4 research outputs found
Laser Spectroscopic Study of β‑Estradiol and Its Monohydrated Clusters in a Supersonic Jet
The structures of 17β-estradiol (estradiol) and
its 1:1 cluster
with water have been investigated in supersonic jets. The S<sub>1</sub>–S<sub>0</sub> electronic spectrum of estradiol monomer shows
four strong sharp bands in the 35050–35200 cm<sup>–1</sup> region. Ultraviolet–ultraviolet hole-burning (UV–UV
HB) and infrared-ultraviolet double-resonance (IR-UV DR) spectra of
these bands indicate that they are due to four different conformers
of estradiol originating from the different orientation of the OH
groups in the <i>A</i>- and <i>D</i>-rings. The
addition of water vapor to the sample gas generates four new bands
in the 34700–34800 cm<sup>–1</sup> region, which are
assigned to the estradiol–H<sub>2</sub>O 1:1 cluster with the <i>A</i>-ring (phenyl ring) OH acting as a hydrogenÂ(H)-bond donor.
In addition, we found very weak bands near the origin bands of bare
estradiol upon the addition of water vapor. These bands are assigned
to the isomers of estradiol–H<sub>2</sub>O 1:1 cluster having
an H-bond at the <i>D</i>-ring OH. We determine the conformation
of bare estradiol and the structures of its monohydrated clusters
with the aid of density functional theory calculation and discuss
the relationship between the stability of hydrated clusters and the
conformation of estradiol
Structure and Hydrogen-Bonding Ability of Estrogens Studied in the Gas Phase
The structures of estrogens (estroneÂ(E1),
β-estradiolÂ(E2), and estriolÂ(E3)) and their
1:1 hydrogen-bonded (hydrated) clusters with water formed in supersonic
jets have been investigated by various laser spectroscopic methods
and quantum chemical calculations. In the S<sub>1</sub>–S<sub>0</sub> electronic spectra, all three species exhibit the band origin
in the 35 050–35 200 cm<sup>–1</sup> region.
By use of ultraviolet–ultraviolet hole-burning (UV–UV
HB) spectroscopy, two conformers, four conformers, and eight conformers,
arising from different orientation of OH group(s) in the A-ring and
D-ring, are identified for estrone, β-estradiol, and estriol,
respectively. The infrared–ultraviolet double-resonance (IR–UV
DR) spectra in the OH stretching vibration are observed to discriminate
different conformers of the D-ring OH for β-estradiol and estriol,
and it is suggested that in estriol only the intramolecular hydrogen
bonded conformer exists in the jet. For the 1:1 hydrated cluster of
estrogens, the S<sub>1</sub>–S<sub>0</sub> electronic transition
energies are quite different depending on whether the water molecule
is bound to A-ring OH or D-ring OH. It is found that the water molecule
prefers to form an H-bond to the A-ring OH for estrone and β-estradiol
due to the higher acidity of phenolic OH than that of the alcoholic
OH. On the other hand, in estriol the water molecule prefers to be
bound to the D-ring OH due to the formation of a stable ring-structure
H-bonding network with two OH groups. Thus, the substitution of one
hydroxyl group to the D-ring drastically changes the hydrogen-bonding
preference of estrogens
Anomalous Cage Effect of the Excited State Dynamics of Catechol in the 18-Crown-6–Catechol Host–Guest Complex
We
determined the number of isomers and their structures for the 18-crown-6
(18C6)–catechol host–guest complex, and examined the
effect of the complex formation on the S<sub>1</sub> (<sup>1</sup>ππ*) dynamics of catechol under a supersonically cooled
gas phase condition and in cyclohexane solution at room temperature.
In the gas phase experiment, UV–UV hole-burning spectra of
the 18C6–catechol 1:1 complex indicate that there are three
stable isomers. For bare catechol, it has been reported that two adjacent
OH groups have an intramolecular hydrogen (H) bond. The IR–UV
double resonance spectra show two types of isomers in the 18C6–catechol
1:1 complex; one of the three 18C6–catechol 1:1 isomers has
the intramolecular H-bond between the two OH groups, while in the
other two isomers the intramolecular H-bond is broken and the two
OH groups are H-bonded to oxygen atoms of 18C6. The complex formation
with 18C6 substantially elongates the S<sub>1</sub> lifetime from
7 ps for bare catechol and 2.0 ns for the catechol–H<sub>2</sub>O complex to 10.3 ns for the 18C6–catechol 1:1 complex. Density
functional theory calculations of the 18C6–catechol 1:1 complex
suggest that this elongation is attributed to a larger energy gap
between the S<sub>1</sub> (<sup>1</sup>ππ*) and <sup>1</sup>πσ* states than that of bare catechol or the catechol–H<sub>2</sub>O complex. In cyclohexane solution, the enhancement of the
fluorescence intensity of catechol was found by adding 18C6, due to
the formation of the 18C6–catechol complex in solution, and
the complex has a longer S<sub>1</sub> lifetime than that of catechol
monomer. From the concentration dependence of the fluorescence intensity,
we estimated the equilibrium constant <i>K</i> for the 18C6
+ catechol ⇄ 18C6–catechol reaction. The obtained value
(log <i>K</i> = 2.3) in cyclohexane is comparable to those
for alkali metal ions or other molecular ions, indicating that 18C6
efficiently captures catechol in solution. Therefore, 18C6 can be
used as a sensitive sensor of catechol derivatives in solution with
its high ability of fluorescence enhancement
Ultraviolet Photodissociation Spectroscopy of the Cold K<sup>+</sup>·Calix[4]arene Complex in the Gas Phase
The
cooling of ionic species in the gas phase greatly simplifies
the UV spectrum, which is of special importance when studying the
electronic and geometric structures of large systems, such as biorelated
molecules and host–guest complexes. Many efforts have been
devoted to achieving ion cooling with a cold, quadrupole Paul ion
trap (QIT), but one problem was the insufficient cooling of ions (up
to ∼30 K) in the QIT. In this study, we construct a mass spectrometer
for the ultraviolet photodissociation (UVPD) spectroscopy of gas-phase
cold ions. The instrument consists of an electrospray ion source,
a QIT cooled with a He cryostat, and a time-of-flight mass spectrometer.
With great care given to the cooling condition, we can achieve ∼10
K for the vibrational temperature of ions in the QIT, which is estimated
from UVPD spectra of the benzo-18-crown-6 (B18C6) complex with a potassium
ion, K<sup>+</sup>·B18C6. Using this setup, we measure a UVPD
spectrum of cold calix[4]Âarene (C4A) complex with potassium ion, K<sup>+</sup>·C4A. The spectrum shows a very weak band and a strong
one at 36018 and 36156 cm<sup>–1</sup>, respectively, accompanied
by many sharp vibronic bands in the 36000–36600 cm<sup>–1</sup> region. In the geometry optimization of the K<sup>+</sup>·C4A
complex, we obtain three stable isomers: one endo and two exo forms.
On the basis of the total energy and UV spectral patterns predicted
by density functional theory calculations, we attribute the structure
of the K<sup>+</sup>·C4A complex to the endo isomer (<i>C</i><sub>2</sub> symmetry), in which the K<sup>+</sup> ion
is located inside the cup of C4A. The vibronic bands of K<sup>+</sup>·C4A at 36 018 and 36 156 cm<sup>–1</sup> are assigned to the S<sub>1</sub>(A)–S<sub>0</sub>(A) and
S<sub>2</sub>(B)–S<sub>0</sub>(A) transitions of the endo isomer,
respectively