Circular dichroism of anthocyanidin 3-glucoside self-aggregates

“NOTICE: this is the author’s version of a work that was accepted for publication in Phytochemistry. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Phytochemistry Volume 88, April 2013, Pages 92–98. DOI 10.1016/j.phytochem.2012.12.011 ."“NOTICE: this is the author’s version of a work that was accepted for publication in Phytochemistry. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Phytochemistry Volume 88, April 2013, Pages 92–98. DOI 10.1016/j.phytochem.2012.12.011 .""Self-association constants for the flavylium cations of the six most common anthocyanidin 3-glucosides were determined by circular dichroism (CD) and UV–Vis spectroscopy. Along with previous 1H NMR results, all measurements were consistent with a monomer–dimer model. The CD spectra of the antho-cyanidin 3-glucosides were similar to the analogues 3,5-diglucosides. All dimers of the anthocyanidin 3-glucosides exhibited left-handed CD signals, with petunidin-3-glucoside and myrtillin having the most intense signals. In addition, the magnitude of the molar ellipticity, [h], was generally higher for the 3-glucosides than for the 3,5-diglucosides. For all six anthocyanins studied, the CD absorption spectra of their dimers showed evidence of the splitting of the monomer absorption into lower (J) and higher (H) energy bands. The angle and the distance between the dipolar moments of the two monomers comprising the dimer were obtained from the lower energy absorption band. While the angle was more or less similar in all six dimers, the separation distance between the monomer dipole moments differed dramatically. The intensity of the CD signal displayed a linear dependence with the inverse square of the dipole moment distances.

Achieving Complexity at the Bottom: Molecular Metamorphosis Generated by Anthocyanins and Related Compounds

Funding Information: This work was supported by the Associated Laboratory for Sustainable Chemistry, Clean Processes and Technologies LAQV through the national funds from UIDB/50006/2020 and UIDP/50006/2020 as well as the European Regional Development Fund within the Operational Programme “Science and Education for Smart Growth 2014–2020” under the Project CoE “National Center of Mechatronics and Clean Technologies” (BG05M2OP001-1.001-0008). N.B. is grateful to FCT for the contract CEECIND/00466/2017, D.S. for the doctoral grant (SFRH/BD/143369/2019), and L.C. for the research contract DL 57/2016/CP1334/CT0008. Publisher Copyright: © 2021 The Authors. Published by American Chemical Society.The concept of molecular metamorphosis is described. A molecule (flavylium cation) generates a sequence of other different molecules by means of external stimuli. The reversibility of the system allows for the flavylium cation to be recovered by other external stimuli, completing one cycle. Differently from supramolecular chemistry, molecular metamorphosis is not a bottom-up approach. All events occur at the bottom. The procedures to characterize the kinetics and thermodynamics of the cycles are summarized. They are based on direct pH jumps (addition of a base to the flavylium cation) and reverse pH jumps (addition of an acid to equilibrated solutions at higher pH values). Stopped flow is an indispensable tool to characterize these systems. The following metamorphic cycles will be described to illustrate the concept: (i) introducing the flavanone in the metamorphic system and illustrating the concept of a timer at the molecular level; (ii) response of the flavylium-based metamorphosis to light inputs and the write-lock-read-unlock-erase molecular system; (iii) a one-way cycle of direct-reverse pH jumps; (iv) interconversion of the flavylium cation with 2,2′-spirobis[chromene] derivatives; (v) 6,8 A-ring substituent rearrangements.publishersversionpublishe

Impacts of hydroxylation on the photophysics of chalcones: Insights into the relation between the chemical composition and the electronic structure

A combined theoretical/experimental study of the photoreactivity of two flavylium-derived chalcones, 2,4,4\u2032-trihydroxychalcone and 2,4\u2032-dihydroxychalcone, at the multiconfigurational wavefunction level of theory (CASSCF//CASPT2) in vacuo and in an implicit solvent (water, treated as a polarisable continuum) and by means of linear absorption spectroscopy is presented. The photosensitivity of flavium salts is expressed in the ability of their chalcone form to undergo a cis-trans isomerisation which has found application in logical networks. Despite a considerable amount of experimental data documenting the dependence of the isomerisation on solvent, pH and temperature, the knowledge of how chalcones process energy under various conditions at the molecular level is still scarce. On the example of 2,4,4\u2032-trihydroxychalcone we unravel the complex excited state deactivation mechanism in vacuo involving ultrafast decay through conical intersections, formation of twisted intramolecular charge transfer species, intramolecular proton transfer and inter system crossings. Furthermore, we rationalise the observed discrepancies in the linear absorption spectra of 2,4,4\u2032-trihydroxychalcone and 2,4\u2032-dihydroxychalcone, thereby establishing a link between the functionalisation pattern and the observed spectral properties

