2 research outputs found
On the Effect of a Single Solvent Molecule on the Charge-Transfer Band of a Donor–Acceptor Anion
Many biochromophore anions located
within protein pockets display
charge-transfer (CT) transitions that are perturbed by the nearby
environment, such as water or amino acid residues. These anions often
contain the phenolate moiety as the electron donor and an acceptor
group that couples to the donor via a π-conjugated system. Here
we show using action spectroscopy that single molecules of water,
methanol, and acetonitrile cause blue shifts in the electronic transition
energy of the bare <i>m</i>-nitrophenolate anion by 0.22,
0.22, and 0.12 eV, respectively (uncertainty of 0.05 eV). These shifts
are similar to CC2-predicted ones and are in accordance with the weaker
binding to the phenolate end of the ion by acetonitrile in comparison
with water and methanol. The nitro acceptor group is almost decoupled
from the phenolate donor, and this ion therefore represents a good
model for CT excitations of an anion. We found that the shift caused
by one acetonitrile molecule is almost half of that experienced in
bulk acetonitrile solution, clearly emphasizing the important role
played by the microenvironment. In protic solvents, the shifts are
larger because of hydrogen bonds to the phenolate oxygen. Finally,
but not least, we provide experimental data that serve to benchmark
calculations of excited states of ion–solvent complexes
On the Influence of Water on the Electronic Structure of Firefly Oxyluciferin Anions from Absorption Spectroscopy of Bare and Monohydrated Ions in Vacuo
A complete
understanding of the physics underlying the varied colors
of firefly bioluminescence remains elusive because it is difficult
to disentangle different enzyme–lumophore interactions. Experiments
on isolated ions are useful to establish a proper reference when there
are no microenvironmental perturbations. Here, we use action spectroscopy
to compare the absorption by the firefly oxyluciferin lumophore isolated
in vacuo and complexed with a single water molecule. While the process
relevant to bioluminescence within the luciferase cavity is light
emission, the absorption data presented here provide a unique insight
into how the electronic states of oxyluciferin are altered by microenvironmental
perturbations. For the bare ion we observe broad absorption with a
maximum at 548 ± 10 nm, and addition of a water molecule is found
to blue-shift the absorption by approximately 50 nm (0.23 eV). Test
calculations at various levels of theory uniformly predict a blue-shift
in absorption caused by a single water molecule, but are only qualitatively
in agreement with experiment highlighting limitations in what can
be expected from methods commonly used in studies on oxyluciferin.
Combined molecular dynamics simulations and time-dependent density
functional theory calculations closely reproduce the broad experimental
peaks and also indicate that the preferred binding site for the water
molecule is the phenolate oxygen of the anion. Predicting the effects
of microenvironmental interactions on the electronic structure of
the oxyluciferin anion with high accuracy is a nontrivial task for
theory, and our experimental results therefore serve as important
benchmarks for future calculations