Florent Di Meo

Free radical scavenging by natural polyphenols: atom versus electron transfer.

Polyphenols (synthetically modified or directly provided by human diet) scavenge free radicals by H-atom transfer and may thus decrease noxious effects due to oxidative stress. Free radical scavenging by polyphenols has been widely theoretically studied from the thermodynamic point of view whereas the kinetic point of view has been much less addressed. The present study describes kinetic-based structure-activity relationship for quercetin. This compound is very characteristic of the wide flavonoid subclass of polyphenols. H-atom transfer is a mechanism based on either atom or electron transfer. This is analyzed here by quantum chemical calculations, which support the knowledge acquired from experimental studies. The competition between the different processes is discussed in terms of the nature of the prereaction complexes, the pH, the formation of activated-deprotonated forms, and the atom- and electron-transfer efficiency. The role of the catechol moiety and the 3-OH group of quercetin as scavengers of different types of free radicals (CH3OO(•), CH3O(•), (•)OH, and (•)CH2OH) is rationalized. Identifying the exact mechanism and accurately evaluating kinetics is of fundamental importance to understand antioxidant behavior in physiological environments.

Highlights on Anthocyanin Pigmentation and Copigmentation: A Matter of Flavonoid π-Stacking Complexation To Be Described by DFT-D.

Anthocyanidins are a class of π-conjugated systems responsible for red, blue, and purple colors of plants. They exhibit the capacity of aggregation in the presence of other natural compounds including flavonols. Such complexations induce color modulation in plants, which is known as copigmentation. It is largely driven by π-interactions existing between pigments and copigments. In this work, the energies of copigmentation-complexation and self-association are systematically evaluated for an anthocyanidin/flavonol couple prototype (3-O-methylcyanidin/quercetin). To describe noncovalent interactions, DFT-D appears mandatory to reach a large accuracy. Due to the chemical complexity of this phenomenon, we also aim at assessing the relevance of both B3P86-D2 and ωB97X-D functionals. The benchmarking has shown that B3P86-D2 possesses enough accuracy when dealing with π-π interactions with respect to both spin component scaled Møller-Plesset second-order perturbation theory post Hartree-Fock method and experimental data. UV-vis absorption properties are then evaluated with time-dependent DFT for the different complexes. The use of range-separated hybrid functionals, such as ωB97X-D, helped to correctly disentangle and interpret the origin of the UV-vis experimental shifts attributed to the subtle copigmentation phenomenon.

NHC Copper(I) Complexes Bearing Dipyridylamine Ligands: Synthesis, Structural, and Photoluminescent Studies

We describe the synthesis of new cationic tricoordinated copper complexes bearing bidentate pyridine-type ligands and N-heterocyclic carbene as ancillary ligands. These cationic copper complexes were fully characterized by NMR, electrochemistry, X-ray analysis, and photophysical studies in different environments. Density functional theory calculations were also undertaken to rationalize the assignment of the electronic structure and the photophysical properties. These tricoordinated cationic copper complexes possess a stabilizing CH-π interaction leading to high stability in both solid and liquid states. In addition, these copper complexes, bearing dipyridylamine ligands having a central nitrogen atom as potential anchoring point, exhibit very interesting luminescent properties that render them potential candidates for organic light-emitting diode applications.

Antioxidant properties of phenolic Schiff bases: Structure-activity relationship and mechanism of action

AbstractPhenolic Schiff bases are known for their diverse biological activities and ability to scavenge free radicals. To elucidate (1) the structure–antioxidant activity relationship of a series of thirty synthetic derivatives of 2-methoxybezohydrazide phenolic Schiff bases and (2) to determine the major mechanism involved in free radical scavenging, we used density functional theory calculations (B3P86/6-31+(d,p)) within polarizable continuum model. The results showed the importance of the bond dissociation enthalpies (BDEs) related to the first and second (BDEd) hydrogen atom transfer (intrinsic parameters) for rationalizing the antioxidant activity. In addition to the number of OH groups, the presence of a bromine substituent plays an interesting role in modulating the antioxidant activity. Theoretical thermodynamic and kinetic studies demonstrated that the free radical scavenging by these Schiff bases mainly proceeds through proton-coupled electron transfer rather than sequential proton loss electron transfer, the latter mechanism being only feasible at relatively high pH.

Tuning color variation in grape anthocyanins at the molecular scale.

Anthocyanins are the main grape pigments. Due to their aromatic cyclic arrangements, they are able to absorb the radiation in the low energy range of the visible spectrum. In the fruit of Vitis vinifera L., the five main anthocyanidins (cyanidin, peonidin, delphinidin, petunidin and malvidin) are present as 3-O-glucosides, as well as their acetyl, p-coumaroyl and caffeoyl ester forms. Despite the huge number of experimental studies dedicated to the anthocyanin profile analysis of grapes and wines, the complete theoretical elucidation of the optical properties of grape anthocyanins is missing. The present work carried out this task through quantum chemistry calculations based on time-dependent density functional theory (TD-DFT), compared to experimental spectra. The differences in visible absorption spectra between the most common grape anthocyanins were rationalized according to B-ring substitution, glucosylation and esterification. A particular attention was given to the intra-molecular copigmentation effect, demonstrating the existence of an intra-molecular charge transfer excited state for the p-coumaroyl and caffeoyl ester forms.

