Raquel Gavara

The effect of self-aggregation on the determination of the kinetic and thermodynamic constants of the network of chemical reactions in 3-glucoside anthocyanins.

The six most common 3-glucoside anthocyanins, pelargonidin-3-glucoside, peonidin-3-glucoside, delphinidin-3-glucoside, malvidin-3-glucoside, cyanidin-3-glucoside and petunidin-3-glucoside were studied in great detail by NMR, UV-vis absorption and stopped flow. For each anthocyanin, the thermodynamic and kinetic constants of the network of chemical reactions were calculated at different anthocyanin concentration, from 6 × 10⁻⁶ M up to 8 × 10⁻⁴ M; an increasing of the flavylium cation acidity constant to give quinoidal base and a decreasing of the flavylium cation hydration constant to give hemiketal were observed by increasing the anthocyanin concentration. These effects are attributed to the self-aggregation of the flavylium cation and quinoidal base, which is stronger in the last case. The UV-vis and ¹H NMR spectral variations resulting from the increasing of the anthocyanin concentration were discussed in terms of two aggregation models; monomer-dimer and isodesmic, the last one considering the formation of higher order aggregates possessing the same aggregation constant of the dimer. The self-aggregation constant of flavylium cation at pH=1.0, calculated by both models increases by increasing the number of methoxy (-OCH₃) or hydroxy (-OH) substituents following the order: myrtillin (2 -OH), oenin (2 -OCH₃), 3-OGl-petunidin (1 -OH, 1 -OCH₃), kuromanin (1 -OH), 3-OGl-peonidin (1 -OCH₃) and callistephin (none). Evidence for flavylium aggregates possessing a shape between J and H was achieved, as well as for the formation of higher order aggregates.

Use of fluorescence spectroscopy to study polymeric materials with porous structure based on imprinting by self-assembled fibrillar networks.

Different polymeric materials have been prepared from the organogels formed by a polymerizable methacrylic mixture (methyl methacrylate/ethylene glycol dimethacrylate, 1:1, w/w) and the macrocyclic pseudopeptide 1. The use of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide as a very efficient radical initiator allows polymeric materials in which the structure of the fibrils formed by self-assembly of the organogelator 1 is truly preserved to be obtained. Removal of the pseudopeptidic molecule provides materials with a porous structure reflecting that of the original self-assembled fibrils. The use of fluorescent probes such as rhodamine B and pyrene greatly facilitate the study of the porous structures formed and, accordingly, that of the morphology of the original fibrils. Those studies reveal the presence of a permanent porosity and the organization of the substructures as a porous network. This confirms the existence of a nucleation and growth mechanism for the generation of the fibrils, giving rise to the formation of spherulitic structures. Those spherulites are additionally linked by connections of variable size. A series of diffusion experiments allowed establishment of a direct dependence of the inner porosity of the materials on the amount of self-organizing template used for their preparation.

From Au(I) organometallic hydrogels to well-defined Au(0) nanoparticles

The reaction of water soluble phosphine 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA) with [Au(CC–C5H4N)]n yields a luminescent water soluble phosphine gold(I) alkynyl complex [Au(4-pyridylethynyl)(DAPTA)] that leads to the formation of a luminescent hydrogel. A small variation of the phosphine structure (introduction of acetyl groups) with respect to the previously reported complex [Au(4-pyridylethynyl)(PTA)] has a clear effect on the observed properties: (i) a great increase in the hydrogel entanglement structure has been observed; (ii) the different cross-linking structures give rise to specific emission properties. The hydrogel was characterized by different techniques (1H-NMR, absorption and emission spectroscopy, optical microscopy, fluorescent microscopy (FM) and SEM). FM together with microspectrofluorimetry measurements has determined different emissive properties that vary according to the cross-linking process. Thermal treatments of the hydrogel produce well-defined metallic gold nanoparticles with a remarkable narrow size distribution, which have been characterized by SEM, TEM and X-ray photoelectron spectroscopy (XPS). At 200 °C, Au particles measuring 1.0 ± 0.2 nm were obtained. Au nanoparticles were also formed as a result of electron beam irradiation.

Thermodynamic aspects of aurophilic hydrogelators.

The complexes [Au(4-pyridylethynyl)(phosph)] (phosph = PTA (1), DAPTA (2)) are known to produce supramolecular aggregates and gels in water. We studied the impact of these aggregation processes in the absorption spectra, (1)H NMR (at different temperatures and concentrations), and DLS and estimated the equilibrium constant for a single step aggregation of the molecule (K = 26760 and 2590 M(-1) for 1 and 2, respectively, at 25 °C). We present spectroscopic evidence for the presence of Au···Au contacts in the aggregates: the recorded changes on (1)H NMR and the appearance of new absorption bands assigned to (σ*Au···Au-π*) have been attributed to the short (Au···Au) average distances in the aggregates. Relativistic density functional theory computations support the existence of short Au···Au distances and reveal charge-transfer in the aurophilic interactions. The free energy for a single step aggregation was calculated from the experimental data, and the value obtained (ΔG ∼ -20 kJ/mol) is in good agreement with the expected values in the order of the energies found for hydrogen bonds. The DFT computations confirm the experimental findings that aggregation of monomer 1 is stronger than the aggregation of monomer 2 and the existence of aurophilic interactions.

Circular dichroism of anthocyanidin 3-glucoside self-aggregates

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 (1)H NMR results, all measurements were consistent with a monomer-dimer model. The CD spectra of the anthocyanidin 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, [θ], 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 aggregates) and higher (H aggregates) 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.

