Assessing Electronically Excited States of Cobalamins …
247
π /d→d character. For BP86, these transitions are d/π →π
∗ and d→π
∗ character.
BP86 indicates more transitions in the α/β region than does B3LYP. LRCT at low
energies occurs more noticeably when BP86 is employed, but the use of a solvent
model eliminates these. Overall, both functionals pick up on the shift of the lowest
intense transition to higher energies that occurs when the Im is exchanged for water
in low pH. Again, it is apparent the assessment of the electronically excited states of
cobalamins is not completely straightforward but requires careful interpretation of
TD-DFT results.
In TD-DFT studies of the photodynamics of AdoCbl, which can be experimentally probed through TAS, a simplified model of AdoCbl named ribosylcobalamin
(RibCbl) was used [50]. A major concern of theoretical studies of the photocleavage
of the Co–C bond in AdoCbl is predicting the BDE properly. As even truncated
models of AdoCbl are computationally demanding, it was important to consider if
a RibCbl model could provide a reliable representation of AdoCbl. Comparisons
between RibCbl simulated Abs spectrum and the experimental AdoCbl Abs spectrum offered a good opportunity to investigate the relevance of this simple structural
model to excited-state descriptions. Later studies of the photodissociation of AdoCbl
would involve the full Ado ligand [22]. The simulated Abs spectra of RibCbl calculated with TD-DFT/BP86 in both the gas phase and in the water PCM are pertinent
to discuss in this chapter [50]. As was pointed out in the study, the simulated Abs
spectrum for RibCbl cannot be directly compared to the experimental Abs spectrum
of AdoCbl, but important correlations can be noted. First, the simulated Abs spectrum of AdoCbl discussed above and the RibCbl spectrum reasonably agree. So, it
would seem that the RibCbl model can provide information related to the experimental spectroscopic features of AdoCbl. For the gas-phase simulated spectrum, the
assignments are as follows. The α/β bands primarily involved d/π →π
∗ transitions
with the LUMO being a pure corrin π
∗ orbital. The D/E and γ bands resulted from
several transitions with contributions from the ribose group playing a significant role.
The energy of the γ band is the same in gas and in water. The α/β band for the RibCbl
simulated spectrum in water is narrower compared to the gas-phase simulation.
9.5 Antivitamins B 12
Antivitamins B 12 are cobalamin derivatives that counteract the physiological effects
of their cobalamin analogues. They have received attention in recent years due
to their potential applications in the medical field [90, 96]. Two antivitamins
B 12 , alkynylcobalamin phenylethynylcobalamin (PhEtyCbl) and arylcobalamin
4-ethylphenylcobalamin (EtPhCbl), have been developed [72, 73]. These strongly
resemble the structures of the biochemically relevant derivatives while being metabolically inert themselves. These maintain all the structural features of cobalamins with
the only difference in the nature of the upper axial ligand. Like the other well-studied
vitamin B 12 derivatives, the Abs spectra of EtPhCbl and PhEtyCbl have been collected and the photodynamics have been assessed with TAS [60].
247
π /d→d character. For BP86, these transitions are d/π →π
∗ and d→π
∗ character.
BP86 indicates more transitions in the α/β region than does B3LYP. LRCT at low
energies occurs more noticeably when BP86 is employed, but the use of a solvent
model eliminates these. Overall, both functionals pick up on the shift of the lowest
intense transition to higher energies that occurs when the Im is exchanged for water
in low pH. Again, it is apparent the assessment of the electronically excited states of
cobalamins is not completely straightforward but requires careful interpretation of
TD-DFT results.
In TD-DFT studies of the photodynamics of AdoCbl, which can be experimentally probed through TAS, a simplified model of AdoCbl named ribosylcobalamin
(RibCbl) was used [50]. A major concern of theoretical studies of the photocleavage
of the Co–C bond in AdoCbl is predicting the BDE properly. As even truncated
models of AdoCbl are computationally demanding, it was important to consider if
a RibCbl model could provide a reliable representation of AdoCbl. Comparisons
between RibCbl simulated Abs spectrum and the experimental AdoCbl Abs spectrum offered a good opportunity to investigate the relevance of this simple structural
model to excited-state descriptions. Later studies of the photodissociation of AdoCbl
would involve the full Ado ligand [22]. The simulated Abs spectra of RibCbl calculated with TD-DFT/BP86 in both the gas phase and in the water PCM are pertinent
to discuss in this chapter [50]. As was pointed out in the study, the simulated Abs
spectrum for RibCbl cannot be directly compared to the experimental Abs spectrum
of AdoCbl, but important correlations can be noted. First, the simulated Abs spectrum of AdoCbl discussed above and the RibCbl spectrum reasonably agree. So, it
would seem that the RibCbl model can provide information related to the experimental spectroscopic features of AdoCbl. For the gas-phase simulated spectrum, the
assignments are as follows. The α/β bands primarily involved d/π →π
∗ transitions
with the LUMO being a pure corrin π
∗ orbital. The D/E and γ bands resulted from
several transitions with contributions from the ribose group playing a significant role.
The energy of the γ band is the same in gas and in water. The α/β band for the RibCbl
simulated spectrum in water is narrower compared to the gas-phase simulation.
9.5 Antivitamins B 12
Antivitamins B 12 are cobalamin derivatives that counteract the physiological effects
of their cobalamin analogues. They have received attention in recent years due
to their potential applications in the medical field [90, 96]. Two antivitamins
B 12 , alkynylcobalamin phenylethynylcobalamin (PhEtyCbl) and arylcobalamin
4-ethylphenylcobalamin (EtPhCbl), have been developed [72, 73]. These strongly
resemble the structures of the biochemically relevant derivatives while being metabolically inert themselves. These maintain all the structural features of cobalamins with
the only difference in the nature of the upper axial ligand. Like the other well-studied
vitamin B 12 derivatives, the Abs spectra of EtPhCbl and PhEtyCbl have been collected and the photodynamics have been assessed with TAS [60].