226
M. J. Toda et al.
additional insights to electronically excited states of cobalamins, but Abs spectra is
widely more understood to date.
4 Transient Absorption Spectroscopy
While UV-visible spectroscopy can be used to identify the oxidation state and the
axial ligation associated with cobalamins, transient absorption spectroscopy (TAS)
can be used to characterize the intermediate states involved in the photolysis mechanism. From several of these time-resolved studies, it is apparent that the photolysis
mechanism related to the cleavage of the Co–C bond is dependent on the nature of
the upper axial ligand and the solvation environment [83]. Generally speaking, nonalkyl cobalamins such as CNCbl are photostable. The important alkyl B 12 cofactors,
such as MeCbl and AdoCbl, are photolabile, and photolysis results in cob(II)alamin
and the corresponding alkyl radical pairs (RP).
5 Early Attempts to Analyze and Assign Electronically
Excited States
In the field of B 12 chemistry, Abs spectroscopy has played an important role in understanding the electronically excited states of cobalamins. There are common features
in Abs spectra across the family of cobalamin derivatives. These were mentioned
briefly in Sect. 3. The α band in the region of 420–600 nm has been associated with
π transitions related to the corrin ring. The β band at 550 nm has been typically
referred to as vibrational fine structure. The combined α/β band has been attributed
to the forbidden π 7 →π
∗
8 transitions based on early studies by Eckert and Kuhn [17].
The intense γ band transitions between 350–420 nm correspond to π transitions
that are symmetry allowed. The antisymmetric forbidden transitions are designated
as C and D. The δ bands (300–330 nm) are of lower intensity than the Soret band
and have been considered as γ vibrational bands. In B 12 Volume 1: Chemistry, C.
Giannotti [24] indicated five transition types that would be present in cobalamins
including:
1. d-d transitions that are specific to the Co ion
2. Internal transitions corresponding to the equatorial corrin ligand
3. Internal transitions of the axial ligand
4. Charge-transfer transitions from the corrin ring to Co or vice versa
5. Charge-transfer transitions from an axial ligand to Co or vice versa.
As spectroscopic techniques and interpretation from simulations have progressed,
the traditional assignment of peaks in Abs spectra of cobalamins has been further
refined. It was once generally accepted that Abs spectra of cobalamins can be interpreted based on the electronic transitions associated entirely with the corrin ring.
M. J. Toda et al.
additional insights to electronically excited states of cobalamins, but Abs spectra is
widely more understood to date.
4 Transient Absorption Spectroscopy
While UV-visible spectroscopy can be used to identify the oxidation state and the
axial ligation associated with cobalamins, transient absorption spectroscopy (TAS)
can be used to characterize the intermediate states involved in the photolysis mechanism. From several of these time-resolved studies, it is apparent that the photolysis
mechanism related to the cleavage of the Co–C bond is dependent on the nature of
the upper axial ligand and the solvation environment [83]. Generally speaking, nonalkyl cobalamins such as CNCbl are photostable. The important alkyl B 12 cofactors,
such as MeCbl and AdoCbl, are photolabile, and photolysis results in cob(II)alamin
and the corresponding alkyl radical pairs (RP).
5 Early Attempts to Analyze and Assign Electronically
Excited States
In the field of B 12 chemistry, Abs spectroscopy has played an important role in understanding the electronically excited states of cobalamins. There are common features
in Abs spectra across the family of cobalamin derivatives. These were mentioned
briefly in Sect. 3. The α band in the region of 420–600 nm has been associated with
π transitions related to the corrin ring. The β band at 550 nm has been typically
referred to as vibrational fine structure. The combined α/β band has been attributed
to the forbidden π 7 →π
∗
8 transitions based on early studies by Eckert and Kuhn [17].
The intense γ band transitions between 350–420 nm correspond to π transitions
that are symmetry allowed. The antisymmetric forbidden transitions are designated
as C and D. The δ bands (300–330 nm) are of lower intensity than the Soret band
and have been considered as γ vibrational bands. In B 12 Volume 1: Chemistry, C.
Giannotti [24] indicated five transition types that would be present in cobalamins
including:
1. d-d transitions that are specific to the Co ion
2. Internal transitions corresponding to the equatorial corrin ligand
3. Internal transitions of the axial ligand
4. Charge-transfer transitions from the corrin ring to Co or vice versa
5. Charge-transfer transitions from an axial ligand to Co or vice versa.
As spectroscopic techniques and interpretation from simulations have progressed,
the traditional assignment of peaks in Abs spectra of cobalamins has been further
refined. It was once generally accepted that Abs spectra of cobalamins can be interpreted based on the electronic transitions associated entirely with the corrin ring.