5 research outputs found
Fine Tuning of Retinal Photoinduced Decay in Solution
Single methylation
at position C<sub>10</sub> of the all-trans
retinal protonated Schiff base switches its excited-state decay in
methanol from a slower picosecond into an ultrafast, protein-like
subpicosecond process. QM/MM modeling in conjunction with on-the-fly
excited-state dynamics provides fundamental understanding of the fine-tuning
mechanics that ācatalyzesā the photoinduced decay of
solvated retinals. Methylation alters the interplay between the ionic
S<sub>1</sub> and covalent S<sub>2</sub> states, reducing the excited-state
lifetime by favoring the formation of a S<sub>1</sub> transient fluorescent
state with fully inverted bond lengths that accounts for the recorded
transient spectroscopy and from which a space-saving conical intersection
seam is quickly (<1 ps) reached. Minimal and apparently innocent
chemical modifications thus affect the characteristic intramolecular
charge-transfer of the S<sub>1</sub> state as well as the interaction
with the covalent S<sub>2</sub> excited state, eventually providing
the high tunability of retinal photophysics and photochemistry and
delivering a new concept for the rational design of retinal-based
photoactive molecular devices
Facile Ecofriendly Synthesis of Monastrol and Its Structural Isomers via Biginelli Reaction
In this paper, a Q-tube equipment is proposed as a valid alternative
to monomode MW technology to synthesize in a simple and economic way
a library of dihydropyrimidine derivatives performing the Biginelli
reaction in solvent-free conditions under pressure and with the catalysis
of a very mild and environmentally benign Lewis acid consisting of
erbium trichloride hexahydrate
Soft Xāray Spectroscopic Properties of Ruthenium Complex Catalyst under CO<sub>2</sub> Electrochemical Reduction Conditions: A First-Principles Study
Solar fuel production through photoelectrochemical
reduction of
CO<sub>2</sub> is a promising route to popularize the use of solar
energy. However, the underlying mechanisms of these complex reactions
are not yet fully resolved, hindering the rational design of novel
photoelectrocatalysts. To shed light on this challenging problem,
the X-ray photoelectron spectroscopy (XPS) and the near edge X-ray
absorption fine structure (NEXAFS) of a number of molecular systems
have been calculated using first-principles theory. First, it was
found that both XPS and NEXAFS display specific features that correlate
with the complex charge state and the coordination number of Ru atom.
Furthermore, through the analysis of C 1s and N 1s XPS and NEXAFS
spectra of key intermediates, we have identified clear fingerprints
for metal-hydride and Ru-CO<sub>2</sub> chemical bonding formation,
two alternative pathways for catalytic CO<sub>2</sub> reduction. These
results indicate that the understanding of the electrochemical properties
of the electrocatalyst, as well as the reaction pathways, could be
significantly advanced through <i>operando</i> X-ray spectroscopy
experiments based on synchrotron radiation. We expect that these theoretical
findings will be the basis of and motivate future experimental initiatives
Electrostatic Effects on Proton Coupled Electron Transfer in Oxomanganese Complexes Inspired by the Oxygen-Evolving Complex of Photosystem II
The influence of electrostatic interactions on the free energy
of proton coupled electron transfer in biomimetic oxomanganese complexes
inspired by the oxygen-evolving complex (OEC) of photosystem II (PSII)
are investigated. The reported study introduces an enhanced multiconformer
continuum electrostatics (MCCE) model, parametrized at the density
