Fine Tuning of Retinal Photoinduced Decay in Solution

Single methylation at position C₁₀ 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 “catalyzes” the photoinduced decay of solvated retinals. Methylation alters the interplay between the ionic S₁ and covalent S₂ states, reducing the excited-state lifetime by favoring the formation of a S₁ 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 S₁ state as well as the interaction with the covalent S₂ 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 X‑ray Spectroscopic Properties of Ruthenium Complex Catalyst under CO₂ Electrochemical Reduction Conditions: A First-Principles Study

Solar fuel production through photoelectrochemical reduction of CO₂ 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-CO₂ chemical bonding formation, two alternative pathways for catalytic CO₂ 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>_a’s and oxidation midpoint potentials (<i>E</i>_m’s) 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>_a’s of terminal water ligands in [Mn­(II/III)­(H₂O)₆]^2+/3+ (<b>1</b>), [Mn­(III)­(P)­(H₂O)₂]^3– (<b>2</b>, P = 5,10,15,20-tetrakis­(2,6-dichloro-3-sulfonatophenyl)­porphyrinato), [Mn₂(IV,IV)­(μ-O)₂(terpy)₂(H₂O)₂]⁴⁺ (<b>3</b>, terpy = 2,2′:6′,2″-terpyridine), and [Mn₃(IV,IV,IV)­(μ-O)₄(phen)₄(H₂O)₂]⁴⁺ (<b>4</b>, phen = 1,10-phenanthroline) and the p<i>K</i>_a’s of μ-oxo bridges and Mn <i>E</i>_m’s in [Mn₂(μ-O)₂(bpy)₄] (<b>5</b>, bpy = 2,2′-bipyridyl), [Mn₂(μ-O)₂(salpn)₂] (<b>6</b>, salpn = <i>N</i>,<i>N′</i>-bis­(salicylidene)-1,3-propanediamine), [Mn₂(μ-O)₂(3,5-di­(Cl)-salpn)₂] (<b>7</b>), and [Mn₂(μ-O)₂(3,5-di­(NO₂)-salpn)₂] (<b>8</b>). The analysis of complexes <b>6</b>–<b>8</b> highlights the strong coupling between electron and proton transfers, with any Mn oxidation lowering the p<i>K</i>_a of an oxo bridge by 10.5 ± 0.9 pH units. The model also accounts for changes in the <i>E</i>_m’s by ligand substituents, such as found in complexes <b>6</b>–<b>8</b>, due to the electron withdrawing Cl (<b>7</b>) and NO₂ (<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 (UV–vis) and theoretical [DFT-QM/MM (SORCI+Q//B3LYP/6-31G­(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 “inter-helical lock” (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