Sequential inclusion of two berberine cations in cucurbit[8]uril cavity: kinetic and thermodynamic studies

A combination of absorption and fluorescence spectroscopic studies with isothermal calorimetric titrations and stopped-flow measurements is a powerful way to reveal the thermodynamics and kinetics of inclusion complex formation with cucurbit[8]uril (CB8). The unique photophysical characteristics of berberine (B), a pharmaceutically important natural alkaloid, was exploited to distinguish the consecutive encapsulation processes, and to examine the confinement in CB8 cavity. The highly environment sensitive fluorescence lifetime of B permitted of the selective detection of various cucurbituril complexes, and indicated to what extent the embedded guest is available for interaction with water. Highly stable 1:1 and 2:1 B-CB8 complexes were produced due to the release of the high energy water molecules from the CB8 interior, and the second binding step proved to be almost 3 times more exothermic. The favorable entropy change appreciably contributed to the driving force of 1:1 encapsulation. Contrarily, the embedment of the second B in CB8 led to substantial entropy diminution. The kinetics of encapsulation was followed in real time by recording the fluorescence intensity change after rapid mixing of B and CB8. No evidence was found for intermediates. The rate constants of (64 ± 9)×10^6, and (5.0 ± 0.5)×10^6 M-1 s-1 were found for 1:1 and 2:1 associations, whereas 3.8 ± 0.6, and 0.6 ± 0.1 s-1 were obtained for the rate constants of the reverse processes at 283 K, respectively

Fény hatására végbemenő folyamatok önszerveződő rendszerekben = Photoinduced processes in self-assembled systems

Hidrogénhíddal, elektrosztatikus illetve hidrofób kölcsönhatással összetartott önszerveződő rendszerekben tanulmányoztuk a fényelnyelést követő folyamatok sebességét befolyásoló tényezőket. Biológiai fontosságú vegyületeket, alkaloidokat, festékeket választva modellvegyületként feltártuk az önszerveződés következtében létrejövő fluoreszcenciás sajátság változások okát. Kimutattuk, hogy a gyógyászati szempontból fontos berberin alkaloid az első fluoreszcenciás jelzőanyag, mely képes a primer és szekunder epesav aggregátumok relatív mennyiségének detektálására. Szupramolekuláris komplex képződésen alapuló nagy érzékenységű, egyszerű fluoreszcenciás eljárást dolgoztunk ki a berberin mennyiségének meghatározására. Tanulmányoztuk szulfonátokalixarén és kukurbit[7]uril makrociklusokba ékelődés hatását a gerjesztett berberin energiavesztési folyamataira. Egy másik rákellenes hatású természetes alkaloidról, az ellipticinről megállapítottuk, hogy egyes OH-vegyületek jelenlétében megfigyelhető kettős fluoreszcenciája nem az irodalomban javasolt tautomerizációtól, hanem gerjesztett állapotban lejátszódó protonátadástól ered. 1-Hidroxi-nílusvörös festék esetén szingulett gerjesztett állapotban nagyon gyors molekulán belüli protonátadást tapasztaltunk, mely tautomerizációra vezetett. Igazoltuk, hogy 1-alkil-3-metilimidazolium-bromid típusú ionfolyadékok vízben aggregálódnak, és vizsgáltuk a molekulaszerkezet változtatásának a hatását a keletkezett micellák tulajdonságaira. | The effects controlling the rate of photoinduced processes were studied in self-assembled systems combined by hydrogen bond, electrostatic and hydrophobic interactions. Biologically important compounds, alkaloids and dyes were chosen as models to clarify why the self-assembly of molecules affects the fluorescence properties. The pharmacological active alkaloid, berberine proved to be the first fluorescent probe, which is able to detect the relative amounts of primary and secondary bile salt aggregates. Utilizing supramolecular complex formation, a highly sensitive, simple fluorescence spectroscopic method was developed for the quantitative analysis of berberine. It was unraveled how the inclusion in the cavity of sulfonatocalixarenes and cucurbit[7]uril influences the kinetics and mechanism of excited berberine deactivation. Studies with another anticancer alkaloid, ellipticine demonstrated that the dual fluorescence in the presence of OH-compounds is due to photoinitiated intermolecular proton transfer instead of the tautomerization proposed in the literature. Singlet excited 1-hydroxy-subsitited Nile Red dye showed extremely fast intramolecular proton transfer leading to tautomerization. Aggregation of 1-alkyl-3-methylimidazolium bromide ionic liquids was found in aqueous solution and the effect of molecular structure alteration on the characteristics of micelles was revealed

Hidrogénhíd-kötés hatása a nílusvörös és az N-(4-piridil)-1,2- naftálimid fluoreszcenciás sajátságaira

