Assessing Electronically Excited States of Cobalamins …
227
This conclusion was based on studies of a FBC, i.e., a cobalamin containing no Co,
which will be discussed more thoroughly later.
As cobalamins have become more intensely scrutinized experimentally and computationally, a number of inconsistencies have been noticed. Consider MeCbl as an
example of cobalamin with an anomalous spectrum, with the α band of lesser intensity than the β band. Traditionally, the α/β band has been viewed as arising from
π →π
∗ excitations associated with vibrational progression of the C=C stretching of
the corrin ring. In other words, the α/β band has been interpreted as a single S 0 →S 1
electronic transition. TAS has indicated that the S 1 state in MeCbl results from metalto-ligand charge-transfer (MLCT) transitions. Wave function-based ab initio methods confer the MLCT description emphasizing importance of Co d orbitals [39]. As
shall be discussed later, hybrid functionals in the TD-DFT framework agree with the
earlier interpretation of an S 1 state arising from π →π
∗ transitions, while gradientcorrected pure functionals give a description consistent with the MLCT assignment
pointing out the importance of the Co d orbitals. This is just one example of how
recent developments in computations have led to a more complete understanding of
cobalamin spectroscopy, and this will be further noted in Sect. 7 where Abs spectra
will be discussed for several cobalamins.
Beyond the theoretical interpretations, there are several experimental considerations that have led to a deeper understanding of cobalamin Abs spectra. Various
parameters can affect the position of Abs bands including temperature, solvation
environment, axial ligation, and modifications to the corrin ring, and these can be
summarized as follows. A reduction in temperature leads to a sharpening of all the
bands. Cooling also leads to an increase in the intensity of the α band in comparison to
the β band. Increases in temperature lead to the displacement of the DBI base. There
is a correlation between the solvent polarity parameter and the absorption maxima of
the α/β band. More simply put, the intensity of the α/β band is affected by the solvation environment. Solvent also plays an important role in terms of time-resolved TAS.
Particularly, cage dynamics in photolytic processes are solvent-dependent, even for
cobalamins [94]. It has been shown that cage escape for RP occurs over various time
scales based on solvent fluidity and radical size [87]. The size of the upper axial ligand does not only have a determinate effect on cage escape time scale, but the nature
of this ligand also effects the static Abs spectrum. Inherently, there will be changes in
properties of cobalamins based on the σ -donor strength of the upper axial ligand. The
σ -donor strength is the amount of negative charge donated to the Co ion through the
Co-R ax bond. Differences in σ -donor strength affect Abs spectra. Shifts of the α band
to longer wavelengths should be proportional to the electron-donating ability of the
axial ligand. In addition, the wavelength of the γ band shifts with electronegativity
of the upper axial ligand [16]. The γ band will shift to shorter wavelengths with more
electronegative ligands like CN
− , whereas less electronegative ligands like CH 3 will
shift the γ band to longer wavelengths. This can be essentially simplified as follows:
The greater the σ -donor strength of the axial ligand, the longer the wavelength of
the γ band. Furthermore, Abs spectra are less altered when changes are made to the
side chains of the corrin macrocycle when compared to the changes noticed in axial
227
This conclusion was based on studies of a FBC, i.e., a cobalamin containing no Co,
which will be discussed more thoroughly later.
As cobalamins have become more intensely scrutinized experimentally and computationally, a number of inconsistencies have been noticed. Consider MeCbl as an
example of cobalamin with an anomalous spectrum, with the α band of lesser intensity than the β band. Traditionally, the α/β band has been viewed as arising from
π →π
∗ excitations associated with vibrational progression of the C=C stretching of
the corrin ring. In other words, the α/β band has been interpreted as a single S 0 →S 1
electronic transition. TAS has indicated that the S 1 state in MeCbl results from metalto-ligand charge-transfer (MLCT) transitions. Wave function-based ab initio methods confer the MLCT description emphasizing importance of Co d orbitals [39]. As
shall be discussed later, hybrid functionals in the TD-DFT framework agree with the
earlier interpretation of an S 1 state arising from π →π
∗ transitions, while gradientcorrected pure functionals give a description consistent with the MLCT assignment
pointing out the importance of the Co d orbitals. This is just one example of how
recent developments in computations have led to a more complete understanding of
cobalamin spectroscopy, and this will be further noted in Sect. 7 where Abs spectra
will be discussed for several cobalamins.
Beyond the theoretical interpretations, there are several experimental considerations that have led to a deeper understanding of cobalamin Abs spectra. Various
parameters can affect the position of Abs bands including temperature, solvation
environment, axial ligation, and modifications to the corrin ring, and these can be
summarized as follows. A reduction in temperature leads to a sharpening of all the
bands. Cooling also leads to an increase in the intensity of the α band in comparison to
the β band. Increases in temperature lead to the displacement of the DBI base. There
is a correlation between the solvent polarity parameter and the absorption maxima of
the α/β band. More simply put, the intensity of the α/β band is affected by the solvation environment. Solvent also plays an important role in terms of time-resolved TAS.
Particularly, cage dynamics in photolytic processes are solvent-dependent, even for
cobalamins [94]. It has been shown that cage escape for RP occurs over various time
scales based on solvent fluidity and radical size [87]. The size of the upper axial ligand does not only have a determinate effect on cage escape time scale, but the nature
of this ligand also effects the static Abs spectrum. Inherently, there will be changes in
properties of cobalamins based on the σ -donor strength of the upper axial ligand. The
σ -donor strength is the amount of negative charge donated to the Co ion through the
Co-R ax bond. Differences in σ -donor strength affect Abs spectra. Shifts of the α band
to longer wavelengths should be proportional to the electron-donating ability of the
axial ligand. In addition, the wavelength of the γ band shifts with electronegativity
of the upper axial ligand [16]. The γ band will shift to shorter wavelengths with more
electronegative ligands like CN
− , whereas less electronegative ligands like CH 3 will
shift the γ band to longer wavelengths. This can be essentially simplified as follows:
The greater the σ -donor strength of the axial ligand, the longer the wavelength of
the γ band. Furthermore, Abs spectra are less altered when changes are made to the
side chains of the corrin macrocycle when compared to the changes noticed in axial