Luminescence of Binary-Doped Silica Aerogel Powders: A Two-Step Sol-Gel Approach

In this study, we report a novel synthesis of hydrophobic silica aerogel powder composites, functionalized and binary-doped with [Tb(phen)2](NO3)3 and [Eu(phen)2](NO3)3 nanocrystals, employing a two-step sol-gel methodology. The investigation delves into the structural elucidation, optical properties and thermal conductivity of these functionalized Tb(III)-Eu(III) composites. Our analysis includes diffuse reflectance spectra and excitation and luminescence spectra, highlighting the quantum yields of composites with varying chemical compositions. Remarkably, these samples exhibit a strong luminescence, with distinct hues of red or green based on the specific doping type and level. The detailed examination of excitation spectra and quantum yields establishes robust energy-transfer mechanisms from the 1,10-phenanthroline molecule to the lanthanide ions. Notably, our study uncovers a Tb3⁺→Eu3⁺ energy-transfer phenomenon within the binary functionalized samples, providing compelling evidence for a structural formation process occurring within the mesoporous framework of the aerogel powders

Isomerization between 2-(2,4-Dihydroxystyryl)-1-benzopyrylium and 7-Hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium

2-Phenyl-1-benzopyrylium (flavylium) and 2-styryl-1-benzopyrylium (styrylflavylium) cations establish in aqueous solution a series of equilibria defining chemical reaction networks responsive to several stimuli (pH, light, redox potential). Control over the mole fraction distribution of species by applying the appropiate stimuli defines a horizontal approach to supramolecular chemistry, in agreement with the customary bottom-up approach toward complex systems. In this work, we designed an asymmetric styrylchalcone able to cyclize in two different ways, producing two isomeric styrylflavylium cations whose chemical reaction networks are thus interconnected. The chemical reaction networks of 2-(2,4-dihydroxystyryl)-1-benzopyrylium (AH⁺) and 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium (AH⁺_iso) comprise the usual species observed in flavylium-derived networks, in this case, the styryl derivatives of quinoidal bases, hemiketals, and chalcones. The thermodynamics and kinetics of the crossed networks were characterized by the use of UV–vis absorption and NMR spectroscopy as well as time-resolved pH jumps followed by stopped-flow. The two styrylflavylium cations are connected (isomerize) through two alternative intermediates, the asymmetric <i>trans</i>-styrylchalcone (Ct) and a spiropyran-type intermediate (SP). At pH = 1, AH⁺ slowly evolves (<i>k</i>_obs ≈ 10^–5 s^–1) to a mixture containing 62% AH⁺_iso through the Ct intermediate, while at pH = 5, the SP intermediate is involved. The observed rate constants for the conversion of the styrylflavylim cations into equilibrium mixtures containing essentially Ct follow a pH-dependent bell-shaped curve in both networks. While at pH = 1 in the dark, AH⁺ evolves to an equilibrium mixture containing predominantly AH⁺_iso, irradiation at λ > 435 nm induces the opposite conversion

Emptying the β‑Cyclodextrin Cavity by Light: Photochemical Removal of the <i>trans</i>-Chalcone of 4′,7-Dihydroxyflavylium

The interaction between the network of chemical reactions of the compound 4′,7-dihydroxyflavylium and β-cyclodextrin was studied by means of pH jumps, followed by UV–vis absorption, flash photolysis, stopped flow, and NMR. The <i>trans</i>-chalcone is the network species exhibiting the strongest interaction with the host. In moderately acidic medium, 95% of the <i>trans</i>-chalcone, 2.5 × 10^–5 M, in the presence of β-cyclodextrin, 9 × 10^–3 M, is expected to fill the host cavity (association constant 2.2 × 10³ M^–1). In contrast, flavylium cation does not interact (association constant ≈ 0). Irradiation of the <i>trans</i>-chalcone in the presence of β-cyclodextrin 9 mM leads to the flavylium cation appearance. Light is thus capable of removing the <i>trans</i>-chalcone from the β-cyclodextrin, leaving the cavity empty. The system is reversible and <i>trans</i>-chalcone goes back to the initial state upon switching off the light due to the thermodynamic favorable conversion of flavylium cation to <i>trans</i>-chalcone in the presence of β-cyclodextrin