A molekulaszerkezet és a gerjesztett állapot energiavesztési módjai közötti kapcsolat tanulmányozása iránt világszerte széleskörű érdeklődés tapasztalható, hiszen a feltárt eredmények nemcsak elméleti szempontból fontosak, hanem elősegítik a fény hatására lejátszódó folyamatok gazdaságos megvalósítását is. A hidrogénhíd-kötés alapvető szerepet játszik például biológiai folyamatokban, önszerveződő szupramolekuláris rendszerekben vagy molekulák szelektív kötődésekor. A szakirodalomban mégis alig találhatók olyan munkák, melyek szisztematikusan tanulmányozták volna a hidrogénhíd-kötés hatását gerjesztett állapotból kiinduló folyamatokra. Nem ismertek általános törvényszerűségek, melyek alapján megjósolható lenne, hogy a hidrogénhídkomplex képződés miként módosítja a gerjesztett molekulák energiavesztési folyamatainak fő irányát és kinetikáját. Az utóbbi években végzett kutatásainkkal e kérdések megválaszolásához kívántunk hozzájárulni

Effect of molecular structure and hydrogen bonding on the fluorescence of hydroxy-substituted naphthalimides

Fluorescence properties of hydroxy-naphthalimides were studied in methylene chloride in the absence and the presence of hydrogen-bonding additives. The position of the HO-substituent only slightly affects the radiative rate, however, the triplet yield and the rate of the radiationless processes are considerably higher for the 3-hydroxy derivative. Addition of nitrogen-heterocyclic compounds leads not only to hydrogen-bonding in the ground state but also Ñuorescence quenching. The parallel change throughout the series of the hydrogen-bond acceptors between the proton affinity and the rate constants of dynamic quenching indicates that proton displacement plays a crucial role in the excited hydrogen-bonded complexes. Interaction of hydroxy-naphthalimides with pyridine and benzoxazole results in rapid radiationless deactivation from the singlet excited state, whereas intense emission as well as long Ñuorescence lifetime characterize imidazole and pyrazole complexes. The dual emission of the imidazole complexes observed in solvents of medium polarity is assigned to two conformers which differ in the extent of the proton shift along the hydrogen-bond

Multiple inclusion complex formation of protonated ellipticine with cucurbit[8]uril: thermodynamics and fluorescence properties

The encapsulation of protonated ellipticine (EH+) in the cavity of cucurbit[8]uril (CB8) was studied in water at pH 4 with spectrophotometric, fluorescence spectroscopic and isothermal calorimetric measurements. The formation of three types of inclusion complexes was observed depending on the host and guest concentrations. Not only one, but also two EH+ was capable of encapsulation in CB8 in 37 μM EH+ solution and the thermodynamics of the binding steps were revealed. The produced very stable complexes showed markedly different absorption and fluorescence properties. When large excess of CB8 was employed in dilute (0.49 μM) EH+ solution, sequential binding of two CB8 occurred to the monomer alkaloid bringing about a substantial alteration in the fluorescence decay kinetics. The driving force of the 1:2 guest:host complex formation was much lower than that of 1:1 encapsulation. © 2016 Informa UK Limited, trading as Taylor & Francis Grou

Nemkovalens kölcsönhatások szerepe biológiai fontosságú vegyületek fényelnyelés hatására végbemenő folyamataiban = Effect of noncovalent interactions on the photoinduced processes of biologically important compounds

Feltártuk, hogy molekulák spontán önszerveződése komplexekké vagy jól definiált sztöchiometriájú aggregátumokká miként befolyásolja a fényelnyelést követő folyamatok kinetikáját és mechanizmusát. Biokompatibilis makrociklusokkal és polielektrolitokkal történő asszociációt tanulmányoztuk, hogy megismerjük a molekulaszerkezet hatását a kötődés erősségére és a fluoreszcenciás sajátságokra. Kukurbiturilok üregébe ékelődéssel módosítani tudtuk fotokróm anyagok tulajdonságait, limikróm tautomerizációját idéztük elő, alkaloid fotooxidációját és nukleofil addiciós átalakulását sikerült gátolnunk, lassítottuk egy merocianin hidrolízisét, megváltoztattuk a saverősséget és a gerjesztett állapotból kiinduló energiavesztés sebességét. Érzékeny fluoreszcenciás módszereket fejlesztettünk ki különféle komplexek kimutatására. Vizsgáltuk a triplett gerjesztett állapot és gyökképződés szerepét alkaloidok és benzoilftálimid származékok fotooxidációs és fotoredukciós reakcióiban. Megállapítottuk, hogy 4-hidroxi-4’-nitrosztilbének fotoiniciált folyamatainak sebessége jelentősen függ az oldószer polaritásától és hidrogen-híd kötő képességétől. 1-Alkil-3-metilimidazólium típusú ionos folyadékok kölcsönhatását vizsgálva kukurbiturilokkal és szulfonátokalixarénekkel összefüggést találtunk a molekulaszerkezet és a keletkezett beékelődési komplexek termodinamikai sajátságai között. | This project revealed how the spontaneous assembly of molecules into complexes or aggregates of well-defined stoichiometry influences the kinetics and mechanism of processes following light absorption. Association with biocompatible macrocycles and polyelectrolytes was studied to understand the effect of molecular structure on the binding strength and the fluorescent properties. It was demonstrated that the inclusion in cucurbiturils modifies photochromic behavior, induces tautomerization of lumichrome, protects alkaloid against photooxidation or nucleophilic addition, hinders the hydrolysis of a merocyanine, alters the acidity, and changes the rate of radiationless energy dissipation from the excited state. Sensitive fluorescence spectroscopic methods were developed to detect the formation of various complexes. The role of the triplet excited state and short-lived radicals in photooxidation, photoreduction of alkaloids and benzoylphthalimide derivatives was clarified. Effect of solvent polarity and hydrogen bonding on the competition among the excited state deactivation pathways of 4-hydroxy-4’-nitrostilbenes was unraveled. The results of systematic studies on the encapsulation of 1-alkyl-3-methylimidazolium type of ionic liquids in cucurbiturils and sulfonatocalixarenes showed how the length of the aliphatic chain of the guest compound and the properties of the macrocycles affect the thermodynamics of inclusion

