Light- and pH-regulated Water-soluble Pseudorotaxanes Comprising a Cucurbit[7]uril and a Flavylium-based Axle.

A linear double pyridinium-terminated thread comprising a central chalcone moiety is shown to provide two independent binding sites with similar affinity for cucurbit[7]uril (CB7) macrocycles in water as judged from NMR, UV-Visible and fluorescence spectroscopies. Association results in [2] and [3]pseudorotaxanes, which are both pH and photosensitive. Switching from the neutral chalcone to the cationic flavylium form upon irradiation at 365 nm under acidic conditions provided an enhanced CB7 association (K1:1 increases from 1.2×105  M-1 to 1.5×108  M-1 ), limiting spontaneous on-thread cucurbituril shuttling. This co-conformational change in the [2]pseudorotaxane is reversible in the dark with kobs =4.1×10-4  s-1 . Threading the flavylium moiety into CB7 leads to a dramatic increase in the fluorescence quantum yield, from 0.29 in the free axle to 0.97 in the [2]pseudorotaxane and 1.0 in the [3]pseudorotaxane.

Toward Light-Controlled Supramolecular Peptide Dimerization.

The selective photodeprotection of the NVoc-modified FGG tripeptide yields the transformation of its 1:1 receptor-ligand complex with cucurbit[8]uril into a homoternary FGG2@CB8 assembly. The resulting light-induced dimerization of the model peptide provides a tool for the implementation of stimuli-responsive supramolecular chemistry in biologically relevant contexts.

Strategies used by nature to fix the red, purple and blue colours in plants: a physical chemistry approach.

While identified by the respective flavylium cation, anthocyanins are much more than this molecule. The flavylium cation (generally appearing only at very acidic pH values) is one of the molecules of a complex sequence of pH dependent molecular species reversibly interconnected by different chemical reactions. These species include the red flavylium cation, purple quinoidal base and blue or bluish anionic quinoidal bases. At the common pH of the vacuoles of simpler anthocyanins, the red flavylium cation is present only at very acidic pH values and at moderately acidic pHs there is no significant colour of the purple quinoidal base. Moreover, the blue or bluish anionic quinoidal base appearing around neutral pH values is not stable. Intermolecular (copigmentation) and intramolecular (in acylated anthocyanins) interactions increase the colour hue and yield bathochromic shifts in the absorption bands, permitting to extend the pH domain of the flavylium cation and increase the mole fraction of the quinoidal bases. Metal complexation is another strategy. In particular, the Al3+ cation plays an essential role in the blue colour of hydrangea. The most sophisticated structures are however the metaloanthocyanins, such as the one that gives the blue colour of commelina communis, constituted of six anthocyanins, six flavanones and two metals. In this work we discuss how physical chemical tools are indispensable to account for the chemical behaviour of these complex systems. The experimental procedures and the equations needed to calculate all equilibrium constants of anthocyanins and the consequent pH dependent mole fraction distributions in the absence or presence of copigments are described in detail. Reverse pH jumps monitored by stopped flow have been shown to be an indispensable tool to calculate these parameters.

Evolution of Flavylium-Based Color Systems in Plants: What Physical Chemistry Can Tell Us.

Anthocyanins are the basis of the color of angiosperms, 3-deoxyanthocyanins and sphagnorubin play the same role in mosses and ferns, and auronidins are responsible for the color in liverworts. In this study, the color system of cyanidin-3-O-glucoside (kuromanin) as a representative compound of simpler anthocyanins was fully characterized by stopped flow. This type of anthocyanin cannot confer significant color to plants without intra- or intermolecular interactions, complexation with metals or supramolecular structures as in Commelina communis. The anthocyanin's color system was compared with those of 3-deoxyanthocyanins and riccionidin A, the aglycone of auronidins. The three systems follow the same sequence of chemical reactions, but the respective thermodynamics and kinetics are dramatically different.

Comparing the Chemistry of Malvidin-3-O-glucoside and Malvidin-3,5-O-diglucoside Networks: A Holistic Approach to the Acidic and Basic Paradigms with Implications in Biological Studies.

The kinetics, thermodynamics, and degradation of malvidin mono- and diglucosides were studied following a holistic approach by extending to the basic medium. In acidic conditions, the reversible kinetics of the flavylium cation toward the equilibrium is controlled by the hydration and cis-trans isomerization steps, while in the basic medium, the OH- nucleophilic addition to the anionic quinoidal bases is the slowest step. There is a pH range (transition pHs), between the acidic and basic paradigms, that includes physiological pH (7.4), where degradation reactions occur faster, preventing the system from reaching the equilibrium. The transition pH of the diglucoside is narrower, and in contrast with the monoglucoside, there is no evidence for the formation of colored oligomers among the degradation products. Noteworthy, OH- addition in position 4 to form B42-, a kinetic product that decreases the overall equilibration rate, was observed only for the diglucoside.

Intermolecular Copigmentation of Malvidin-3-O-glucoside with Caffeine in Water: The Effect of the Copigment on the pH-Dependent Reversible and Irreversible Processes.

Intermolecular copigmentation of malvidin-3-O-glucoside with caffeine was studied using a holistic procedure that includes the extension to basic pH values. In moderately basic solutions (7.5 < pH < 9.5) and independently of the copigment presence, there is a pH region where degradation of the quinoidal base and anionic quinoidal bases is faster than hydration and OH- nucleophilic addition, preventing the system from reaching the equilibrium. Intermolecular copigmentation with caffeine reduces significantly the degradation rate of quinoidal bases. In a more basic medium, the equilibrium is reached and degradation occurs from the anionic chalcones. In this case, the addition of caffeine also reduces the degradation rate in the interval 10 < pH < 11.5.

Intermolecular Copigmentation Between Delphinidin 3-O-Glucoside and Chlorogenic Acid: Taking into Account the Existence of Neutral and Negatively Charged Forms of the Copigment.

Stopped flow corroborated by UV-vis measurements allowed for the calculation of the copigmentation constants of delphinidin 3-O-glucoside with the neutral (CP) and negatively charged CP(-) forms of chlorogenic acid. Solutions of delphinidin 3-O-glucoside in the absence and presence of the copigment were equilibrated at several pH values in the acidic region, pH < 6, and reverse pH jumps monitored by stopped flow were carried out by adding sufficient acid to give flavylium cation at pH ≤ 1. This procedure allows for the separation of three contributions: (i) all flavylium cation and quinoidal base species, (ii) all hemiketal species, and (iii) all cis-chalcone species. Reverse pH jumps can also be performed at fixed pH versus copigment addition. The contribution of trans-chalcone, minor species in the present system, requires reverse pH jumps from the equilibrium followed by a common spectrophotometer. The system was also studied by UV-vis as a function of the copigment addition at different pH values. A global fitting of all experimental data allowed for determination of the copigmentation constants with flavylium cation, KAH+CP = 167 M-1, KAH+CP(-) = 338 M-1; and quinoidal base, KACP = 1041 M-1, KACP(-)= 221 M-1. No significant copigmentation was observed for hemiketal and chalcones. Computational calculations confirm different geometries for the interactions of flavylium cation and quinoidal base with the neutral or the negatively charged forms of the copigment as well as predict identical relative order for the binding energies of the four adducts.