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

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

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

Kinetics and Mechanism of Cation‐Induced Guest Release from Cucurbit[7]uril

Abstract: The release of two organic guests from cucurbit[7]uril (CB7) was selectively monitored by the stoppedflow method in aqueous solutions of inorganic salts to reveal the mechanistic picture in detail. Two contrasting mechanisms were identified: The symmetric dicationic 2,7- dimethyldiazapyrenium shows a cation-independent complex dissociation mechanism coupled to deceleration of the ingression in the presence of alkali and alkaline earth cations (M$^{+n}$) due to competitive formation of CB7–M$^{+n}$ complexes. A much richer, unprecedented kinetic behaviour was observed for the ingression and egression of the monocationic and non-symmetric berberine (B$^{+}$). The formation of ternary complex B+–CB7–M$^{+n}$ was unambiguously revealed. A difference of more than two orders of magnitude was found in the equilibrium constants of Mn+ binding to B$^{+}$–CB7 inclusion complex. Large cations, such as K$^{+}$ and Ba$^{2+}$, also promoted B$^{+}$ expulsion from the ternary complex in a bimolecular process. This study reveals a previously hidden mechanistic picture and motivates systematic kinetic investigations of other host–guest systems

Nanoparticle formation of chitosan induced by 4-sulfonatocalixarenes: utilization for alkaloid encapsulation

Spontaneous formation of positively charged nanoparticles was observed upon mixing more than stoichiometric amount of chitosan with 4-sulfonatocalix[8]arene (SCX8) in acidic solution. The particle size did not change with SCX8 concentration, polymer chain length, and the degree of deacetylation at 0.002 ≤ SCX8/chitosan ≤0.043 molar ratios in 0.01 M HCl. However, larger aggregates were produced when chitosan concentration was increased. The most stable nanoparticles with 160 nm diameter and narrow size distribution were obtained at pH 4 using low molecular weight chitosan. These particles encapsulated coralyne with more than 90 % entrapment efficiency and 15 % loading capacity. A loading ratio of [coralyne]/[SCX8] = 1.7 was achieved without any stability loss. 4-Sulfonatocalix[4]arene induced the formation of slightly smaller nanoparticles than its homologs comprising 6 or 8 phenol units. © 2016, Springer-Verlag Berlin Heidelberg