Thermodynamics and kinetics of cyanidin 3-glucoside and caffeine copigments.

The multiequilibrium system of reactions of cyanidin 3-glucoside at acidic and mildly acidic pH values was studied in the presence of caffeine as a copigment. The thermodynamic and kinetic constants were determined using the so-called direct and reverse pH jump experiments that were followed by conventional UV-vis spectroscopy or stopped flow coupled to a UV-vis detector, depending on the rate of the monitored process. Compared with that of free anthocyanin, the copigmentation with caffeine extends the domain of the flavylium cation up to less acidic pH values, while in a moderately acidic medium, the quinoidal base becomes more stabilized. As a consequence, the hydration to give the colorless hemiketal is difficult over the entire range of pH values. At pH 1, two adducts were found for the flavylium cation-caffeine interaction, with stoichiometries of 1:1 and 1:2 and association constants of 161 M⁻¹ (K₁) and 21 M⁻¹ (K₂), respectively.

Reversible Self-Assembly of Water-Soluble Gold(I) Complexes

The reaction of the gold polymers containing bipyridyl and terpyridyl units, [Au(C≡CC15H10N3)]n and [Au(C≡CC10H7N2)]n, with the water-soluble phosphines 1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane and 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane gives rise to the formation of four gold(I) alkynyl complexes that self-assemble in water (H2O) and dimethyl sulfoxide (DMSO), through different intermolecular interactions, with an impact on the observed luminescence displayed by the supramolecular assemblies. A detailed analysis carried out by NMR studies performed in different DMSO/deuterated H2O mixtures indicates the presence of two different assembly modes in the aggregates: (i) chain assemblies, which are based mainly on aurophilic interactions, and (ii) stacked assemblies, which are based on Au···π and π···π interactions. These different supramolecular environments can also be detected by their intrinsic optical properties (differences in absorption and emission spectra) and are predicted by the changes in the relative binding energy from density functional theory calculations carried out in DMSO and H2O. Small-angle X-ray scattering (SAXS) experiments performed in the same mixture of solvents are in agreement with the formation of aggregates in all cases. The aromatic units chosen, bipyridine and terpyridine, allow the use of external stimuli to reversibly change the aggregation state of the supramolecular assemblies. Interaction with the Zn2+ cation is observed to disassemble the aggregates, while encapsulating agents competing for Zn2+ complexation revert the process to the aggregation stage, as verified by SAXS and NMR. The adaptive nature of the supramolecular assemblies to the metal-ion content is accompanied by significant changes in the absorption and emission spectra, signaling the aggregation state and also the content on Zn2+.

Polarized Supramolecular Aggregates Based on Luminescent Perhalogenated Gold Derivatives

The reaction of [Au(C6F5)(tht)] (tht = tetrahydrothiophene) with 1,3,5-triaza-7-phosphaadamantane (PTA) and 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA) leads to the formation of [Au(C6F5)(phosph)] (phosph = PTA, 1; phosph = DAPTA, 2). The compounds are slightly soluble in water and aggregate at higher concentrations, giving rise to the formation of needle- and rodlike structures (1) and well-organized spherical aggregates (2). Compounds 1 and 2 were reacted with AgPF6, giving rise to the formation in all cases of luminescent water-soluble 1:1 Au···Ag heterometallic complexes, as evidenced by X-ray crystal structure determination. The use of different silver salts that differ on the counterion induces changes in the resulting luminescence and aggregation morphology.

Ternary assemblies comprising metal–salophen complexes and 4,4′-bipyridine

Two series of ternary assemblies comprising metal–salophen [salophen = N,N′-phenylenebis(salicylideneimine)] complexes (M = Zn2+ or UO22+) and 4,4′-bipyridine have been synthesized. Their absorption and emission data have been compared with those of the corresponding mononuclear derivatives. The dinuclear zinc species present lower emission intensities than those corresponding to the mononuclear precursors, while the emission of the analogous uranyl complexes is observed to increase. Absorption and emission titrations indicate that the ternary species are obtained in two independent coordination steps. The important role of the electron donating/withdrawing properties of the substituents is analyzed regarding their photophysical properties and the association constants. A MLCT transition (recorded by emission spectra) is expected and supported by the increase in the emissive properties of the complexes containing the higher electron donating substituent (methoxy). Nevertheless, this electron donating character disfavors the coordination to bipyridine as shown by the lower calculated association constants. DFT calculations have been performed on the zinc derivatives in order to estimate the relative stability of the dinuclear complexes.

Modulation of supramolecular gold(I) aggregates by anion’s interaction

Abstract The gold complexes [Au(4-pyridylethynyl)(PTA)] and [Au(4-pyridylethynyl)(DAPTA)] (PTA = 1,3,5-triaza-7-phosphaadamantane; DAPTA = 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane) were used as host in molecular recognition processes of sodium hexametaphosphate (HMP) and a single-stranded 24-long oligonucleotide. Experiments were performed by absorption titrations and looking at the resulting host:guest adducts by polarised optical, fluorescence and scanning electron microscopy. The resulting information indicates that different types of interactions are present with the two different guest molecules. In the case of HMP, the compounds aggregate giving rise to larger structures, favouring exciton splitting coupling and the formation of head to tail interactions. In the case of oligonucleotide studies, the formation of smaller supramolecular structures is observed, with less contribution of aurophilic contacts and organised in a parallel way (head to head interactions organised by the presence of the oligonucleotide).