6 research outputs found
Spectroscopic Study of Firefly Oxyluciferin in an Enzymatic Environment on the Basis of Stability Monitoring
To understand the influence of the
enzyme microenvironment on the
properties of the emitter oxyluciferin (OL) in firefly bioluminescence,
we investigated the spectroscopic characteristics of OL in a complex
with the enzyme luciferase formed in a consumed reaction mixture.
By monitoring the in situ absorption spectra, we analyzed the enzymatic
synthesis and the stability of OL in luciferase environment. The absorption
spectra of OL in <i>Photinus pyralis</i> luciferase showed
that the dominant form was neutral OL, probably the enol form, which
emitted blue fluorescence (âŒ450 nm). A monoanionic OL emitting
green fluorescence (âŒ560 nm) exhibited a weak pH-dependent
equilibrium with the neutral enol-OL. The red-emitting form of OL
was almost completely absent from the consumed reaction mixture. The
peak wavelengths of the green and red emissions of the fluorescence
and bioluminescence were similar, but the peak intensities, and hence
the spectral shapes, differed greatly. The above characteristics were
also found in the absorption and fluorescence spectra of OL in a complex
with the H433Y mutant of <i>Luciola cruciata</i> luciferase,
which catalyzes pH-independent red bioluminescence. Optical excitation
could not reproduce the excited states of bioluminescence that was
generated from the chemical reaction. The probable reason is that
the chemical excited states formed from a keto-like transition state
after decomposition of a dioxetanone intermediate, whereas the optical
excited states were generated by exciting the neutral enol-OL. Different
luciferases only influenced the chemical transition state during the
bioluminescence reaction; they did not influence the ground states
or optical excited states after the reaction
Reverse Stability of Oxyluciferin Isomers in Aqueous Solutions
We investigated the stability of
oxyluciferin anions (keto, enol,
and enolate isomers) in aqueous solution at room temperature by performing
a nanosecond time scale first-principles molecular dynamics simulation.
In contrast to all previous quantum chemistry calculations, which
suggested the keto-type to be the most stable, we show that the enol-type
is slightly more stable than the keto-type, in agreement with some
recent experimental studies. The simulation highlights the remarkable
hydrophobicity of the keto-type by the cavity formed at the oxyluciferinâwater
interface as well as a reduction in hydrophobicity with the number
of hydrating water molecules. It is therefore predicted that the isomeric
form in a hydrated cluster is size-dependent
Reverse Stability of Oxyluciferin Isomers in Aqueous Solutions
We investigated the stability of
oxyluciferin anions (keto, enol,
and enolate isomers) in aqueous solution at room temperature by performing
a nanosecond time scale first-principles molecular dynamics simulation.
In contrast to all previous quantum chemistry calculations, which
suggested the keto-type to be the most stable, we show that the enol-type
is slightly more stable than the keto-type, in agreement with some
recent experimental studies. The simulation highlights the remarkable
hydrophobicity of the keto-type by the cavity formed at the oxyluciferinâwater
interface as well as a reduction in hydrophobicity with the number
of hydrating water molecules. It is therefore predicted that the isomeric
form in a hydrated cluster is size-dependent
First-Principles Investigation of Strong Excitonic Effects in Oxygen 1s Xâray Absorption Spectra
We
calculated the oxygen 1s X-ray absorption spectra (XAS) of acetone
and acetic acid molecules in vacuum by utilizing the first-principles <i>GW</i>+BetheâSalpeter method with an all-electron mixed
basis. The calculated excitation energies show good agreement with
the available experimental data without an artificial shift. The remaining
error, which is less than 1% or 2â5 eV, is a significant improvement
from those of time-dependent (TD) density functional methods (5% error
or 27â29 eV for TD-LDA and 2.4â2.8% error or 13â15
eV for TD-B3LYP). Our method reproduces the first and second isolated
peaks and broad peaks at higher photon energies, corresponding to
Rydberg excitations. We observed a failure of the one-particle picture
(or independent particle approximation) from our assignment of the
five lowest exciton peaks and significant excitonic or state-hybridization
effects inherent in the core electron excitations
âVisibleâ 5d Orbital States in a Pleochroic Oxychloride
Transition
metal compounds sometimes exhibit attractive colors. Here, we report
a new oxychloride, Ca<sub>3</sub>ReO<sub>5</sub>Cl<sub>2</sub>, that
shows unusually distinct pleochroism; that is, the material exhibits
different colors depending on the viewing direction. This pleochroism
is a consequence of the coincidental complex crystal field splitting
of the 5d orbitals of the Re<sup>6+</sup> ion in a square-pyramidal
coordination of low symmetry in the energy range of the visible spectrum.
Since the relevant dâd transitions show characteristic polarization
dependence according to the optical selection rule, the orbital states
are âvisibleâ in Ca<sub>3</sub>ReO<sub>5</sub>Cl<sub>2</sub>
Atomically Engineered MetalâInsulator Transition at the TiO<sub>2</sub>/LaAlO<sub>3</sub> Heterointerface
We demonstrate that the atomic boundary
conditions of
simple binary
oxides can be used to impart dramatic changes of state. By changing
the substrate surface termination of LaAlO<sub>3</sub> (001) from
AlO<sub>2</sub> to LaO, the room-temperature sheet conductance of
anatase TiO<sub>2</sub> films are increased by over 3 orders of magnitude,
transforming the intrinsic insulating state to a high mobility metallic
state, while maintaining excellent optical transparency