Mónika Megyesi

Considerable Change of Fluorescence Properties upon Multiple Binding of Coralyne to 4-Sulfonatocalixarenes

The interaction of coralyne, an analogue of natural protoberberine alkaloids, with 4-sulfonatocalixarenes (SCXn) was studied in aqueous solution at pH 2 to reveal the major factors determining the stability, stoichiometry, and fluorescent properties of the species formed. Addition of SCXn to coralyne solution brought about remarkable fluorescence intensity diminution and hypochromism in the 300-440 nm absorption domain. SCXn hosts were capable of binding as many coralyne molecules as the number of their hydroxybenzenesulfonate units. The SCXn-promoted interaction among coralyne molecules was evidenced by the appearance of a long-lived fluorescence component. In dilute alkaloid solution, 1:1 and 1:2 coralyne/4-sulfonatocalix[4]arene complexes were formed, but only 1:1 association occurred with 4-sulfonatocalix[8]arene. Time-resolved fluorescence measurements demonstrated that photoinduced electron transfer from a hydroxybenzenesulfonate moiety to the singlet-excited coralyne can compete efficiently with the other deactivation processes.

Dimer-promoted fluorescence quenching of coralyne by binding to anionic polysaccharides

The effects of anionic chondroitin or dextran sulfates on the absorption and fluorescence properties of coralyne, a cationic benzo[c]phenanthridine type alkaloid, were studied in aqueous solution. The remarkably strong binding to both polysaccharides promotes the coralyne dimer formation, which was evidenced by the changes in the absorption and fluorescence spectra and the fluorescence decay. The extent of dimerization, induced by chondroitin, shows a significant pH dependence because the competitive protonation of the carboxylate moieties of the polymer chain decreases the number of binding sites. A larger molecular weight of dextran sulfate stabilizes the coralyne dimer more efficiently.

Photooxidation of Alkaloids: Considerable Quantum Yield Enhancement by Rose Bengal-sensitized Singlet Molecular Oxygen Generation

The photooxidation of sanguinarine, coralyne and berberine was studied in oxygenated alkaline methanol solutions. Rose bengal as photosensitizer significantly accelerates the process, indicating the importance of singlet molecular oxygen in the reaction mechanism. The quantum yield of sensitized oxidation was found to increase significantly with pH and reaches 0.4 for berberine at pH 13.8. The direct oxidation of alkaloids is less efficient, the quantum yield does not exceed 0.01 even in oxygen‐saturated solutions. The photoinduced electron ejection does not play a role in the oxidation. The uncharged pseudobase forms, which are present in alkaline medium, are oxidized much more easily than the alkaloid cations.

Effect of electrolytes, nucleotides and DNA on the fluorescence of flavopereirine natural alkaloid

The absorption and fluorescence characteristics of flavopereirine, a pharmaceutically important natural alkaloid, were studied to reveal how the complex formation and the change of the microenvironment affect the deactivation kinetics from the singlet-excited state. The fluorescence lifetime was not influenced by the ionic strength, but a significant deuterium effect was observed showing that hydrogen bonding in the singlet-excited state promoted energy dissipation. Nucleotides caused both static and dynamic quenching. The rate constant of the latter process increased when the nucleobase was capable of donating electron to the excited flavopereirine. The spectrophotometric measurements provided evidence for non-cooperative binding to double-stranded DNA with an equilibrium constant of 4.6 × 10(5) M(-1). Time-resolved fluorescence signals showed that three kinds of complexes are formed with distinct fluorescence lifetimes. Flavopereirine binding to chondroitin sulfate was also found, which led to different fluorescence characteristics at pH 2 and 6.

Photoreduction and ketone-sensitized reduction of alkaloids.

The photoprocesses of berberine, palmatine, coralyne, sanguinarine, flavopereirine and ellipticine were studied in several solvents. The quantum yields ΦΔ of singlet molecular oxygen formation of berberine, palmatine and sanguinarine are moderate in dichloromethane (0.2–0.6) and much smaller in acetonitrile or trifluoroethanol. For the other alkaloids examined, ΦΔ is rather independent of solvent polarity. The direct and ketone‐sensitized photolysis, using steady‐state irradiation at 313 nm or 248/308 nm laser pulses, was studied by absorption and fluorescence spectroscopy. Thereby, radicals were observed yielding eventually dihydro derivatives as major products, which are thermally back‐converted on admission of oxygen. The quantum yield of conversion of alkaloids to dihydroalkaloids is enhanced in the presence of triethylamine. The reaction in the presence of ketones and electron or H‐atom donors has a quantum yield of close to unity.