Ion pair formation via photoinduced proton transfer in excited hydroxynaphthalimide-N-methylimidazole hydrogen bonded complex: effect of temperature and viscosity on dual fluorescence

The kinetics of photoinduced processes in the hydrogen bonded complex between N-methyl-3- hydroxynaphthalimide and 1-methylimidazole was studied in a wide temperature range in ethyl acetate and glycerol triacetate. The proton transfer within the excited complex was found to be very fast because of its negligible activation energy. The fairly intense dual Ñuorescence was assigned to hydrogen bonded and solvent separated ion pairs. Kinetic parameters for the various deactivation pathways of these excited species were derived from the combined analysis of the steady-state and the time-resolved Ñuorescence results. The Arrhenius pre-exponential factor of the transition from the solvent separated into the hydrogen bonded ion pair proved to be more than two orders of magnitude larger in glycerol triacetate compared with that in ethyl acetate, whereas the other processes showed less viscosity dependence. The radiationless energy dissipation rate of the hydrogen bonded ion pair was insensitive to the experimental conditions. However, thermal enhanced internal conversion was observed for the solvent separated ion pair

Influence of geometry on the emitting properties of 2,3-naphthalimides

The luminescence properties of 2,3-naphthalimides have been studied using picosecond and nanosecond spectroscopies. In acetonitrile solution N-phenyl-2,3-naphthalimid(e3 ) is found to emit dual fluorescence with emission maxima at 385 and 490 nm, respectively. The short-wavelength emission corresponds to the known fluorescence of the naphthalimides and is demonstrated for 3 to originate from a molecular conformation in which the phenyl substituent and the naphthalimide skeleton are orthogonal to each other. The long-wavelength emission is assumed to originate from a singlet excited state formed by a ca. 90° rotation of the phenyl group so that the two moieties are coplanar. Only a small dipole moment change is found between this excited state and the ground state. Only short-wavelength emission is observed with a lifetime in the nanosecond range as in the case of 1 and 2 when phenyl rotation is blocked with a bulky ortho tert-butyl group (compound 4). Increasing the viscosity of a glycerol/ethanol medium enhances both the efficiency and the lifetime of the short-wavelength emission of 3. It appears that at 77 K the emission originates directly from the Franck-Condon state. At room temperature, the other two emitting species are shown to arise from the Franck-Condon state by competitive intramolecular geometrical relaxation processes. Structures 5 and 6 are tentatively put forward to explain the formation of naphthazepinedione 8 by a 2 \pi + 2 \pi photochemical cycloaddition reaction

Kinetics of the reversible inclusion of flavopereirine in cucurbit[7]uril

The temperature dependence of the kinetics of flavopereirine inclusion in cucurbit[7]uril (CB7) was studied by stopped-flow method in water. The substantial blue-shift of the emission band and the ~4-fold fluorescence quantum yield enhancement upon host-guest binding permitted the monitoring of the formation and dissociation of the flavopereirine-CB7 1:1 complex in real time. The competitive binding of 1-adamantylammonium cation with extremely high affinity was exploited to selectively and very accurately determine the kinetic parameters of the flavopereirine exit from CB7 cavity. The rate constants of the ingression into and the egression from CB7 were found to be 9.0 ×10e7 M-1s-1 and 1.6 s-1 at 298 K, respectively. Both processes had substantial activation enthalpy implying that a steric barrier had to be overcome in the course of the reversible encapsulation. The 31 ± 2 kJ mol-1 activation enthalpy of the entry into CB7 was comparable to the 37 ± 2 kJ mol-1 enthalpy change upon the complex dissociation