functional theory (DFT) level with a classical valence model for the
oxomanganese core. The calculated p<i>K</i><sub>a</sub>ās
and oxidation midpoint potentials (<i>E</i><sub>m</sub>ās)
match experimental values for eight complexes, indicating that purely
electrostatic contributions account for most of the observed couplings
between deprotonation and oxidation state transitions. We focus on
p<i>K</i><sub>a</sub>ās of terminal water ligands
in [MnĀ(II/III)Ā(H<sub>2</sub>O)<sub>6</sub>]<sup>2+/3+</sup> (<b>1</b>), [MnĀ(III)Ā(P)Ā(H<sub>2</sub>O)<sub>2</sub>]<sup>3ā</sup> (<b>2</b>, P = 5,10,15,20-tetrakisĀ(2,6-dichloro-3-sulfonatophenyl)Āporphyrinato),
[Mn<sub>2</sub>(IV,IV)Ā(Ī¼-O)<sub>2</sub>(terpy)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>4+</sup> (<b>3</b>, terpy = 2,2ā²:6ā²,2ā³-terpyridine),
and [Mn<sub>3</sub>(IV,IV,IV)Ā(Ī¼-O)<sub>4</sub>(phen)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>4+</sup> (<b>4</b>, phen
= 1,10-phenanthroline) and the p<i>K</i><sub>a</sub>ās
of Ī¼-oxo bridges and Mn <i>E</i><sub>m</sub>ās
in [Mn<sub>2</sub>(Ī¼-O)<sub>2</sub>(bpy)<sub>4</sub>] (<b>5</b>, bpy = 2,2ā²-bipyridyl), [Mn<sub>2</sub>(Ī¼-O)<sub>2</sub>(salpn)<sub>2</sub>] (<b>6</b>, salpn = <i>N</i>,<i>Nā²</i>-bisĀ(salicylidene)-1,3-propanediamine),
[Mn<sub>2</sub>(Ī¼-O)<sub>2</sub>(3,5-diĀ(Cl)-salpn)<sub>2</sub>] (<b>7</b>), and [Mn<sub>2</sub>(Ī¼-O)<sub>2</sub>(3,5-diĀ(NO<sub>2</sub>)-salpn)<sub>2</sub>] (<b>8</b>). The analysis of complexes <b>6</b>ā<b>8</b> highlights the strong coupling between
electron and proton transfers, with any Mn oxidation lowering the
p<i>K</i><sub>a</sub> of an oxo bridge by 10.5 Ā± 0.9
pH units. The model also accounts for changes in the <i>E</i><sub>m</sub>ās by ligand substituents, such as found in complexes <b>6</b>ā<b>8</b>, due to the electron withdrawing Cl
(<b>7</b>) and NO<sub>2</sub> (<b>8</b>). The reported
study provides the foundation for analysis of electrostatic effects
in other oxomanganese complexes and metalloenzymes, where proton coupled
electron transfer plays a fundamental role in redox-leveling mechanisms
Spectral Tuning of Ultraviolet Cone Pigments: An Interhelical Lock Mechanism
Ultraviolet
(UV) cone pigments can provide insights into the molecular
evolution of vertebrate vision since they are nearer to ancestral
pigments than the dim-light rod photoreceptor rhodopsin. While visible-absorbing
pigments contain an 11-<i>cis</i> retinyl chromophore with
a protonated Schiff-base (PSB11), UV pigments uniquely contain an
unprotonated Schiff-base (USB11). Upon F86Y mutation in model UV pigments,
both the USB11 and PSB11 forms of the chromophore are found to coexist
at physiological pH. The origin of this intriguing equilibrium remains
to be understood at the molecular level. Here, we address this phenomenon
and the role of the USB11 environment in spectral tuning by combining
mutagenesis studies with spectroscopic (UVāvis) and theoretical
[DFT-QM/MM (SORCI+Q//B3LYP/6-31GĀ(d): Amber96)] analysis. We compare
structural models of the wild-type (WT), F86Y, S90A and S90C mutants
of Siberian hamster ultraviolet (SHUV) cone pigment to explore structural
rearrangements that stabilize USB11 over PSB11. We find that the PSB11
forms upon F86Y mutation and is stabilized by an āinter-helical
lockā (IHL) established by hydrogen-bonding networks between
transmembrane (TM) helices TM6, TM2, and TM3 (including water w2c
and amino acid residues Y265, F86Y, G117, S118, A114, and E113). The
findings implicate the involvement of the IHL in constraining the
displacement of TM6, an essential component of the activation of rhodopsin,
in the spectral tuning of UV pigments