Photoprotective capacities of lichen metabolites: A joint theoretical and experimental study

The adaptative capacity of lichens to UV radiation could be expressed by the production of photo-absorbing secondary metabolites in thalli. A preliminary screening performed on twelve lichen species by high performance thin layer chromatography (HPTLC) revealed five major compounds absorbing in the UVA (315-400nm) or UVB (280-315nm) ranges in Diploicia canescens. After phytochemical investigation of this lichen, twelve metabolites have been isolated and characterized. Those obtained in sufficient quantities were evaluated for their photoprotective capacities and compared to three referent sunscreens. Experimental spectra were compared to theoretical spectra as obtained at the TD-DFT level of theory. Different DFT functionals were tested to accurately reproduce the UV/Vis spectra of five depsidones, one diphenylether and two bisxanthones. Results indicate that absorption wavelengths and molecular extinction coefficients (oscillator strengths) obtained for the bisxanthones were similar to those of the UVA referent sunscreen.

Binding modes of a core-extended metalloporphyrin to human telomeric DNA G-quadruplexes.

The molecular recognition of human telomeric G-quadruplexes by a novel cationic π-extended Ni(II)-porphyrin (Ni(II)-TImidP4) is studied in aqueous solutions via (chir)optical spectroscopy, Fluorescence Resonance Energy Transfer (FRET) melting assay, and computational molecular modeling. The results are systematically compared with the recognition by a conventional meso-substituted Ni(II)-porphyrin (Ni(II)-TMPyP4), which allows us to pinpoint the differences in binding modes depending on the G-quadruplex topology. Importantly, FRET melting assays show the higher selectivity of Ni(II)-TImidP4 towards human telomeric G4 than that of Ni(II)-TMPyP4.

Bottom-Up Hierarchical Self-Assembly of Chiral Porphyrins through Coordination and Hydrogen Bonds

A series of chiral synthetic compounds is reported that shows intricate but specific hierarchical assembly because of varying positions of coordination and hydrogen bonds. The evolution of the aggregates (followed by absorption spectroscopy and temperature-dependent circular dichroism studies in solution) reveal the influence of the proportion of stereogenic centers in the side groups connected to the chromophore ring in their optical activity and the important role of pyridyl groups in the self-assembly of these chiral macrocycles. The optical activity spans 2 orders of magnitude depending on composition and constitution. Two of the aggregates show very high optical activity even though the isolated chromophores barely give a circular dichroism signal. Molecular modeling of the aggregates, starting from the pyridine-zinc(II) porphyrin interaction and working up, and calculation of the circular dichroism signal confirm the origin of this optical activity as the chiral supramolecular organization of the molecules. The aggregates show a broad absorption range, between approximately 390 and 475 nm for the transitions associated with the Soret region alone, that spans wavelengths far more than the isolated chromophore. The supramolecular assemblies of the metalloporphyrins in solution were deposited onto highly oriented pyrolitic graphite in order to study their hierarchy in assembly by atomic force microscopy. Zero and one-dimensional aggregates were observed, and a clear dependence on deposition temperature was shown, indicating that the hierarchical assembly took place largely in solution. Moreover, scanning electron microscopy images of porphyrins and metalloporphyrins precipitated under out-of-equilibrium conditions showed the dependence of the number and position of chiral amide groups in the formation of a fibrillar nanomaterial. The combination of coordination and hydrogen bonding in the complicated assembly of these molecules-where there is a clear hierarchy for zinc(II)-pyridyl interaction followed by hydrogen-bonding between amide groups, and then van der Waals interactions-paves the way for the preparation of molecular materials with multiple chromophore environments.

Influence of a Flavan-3-ol Substituent on the Affinity of Anthocyanins (Pigments) toward Vinylcatechin Dimers and Proanthocyanidins (Copigments)

The aim of this study is to investigate interactions possibly taking place in red wine between three flavanols (copigments, CP), i.e., two epimeric vinylcatechin dimers (CP1 and CP2) and catechin dimer B3 (CP3), and a specific pigment resulting from the condensation between the main grape anthocyanin malvidin 3-O-glucoside (oenin) and catechin, catechin-(4→8)-oenin. By comparison with our previous work on oenin itself, the influence of the catechin moiety of the anthocyanin in the binding is established. The thermodynamic parameters show that both vinylcatechin dimers exhibit a higher affinity for catechin-(4→8)-oenin, in comparison with proanthocyanidin B3, as previously observed with oenin. However, the corresponding binding constants are weaker, probably due to steric hindrance in the anthocyanin brought by the flavanol nucleus. Consequently, catechin-(4→8)-oenin should be much less stabilized by copigmentation in hydroalcoholic solution than oenin. Quantum mechanics and molecular dynamics simulations are also performed to interpret the binding data, to specify the relative arrangement of the pigment and copigment molecules within the complexes, and to interpret their absorption properties in the visible range.

DNA electronic circular dichroism on the inter-base pair scale: An experimental-theoretical case study of the at homo-oligonucleotide

A successful elucidation of the near-ultraviolet electronic circular dichroism spectrum of a short double-stranded DNA is reported. Time-dependent density functional theory methods are shown to accurately predict spectra and assign bands on the microscopic base-pair scale, a finding that opens the field for using circular dichroism spectroscopy as a sensitive nanoscale probe of DNA to reveal its complex interactions with the environment.