitoCbl, could be classified as
typical and those of SO 3 Cbl, SeCNCbl, MeCbl, AdoCbl and isoamylCbl as unique.
A switch of the axial group can result in a change in the spectra of the cobalamin.
This can be further explained through the lens of electronic structure theory, but there
are two different explanations of why there is a change in the spectra just by changing
the axial group. One was proposed by Brunold and co-workers [85], while other by
Marques and co-workers, [66] each proposing different reasons for the change.
Brunold and co-workers [85] have indicated that the unique spectra of cobalamins
with an alkyl ligand in the upper axial position arises from a combination of two
effects. The first is that there is a net increase in the number of donor molecular orbitals
available for electronic transitions. The second is that there is a mixing between Co
3d and corrin π orbitals. The increase of intensity of the β band relative to the α band
from typical to unique cobalamin Abs spectra is a result of the additional excited-state
distortion (the corrin-based HOMO has Co–C σ -bonding character which is lost upon
HOMO→LUMO excitation distorting along both coordinates in the corresponding
excited state).
According to Marques and co-workers, [66] there is actually no fundamental
difference between the typical and unique spectra of cobalamins, but the reason for
the difference is the relative position of the γ -region components of the cobalamin
spectra. As the donor power of the axial ligand increases, the electronic transitions
of cobalamins move to longer wavelengths. The γ -region moves apart in response
to the increase in donation of electronic density from the axial ligand, which causes
the difference. Also, the bond length increases as the donor power of the axial ligand
increases.
The differences in Abs spectra as they relate to oxidation state of Co and the
nature of the upper axial ligand can be easily understood upon inspection of Fig. 2.
CNCbl exhibits the normal spectrum with a very intense γ band and a prominent
α/β band. The Abs spectrum for the base-off AdoCbl is markedly different. It does
not contain the Soret band, and the α/β region is blue shifted in comparison with the
α/β band of CNCbl. It has been summarized that strong σ -donor ligands produce
a red-shifted γ band, while weak σ -donor ligands produce γ bands that are blue
shifted (Fig. 3) [91]. Clearly, the nature of the axial ligands contributes to the Abs
spectra, providing evidence that π →π
∗ transitions from the corrin macrocycle are
not the only contributor to spectral features. Further differences are evident when
comparing the spectra of cob(III)alamin species like CNCbl and AdoCbl to other
relevant oxidation states like Co(II) and Co(I). The cob(I)alamin spectrum is similar
to that of CNCbl with respect to the intensity of the γ band, although this band is
slightly red shifted compared to CNCbl. Although the Co(I) species does not contain
axial ligands, the intensity of the γ band can be attributed to the lone pair of electrons
acting as a sort of strong ligand which shifts the γ band further to the red [91]. The
major difference between cob(I)alamin and CNCbl is with the intensity of the α/β
band. The α/β band is much less pronounced for cob(I)alamin. The blue-shifted α/β
M. J. Toda et al.
are representative of the normal type, while MeCbl and AdoCbl Abs spectra can be
classified as anomalous. The spectra of some lesser known derivatives, NO 2 Cbl,
SCNCbl, ClCbl, N 3 Cbl, CNCbl, and (i-Pr) 2 -phosphitoCbl, could be classified as
typical and those of SO 3 Cbl, SeCNCbl, MeCbl, AdoCbl and isoamylCbl as unique.
A switch of the axial group can result in a change in the spectra of the cobalamin.
This can be further explained through the lens of electronic structure theory, but there
are two different explanations of why there is a change in the spectra just by changing
the axial group. One was proposed by Brunold and co-workers [85], while other by
Marques and co-workers, [66] each proposing different reasons for the change.
Brunold and co-workers [85] have indicated that the unique spectra of cobalamins
with an alkyl ligand in the upper axial position arises from a combination of two
effects. The first is that there is a net increase in the number of donor molecular orbitals
available for electronic transitions. The second is that there is a mixing between Co
3d and corrin π orbitals. The increase of intensity of the β band relative to the α band
from typical to unique cobalamin Abs spectra is a result of the additional excited-state
distortion (the corrin-based HOMO has Co–C σ -bonding character which is lost upon
HOMO→LUMO excitation distorting along both coordinates in the corresponding
excited state).
According to Marques and co-workers, [66] there is actually no fundamental
difference between the typical and unique spectra of cobalamins, but the reason for
the difference is the relative position of the γ -region components of the cobalamin
spectra. As the donor power of the axial ligand increases, the electronic transitions
of cobalamins move to longer wavelengths. The γ -region moves apart in response
to the increase in donation of electronic density from the axial ligand, which causes
the difference. Also, the bond length increases as the donor power of the axial ligand
increases.
The differences in Abs spectra as they relate to oxidation state of Co and the
nature of the upper axial ligand can be easily understood upon inspection of Fig. 2.
CNCbl exhibits the normal spectrum with a very intense γ band and a prominent
α/β band. The Abs spectrum for the base-off AdoCbl is markedly different. It does
not contain the Soret band, and the α/β region is blue shifted in comparison with the
α/β band of CNCbl. It has been summarized that strong σ -donor ligands produce
a red-shifted γ band, while weak σ -donor ligands produce γ bands that are blue
shifted (Fig. 3) [91]. Clearly, the nature of the axial ligands contributes to the Abs
spectra, providing evidence that π →π
∗ transitions from the corrin macrocycle are
not the only contributor to spectral features. Further differences are evident when
comparing the spectra of cob(III)alamin species like CNCbl and AdoCbl to other
relevant oxidation states like Co(II) and Co(I). The cob(I)alamin spectrum is similar
to that of CNCbl with respect to the intensity of the γ band, although this band is
slightly red shifted compared to CNCbl. Although the Co(I) species does not contain
axial ligands, the intensity of the γ band can be attributed to the lone pair of electrons
acting as a sort of strong ligand which shifts the γ band further to the red [91]. The
major difference between cob(I)alamin and CNCbl is with the intensity of the α/β
band. The α/β band is much less pronounced for cob(I)alamin. The blue-shifted α/β